Gene interaction
Two or more pairs of alleles located on different pair of homologous chromosome may interact in various ways to control a single trait or even the phenotypic expression of one gene is altered by the influence of other gene. Such complex interactions are called gene interactions. These interactions may be:-
Intragenic interactions – The interactions between or among two or more alleles of a single gene are called intragenic or intrallelic interactions.
Intergenic interactions - The interactions between or among two or more alleles located on the different loci on the same chromosome or different chromosome is called intrgenic interactions.
Monogenes – Majority of heritable traits are governed by a pair of alleles that controls the expression of a complete trait. Such genes are called monogenes.
Qualitative Inheritance or discontinuous inheritance – The inheritance governed by monogenes is non metric and is called qualitative Inheritance. Eg. The fruit colour is red or green; seed shape is round or wrinkled; a cattle may have horn or no horn etc.
Polygenes – Many characters such as height, weight, intelligence, colour etc exhibit continuous variations which are determined by number of genes . Each gene controls only a fraction or unit of expression of a trait and all the genic pairs have cumulative effect. Such genes are called polygenes or cumulative factors.
Quantitative inheritance - The inheritance of these traits controlled by polygenes is called quantitative Inheritance.
INTERALLELIC OR INTERAGENIC INTERACTIONS
1. INCOMPLETE DOMINANCE OR SEMI DOMINANCE
Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele. This results in a third phenotype in which the expressed physical trait is a combination of the phenotypes of both alleles. Unlike complete dominance inheritance, one allele does not dominate or mask the other.
Example - Flower colour in Snapdragon – In Snapdragon a cross between pure breeds of red flowered and white flowered plants of Snapdragon produces all pink flowered plants in F1. On selfing of F1 hybrid the F2 generation consists of red, pink white flowers in ratio of 1:2:1 showing that there is absence of complete dominance of one allele over the other in the same allelic pair. 2.CO- DOMINANCE - Co-dominance is a type of non-Mendelian inheritance pattern that finds the traits expressed by the alleles to be equal in the phenotype. There is neither a complete dominance or incomplete dominance of one trait over the other for that given characteristic. Co- dominance would show both alleles equally instead of a blending of the traits as is seen in incomplete dominance.In the case of co-dominance, the heterozygous individual expresses both alleles equally. There is no mixing or blending involved and each is distinct and equally shown in the phenotype of the individual.
Eg. Blood alleles in man- One example of co-dominance in humans is the AB blood type. Red blood cells have antigens on them that are designed to fight off other foreign blood types, which is why only certain types of blood can be used for blood transfusions based on the recipient's own blood type. A type blood cells have one kind of antigen, while the B type blood cells have a different type. Normally, these antigens would signal that they are a foreign blood type to the body and would be attacked by the immune system. People with AB blood types have both antigens naturally in their systems, so their immune system will not attack those blood cells.
This makes people with the AB blood type "universal recipients" due to the co-dominance displayed by their AB blood type. The A type does not mask the B type and vice versa. Therefore, both the A antigen and B antigen are equally expressed in a display of co-dominance.
LETHAL GENES
It has been observed that all genes or genetic factors are not useful to the organism. There are
some genetic factors or genes, when present in any organism cause its death during early stage of development, are called lethal genes. They may even cause death of the individual either in homozygous dominant or homozygous recessive condition.
Types of Lethal Genes: Lethel genes may be classified in to the following groups: 1. Recessive lethals
2. Dominant lethals
3. Conditional lethals 1. Recessive lethal: Most of the lethal genes are recessive lethals. It is expressed only when they are in homozygous condition. The survival of heterozygotes is not affected e.g., coat colour in mice and Brachyphalangy in human beings – It is the occurrence of short digits in human beings due to the absence of one in the phalanges. A marriage between two persons having brachyphalangy results in 3 types of children. (1) ¼ normal due to homozygous for normal genes (2) 2/4 with brachyphalangy, a heterozygous condition. (3) ¼ children without phalanges die due to homozygous condition.
2. Dominant lethal: There are some lethal genes which reduce viability even in heterozygotes, are said as dominant lethals. e.g., epiloia gene in human beings. This cause mental defects, abnormal skin growth and tumors in heterozygotes, therefore, they die before reaching adulthood. The dominant lethals may be produced in every generation through mutation. Eg. Sickle shaped Anemia in man- This disease is an autosomal hereditary disorder in which erythrocytes become sickle shaped under oxygen deficiency due to the formation of abnormal haemoglobin by a gene Hbs. The gene for normal haemoglobin is represented as Hba. Homozygous individuals ( HbsHbs) die of fatal Anemia before reaching maturity whereas heterozygous individuals also called carriers have both Hbs and Hba. They show mild anemia because only a few RBC become sickle shaped.
3. Conditional lethal: The lethal genes require a definite or specific condition for their lethal action are said as conditional lethals. Many mutants of barley, maize, Neurospora, Drosophila and many other organisms are termed as temperature sensitive mutations. Each of them needs a definite, generally high temperature to express their lethal action. A chlorophyll mutant of barley allows normal chlorophyll development at a temperature of 19°C or above, but it develops albina or abnormal white seedlings at temperature below 8°C. Temperature is not only responsible to bring out conditional lethals. Some conditional lethals require light, nutrition etc.
NON ALLELIC OR INTERGENIC INTERACTIONS
COMPLEMENTARY GENES – These are two pairs of non allelic genes present on separate gene loci, interact to express for one phenotypic character but neither of them is able to express this character independently in the absence of other. Eg. Flower colour in sweet pea- In sweet pea flower is governed by interaction of two non- allelic genes WW and CC which are dominant over their recessive alleles ww and cc respectively.
Independently both genes produce white coloured flowers. A cross between two white coloured varieties pure for gene W and C respectively produce purple coloured flowered in F1 hybrids. When these F1 plants were self pollinated , both purple and white flowered plants appeared in F2 generation in the ratio of 9:7. SUPPLIMENTARY GENES - Supplementary genes are two independent pairs of genes interacting in such a manner that one dominant factor produces its effect whether the other is present or not, while the second gene can produce its effect only in the presence of the first. Eg.- In lablab a dominant a dominant gene B express for khaki seed coat in homozygous or heterozygous condition while it’s recessive allele b express for buff colour in homozygous
recessive condition. Another dominant gene L when interact with the dominant gene B produces chocolate colour.
Features of Selaginella
Observe the external features of the plant, and arrangement, types and shape of leaf, ligule, rhizophore and roots. 1. Plant body is sporophytic and the sporophyte is evergreen and perennial. 2. Most of the species are prostrate but Selaginella trachyphylla is sub-erect and S.erythropus is erect 3. Size of the sporophyte ranges from few centimetres to several feet in different species. 4. Plant body is differentiated into stem, leaves, rhizophore and roots. 5. Two sub-genera, namely Heterophyllum and Homoeophyllum, have been recognized in the genus Selaginella on the basis of characters of stem and leaves. 6. In sub-genus Heterophyllum the stem is prostrate, dorsiventral with lateral branching. It contains two types of leaves. But in case of sub-genus Homoeophyllum the stem is somewhat erect showing dichotomous type of branching and all the leaves are of only one type. 7. Leaves are simple, small, thin and ovate to lanceolate in shape 8. Each leaf contains a midrib. 9. In most of the species, leaves are of two types, i.e., smaller and larger 10. Leaves are arranged on the stem in four longitudinal rows. 11. Two rows of smaller leaves are present on the dorsal surface of stem while remaining two rows of larger leaves on the ventral surface. 12. A pair of leaves comprises of a small leaf on the dorsal surface and a large leaf on the ventral surface of the stem. 13. At the base of upper or adaxial surface of each leaf is present a thin membranous finger- like structure called ligule 14. Each ligule (Fig. 212) consists of a basal hemispherical glossopodium made up of large thin-walled cells and surrounded by a glossopodial sheath. Above the glossopodium is the body of ligule made up of many small and large cells. 15. At the place of branching in stem, arises a long, unbranched, leafless structure towards the lower side. This is known as rhizophore16. Rhizophore becomes branched at its tip and forms many adventitious roots.
Pteridophyta Before the flowering plants, the landscape was dominated with plants that looked like ferns for hundreds of millions of years. Today, their massive lineage have descendants that have almost the same characteristics as their ancient ancestors. Unlike most other members of the Plant Kingdom, pteridophytes don’t reproduce through seeds; they reproduce through spores instead.
Pteridophyta Classification Pteridophyta can be classified into:
Lycopodiopsida
• 1. Lycopodiidae (club mosses) 2. Selaginellidae (quillworts, spike mosses)
Polypodiopsida
• 1. Psilotidae: Ophioglossales (e.g. grape ferns) and Psilotales (whisk ferns). 2. Equisetidae (horsetails) 3. Polypodiidae (leptosporangiate ferns, the most species-rich group) 4. Marattiidae (marattioid ferns)
Pteridophyta Characteristics 1. Pteridophytes are the first true land plants: It is speculated that life began in the oceans, and through millions of years of evolution, life slowly adapted on to dry land. And among the first of the plants to truly live on land were the Pteridophytes. 2. They are seedless, vascular cryptogams: Pteridophytes are seedless, and they procreate through spores. They don’t have conducting tissues for transportation of water and minerals. Instead, the water and minerals flow from the surface of the plant- cell to cell in the plant’s body. This is also one of the reasons why these plants need a constantly moist environment to survive. 3. They show true alternation of generations: The sporophyte generation and the gametophyte generation are observed in Pteridophytes. 4. Sporophyte has true roots, stem and leaves: They contain vascular tissues. 5. Spores developed in sporangia are homosporous or heterosporous: The sporangium is the structures in which spores are formed. They are usually homosporous (meaning: one type of spore is produced) and are also heterosporous, (meaning: two kinds of spores are produced.) Read More: Sporulation 6. Sporangia are produced in groups on sporophylls: Leaves that bear the sporangia are termed as sporophylls. 7. Young leaves of sporophyte show circinate vernation: The tip of the leaves tends to curl inwards to protect the vulnerable growing parts. 8. Sex organs multicellular and jacketed: The male sex organs are called antheridia, while the female sex organs are called archegonia.
Life Cycle of Pteridophyta Similar to the life cycle of seed plants, the pteridophytes also involves the alternation of generations in its life cycle. However, the pteridophytes differ from mosses and seed plants in that both generations are independent and free-living. The sexuality of pteridophytic gametophytes can be classified as follows:
1. Dioicous: the individual gametophyte is either a male producing antheridia and sperm, or a female producing archegonia and egg cells. 2. Monoicous: every individual gametophyte may produce both antheridia and archegonia and it can function both as a male as well as a female. 3. Protandrous: the antheridia matures before the archegonia. 4. Protogynous: the archegonia matures before the antheridia.
Pteridophyta Examples Following are the important examples of Pteridophyta:
• Whisk Fern • Flying Spider • Dicksonia sellowiana • Salvinia natans • Lophosoria quadripinnata • Equisetidae • Man fern • Silver fern
Economic Importance of Pteridophytes Pteridophytes commonly known as Vascular Cryptogams, are the seedless vascular plants that evolved after bryophytes. Besides being a lower plant, pteridophytes are economically very important. Dry fronds of many ferns are used as a cattle feed. Pteridophytes are also used as a medicine. The decoction of foliage of Lycopodium is used in home¬opathy to treat diarrhoea, bladder irritability, eczema, rheumatism, constipation and inflammation of liver. The Flavonoids and saponins present in Equisetum have diuretic affect. The fern, Dryopteris yield an antihelminthic drug. Sporocarps of Marsilea are rich source of starch and eaten for their nutritive value as food. Osmunda cinnamomea is used externally for rheumatism and internally for joint pain. The chemically active principal ‘Marsiline’ isolated from Marsilea is found to be very effective against sedative and anti- convulsant principal. Aquatic pteridophyte Azolla is used as a biofertilizer. Azolla forms a symbiotic relationship with cyanobacteria and therefore, former is used in water¬logged rice fields as a green manure which provides nitrogen to the plant. Pteridophytes are also used as a indicator plants. Equisetum accumulates minerals, especially gold, in their stem. Similarly, Asplenium adulterinum is an indicator of nickel and Actinopteris australis is a cobalt indicator plant. Several ferns such as Angiopteris, Asple¬nium, Marattia, Microsorium, Nephrolepis, Phymatodes, etc., have aesthetic values for their beautiful habit, graceful shape of the leaves, and beautiful soral arrangement. Thus, these characte¬ristics make them horticulturally important plants.
Systematic position: Kingdom: plantae Division:Bryophyta Class: Hepaticopsida Order: Marchantiales Family: Marchantiacea Genus: Marchantia Morphology: The plant body of marchantia is of gametophyte which is called a thallus. The mature thallus may attain the length of 1-10cm. it is a dark green in color and possess the prominent midrib. The midrib is marked by the shallow groove in the dorsal surface and shows the rosette type. Along the midrib, special cup like structures called gemma cups is present. The ventral surface of the thallus bears two or more rows of violet, multicellular plate likes scales on the either side of the midrib. Scales are of two types: Ligulate and appendiculate. The ligulate scales are small and have no appendages. Scales gives the protection to the growing point. The ventral surface of the thallus also bears numerous rhizoids. They are pale brown, unicellular and branched. The rhizoid are of two types; smooth walled and tuberculated walled rhizoid. In the smooth walled rhizoid contain the smooth wall in the inner wall and in tuberculated rhizoids the inner wall possesses the peg like structure. They help in plant fixation and absorption of water and minerals. In the reproductive stages thalli bears small upright, stalked structure called antheridiophores and archegoniophores. This bears male sex organ and female sex organ respectively. Internal structure of the thallus: A v.s. of the thallus shows the two distinct regions, the upper photosynthetic and storage region. The upper epidermis is single layered of thin walled cells. It is interrupted by several air pores. Air pores are barrel shaped consisting of 4-5 superimposed cells having both an upper and lower opening. The air pores are helps to exchange the gaseous during the respiration and photosynthesis the photosynthesis region consist of large no. of air chambers surrounded by the single celled thick 4-8 cells high septa. The storage region consists of compactly arranged thin walled parenchymatous cells. Some cells may have single oil body or filled by the mucilage. The lowermost layer of the storage region is lower epidermis. Rhizoids and scales are borne form certain cells of lower epidermis. Sexual reproduction: The sexual reproduction of the marchantia is oogamous type. Marchantia plants are dioecious , i.e. male and female sex organ develop in separate thalli. The male sex organ is the antheridia and female sex organ are called archegonia. The sex organ of marchatia is borne on special erect and stalked branches called the gametophores. The gametophore bearing the antheridia is called antheriaophore or stalk of Male receptacle whereas the gametophore bearing archegonia is called archegoniophore or stalk of female receptacle. Antheriodiophore: Antheridiophore has an erect stalk which is about 2-3 cm in length. The top of antheridiophore is a flattened, slightly convex; eight lobed pelate discs each lobe of the disc has a growing point at its tip and this represent the apex of branch. On each lobe of the pellate disc, 10-12 antheridia develop in acropetl succession is the oldest being near the center and the youngest towards the tips of lobe. The ventral surface of stalk of antherodiosphore bears the scale and the rhizoids along the two grooves. The internal structure of antheridiospore is similar to that of the thallus. The antheridial chamber open outside by the pores called ostioles. Structure of mature antheridium: The mature antheridium is club shape structure and consists of a short multicellular and rounded or ovoid antheridium proper. The antheridium proper has a single jacket. The jacket layer encloses the single androcytes. The androcytes gets metamorphosed into motile biflagellated antherozoids or male gametes. Dehiscence of the antheridium: The mature antheridium dehiscence in the presence of water. When the water enter into the antheridial chamberthrough the outside, the antheridium become swell up. Some of the terminal cells the wall on coming in contact with water disintegrated. Thus the antheridium raptureand the mass of the antherozoid emerge out. The antherozoids are sets free from the mass and swim in water. Each antherozoid is long slightly coiled rod shaped st. with two flagella attach to the anterior end. The antherozoids swims in water present in the grooves of receptacles with the help of flagella. Archegoniospore: The archegoniospore consists of slender stalk and female receptacle. The stalks of mature archegoniosphore are comparatively longer and stouter then the antheridiospphore. The stalk is about 5-7cm in long and each surmounted by the lobe disc called the female receptacle. The disc is eight lobed with nine umbrellaslike rays dropping down. All the archegonia are covered by the two-lipped membrane known as perichaetium or involucre. A cup shaped outgrowth that surround the archegonium called perigynium, give the protection to the archegonium. Structure of the mature archegonium: The mature archegonium is flask shaped structure developed on short stalk. It has swollen venter and long neck. The neck consists of the six vertical rows of jacket cells. The apical parts of neck consistof four covered cells or lid cells. A single layer of jackets surrounds the venter. The venter has one egg and one venter canal cell. The fully mature, the neck canal cell and venter canal cell disorganize and to form the mucilage mass. Which absorb the moisture and swells. The swelling exerts the pressure on lid cells so that they separate apart and the passage down to the egg is developed. Fertilization: It takes place in presence of water. The mucilage with the malic acids starts oozing through the mouth and that attract the antherozoids. The movement of the antherozoid in response of chemical is called chemotaxis. During the fertilization the antherozoids moving into water with female receptacle. Then a no. Of antherozoids enter the archegonium, but only one of them fuses with the egg to form the diploid zygote. The zygote developed the wall around itself and called the oospore. The fertilized egg or oospores represent the first stage of sporophytic generation. Sporophyte: After the fertilization the archegonial cells shrivels. The wall of venter gives the 1- 4 layered investment called calyptra which is of the gametophytic origin. In addition, perigynium also grows rapidly and covers the calyptra. The third covering called the involucre or perichetium is already present around the archegonia of each lobe. Thus the mature sporophyte is enveloped by three distinct protective covering; thecalyptra, the perigynium, the perichaetium. These three covering layer protect young sporogonium from desiccation. The mature sporogonium of marchantia is completely parasite on gametophyte because of the lack of chloroplast; ventral position and the covering layer avoid the light penetration. It is differentiated into foot, seta and capsule: 1. Foot: it is made up of parenchymatous cells, which are situated towards the base of archegonium .It helps in absortion of food material from the gametophyte for developing sporophyte. 2. Seta: the seta is short stalked, junction of connecting link between the foot and the capsule. It helps in the dispersal of spores. 3. Capsule: it is situated towards the neck of archegonium. It has single jacket layer of cell. Spores and germination of spores: When the spores fall on the suitable condition they germinated. The swell by the absorption of water. The exine ruptures and intine produce the small germ tube. The filament like structure divided and redivided to form a multicellular structure and finally gets changed into dichotomously branched thallus of marchntia. Alternation of generation: The life cycle of marchantia shows the distict alternation of generation. The lifecycle is completed only when the plant passes through the both the stages. The dominant phase is haploid gametophytic phase. The gametophyte reproduces sexually by male and female gametes and the result in the formation of sporophyte. The sporophyte reproduces asexually by the formation of spores. These spores give rise to the gametophyte. Themarchantia plant is a gametophyte which develops from haploid sporo. The plant body consists of dorsoventrally differentiated thallus. The antheridia produce the female gametes or egg. The antherozoids swim in the water and reach to the egg of an archegonium. Out of them only one fuses with egg together to form a diploid zygote. The diploid zygote represent of first stage of sporophyteic generation. The diploid zygote develops into complicating and elaborated structure the sporogonium or sporophyte. The sporogenous cell inside the sporophyte divided by meiosis to form a tetrad of haploid. This method of spore formation involving the meiosis is an asexual reproduction. The spores gametophytic stage. Each sporegerminates and forms a haploid gametophytic thallus of marchantia.These two generation regularly alternate to each other. This is known as alternation of generation.
EQUISETUM CHARACTERS
1. Plant body is sporophytic and the sporophyte is a well-branched perennial herb.
2. Size of the plant body ranges from a few centimeters as in Equisetum scirpoides to several metres as in E. giganteum (up to 13 metres). Most of the species are less than a meter in height.
3. Plant body consists of a long, horizontal, underground rhizome, from which arise many roots towards the lower side and many erect aerial shoots towards upper side
4. Rhizome is long, creeping and well-branched. It is divisible into nodes and internodes.
5. Roots, which develop from the node of rhizome, are long, slender, well-branched and adventitious.
6. Aerial shoots, which arise from the rhizome towards upper side, are of two types, i.e., sterile or vegetative shoots and fertile or reproductive shoots.
7. Both the sterile and fertile aerial shoots are ribbed and divisible into nodes and internodes, but the former is well-branched and long-lived while the latter (fertile shoots) are generally unbranched and short-lived structures.
8. Aerial shoots as well as rhizome are articulated (i.e., jointed).
9. From the nodes of aerial sterile shoots arise two types of branches in whorls. Some are long, unlimited in growth, well-branched and contain the same structure as the main axis of aerial sterile shoot. Others are short, also bear nodes and inter- nodes but are limited in growth and unbranched10. Fertile shoots are unbranched, colourless or pale- yellow coloured branches, each of which bears a strobilus at the tip.
11. On the nodes of rhizome, sterile shoot and fertile shoots are present many scaly leaves.
12. Scaly leaves are minute, thin, uninerved, present in the form of a whorl and vary in number from 3 to 40 in different species.
13. These leaves are green when young but become yellow or red-coloured at maturity. 14. The upper end of each leaf is free and pointed but all of them unite below at the base to form a sheath on the node.
15. The number of leaves represents the number of ridges on the internode.
16. Many round or irregular bodies are present on the rhizome. These are thick-walled and meant for vegetative reproduction. These are called tubers
17. Functions of various parts of the sporophyte are as follows: (a) Roots – Absorption and fixation;
(b) Rhizome – Storage;
(c) Sterile shoots – Photosynthetic;
(d) Fertile shoot – Reproductive.
Thus, Equisetum shows an example of physiological division of labour.
Moss: Funaria, polytrichum Distribution: Worldwide in distribution, cold region, moist wall, rainy season and are valvate like, dry-winter season. Structure: Simple leafy plant 1-more cm in the length. Green in color, root like rhizoids, unicellular, branched, and only one type and helps in fixation, absorption of water mineral, stout and cylindrical stem leaves are small, membranous green with single rib, spiral manner. Reproduction: Sexual reproduction: It takes place by the formation of male and female gamete. The male and female gamete. The male and female female reproductive organ are antheridia and archegonia which lies in the association with sterile hair like St. called the paraphysis. The male and female reproductive organ are developed at the different branches of same plant,i.e . monoecious. The male branch has diversed leaves consisting group of antheridia at its apex which are usually arranged to yellow in color. The female branch has covered leaves usually produce the capsule as sporogonium. Antheridia of the male branch are club and covered by the jacket like st. they encloses the androcytes, where the spermatozoids are formed which comes out after the brusting of antheridia. Each antherozoids are coiled and biflagellated. Similarly archegonia of female branch are flask shape having narrow elongated neck with a row of neck canal cell and swollen venter. Inside the enter an oval ovum is formed. At the time of fertilization , the N.C.C degenerate to formed mucilaginous mass, which produce the a cane sugar that attract the antherozoid, and fuses with egg to form diploid zygote. The zygote soon develop a covering wall and called oospore. Thus the end of gametophytic and beguning of sporophytic phase . The oospore germinate to give rise to sporophyte or sporogonium. The sporogonium consist of 3 main part : 1. Foot: Itremain buried inside the archegonium nad absorb the food material for developing sporophyte. 2. Seta: it acts as stalk that helps in transportation of food as well as it helps in dehiscence of capsule. 3. Capsule:the capsule is pear shaped structure covered by the calyptra. It is the apical region consist of cap like uperculim with teeth like st. called the peristome, attach to the annukus. The peristome is hydrophobic in nature. FEMALE CAPSULE The main body of capsule supported by a solid base called the apophysis, consisting of conducting cells. The body of the capsule is covered by a single layer called the epidermis; they form a small gap called the stroma which helps in ex- change of gaseous. It consists of solid sterile tissue called columella, surrounded by the spore sac wall attach to the air chamber with trabaculee. Each spore sac bears a spore mother cell, which undergoes the meiosis and result in the formation of spores, these spore dispersed in dry season and when it gets a favourable condition it germinates. Each spore germinates to form the free filamentous st. called the protonema, which produce numerous bud called protonemal buds. These buds give to new gametophyte. Alternation of generation: In the life cycle of it is clearly seen that there are two phases. The gametophytic phase and sporophytic phase, or germination of oospore represents the sporophytic phaseswhereas gametophytic phase is the formation of gametophyte. These two phase alter to each other. That is called as alternation of generation. Gametophytic phase is dominated over the sporophytic phase and gametophytic phase last for long period of time and is independed.
Bryophyta: Features, Classification and Economic Importance
The division Bryophyta (Gr. bryon=moss) includes over 25000 species of non-vascular embryophytes such as mosses, liverworts and hornworts.
Bryophytes are small plants (2cm to 60cm) that grow in moist shady places. They don’t attain great heights because of absence of roots, vascular tissues, mechanical tissues and cuticle. They are terrestrial but require external water to complete their life cycle.
Hence, they are called “Amphibians of plant kingdom”.
The fossil record indicates that bryophytes evolved on earth about 395 – 430 million years ago (i.e. during Silurian period of Paleozoic era). The study of bryophytes is called bryology. Hedwig is called ‘Father of Bryology’. Shiv Ram Kashyap is the ‘Father of Indian Bryology’.
Salient features of Bryophytes: 1. Bryophytes grow in damp and shady places.
2. They follow heterologous haplodiplobiontic type of life cycle.
3. The dominant plant body is gametophyte on which sporophyte is semiparasitic for its nutrition.
4. The thalloid gametophyte differentiated in to rhizoids, axis (stem) and leaves.
5. Vascular tissues (xylem and phloem) absent.
6. The gametophyte bears multi-cellular and jacketed sex organs (antheridia and archegonia).
7. Sexual reproduction is oogamous type.
8. Multi-cellular embryo develops inside archegonium.
9. Sporophyte differentiated into foot, seta and capsule.
10. Capsule produces haploid meiospores of similar types (homosporous).
11. Spore germinates into juvenile gametophyte called protonema. 12. Progressive sterilization of sporogenous tissue noticed from lower to higher bryophytes.
13. Bryophytes are classified under three classes: Hepaticae (Liverworts), Anthocerotae (Hornworts) and Musci (Mosses).
Classification of Bryophytes: According to the latest recommendations of ICBN (International Code of Botanical Nomenclature), bryophytes have been divided into three classes.
1. Hepaticae ( Hepaticopsida = Liverworts)
2. Anthocerotae (Anthocertopsida= Hornworts)
3. Musci (Bryopsida= Mosses)
Class 1. Hepaticae or Hepaticopsida: 1. Gametophytic plant body is either thalloid or foliose. If foliose, the lateral appendages (leaves) are without mid-rib. Always dorsiventral.
2. Rhizoids without septa.
3. Each cell in the thallus contains many chloroplasts; the chloroplasts are without pyrenoi.
4. Sex organs are embedded in the dorsal surface.
5. Sporophyte may be simple (e.g., Riccia) having only a capsule, or differentiated into root, seta and capsule (e.g., Marchantia, Pallia and Porella etc.)
6. Capsule lacks columella.
7. It has 4 orders:
(i) Calobryales
(ii) Jungermanniales
(iii) Spherocarpales
(iv) Marchantiales. Class 2. Anthocerotae or Anthocerotopsid: 1. Gametophytic plant body is simple, thalloid; thallus dorsiventra without air cambers, shows no internal differentiation of tissues.
2. Scales are absent in the thallus.
3. Each cell of the thallus possesses a single large chloroplast with a pyrenoid.
4. Sporophyte is cylindrical only partly dependent upon gametophyte for its nourishment. It is differentiated into bulbous foot and cylindrical capsule. Seta is meristematic.
5. Endothecium forms the sterile central column (i.e., columella) in the capsule (i.e. columella is present).
6. It has only one order-Anthocerotales.
Class 3. Musci or Bryopsida: 1. Gametophyte is differentiated into prostrate protonema and an erect gametophores
2. Gametophore is foliose, differentiated into an axis (=stem) and lateral appendages like leaves but without midrib.
3. Rhizoids multi-cellular with oblique septa.
4. Elaters are absent in the capsule of sporangium.
5. The sex organs are produced in separate branches immersed in a group of leaves.
6. It has only three orders:
(i) Bryales,
(ii) Andriales and
(iii) Sphagnales.
Economic importance of Bryophytes: 1. Protection from soil erosion: Bryophytes, especially mosses, form dense mats over the soil and prevent soil erosion by running water.
2. Soil formation: Mosses are an important link in plant succession on rocky areas. They take part in binding soil in rock crevices formed by lichens. Growth of Sphagnum ultimately fills ponds and lakes with soil.
3. Water retention: Sphagnum can retain 18-26 times more water than its weight. Hence, used by gardeners to protect desiccation of the seedling during transportation and used as nursery beds.
4. Peat: It is a dark spongy fossilized matter of Sphagnum. Peat is dried and cut as cakes for use as fuel. Peat used as good manure. It overcomes soil alkalinity and increases its water retention as well as aeration. On distillation and fermentation yield many chemicals.
5. As food: Mosses are good source of animal food in rocky and snow-clad areas.
6. Medicinal uses: Decoction of Polytrichum commune is used to remove kidney and gall bladder stones. Decoction prepared by boiling Sphagnum in water for treatment of eye diseases. Marchantia polymorpha has been used to cure pulmonary tuberculosis.
7. Other uses: Bryophytes arc used as packing material for fragile goods, glass wares etc. Some bryophytes act as indicator plants. For example, Tortell tortusa grow well on soil rich in lime.
The lac operon
Regulation of genes for lactose utilization. lac repressor, catabolite activator protein, and cAMP.
Key points:
• The lac operon of E. coli contains genes involved in lactose metabolism. It's expressed only when lactose is present and glucose is absent. • Two regulators turn the operon "on" and "off" in response to lactose and glucose levels: the lac repressor and catabolite activator protein (CAP). • The lac repressor acts as a lactose sensor. It normally blocks transcription of the operon, but stops acting as a repressor when lactose is present. The lac repressor senses lactose indirectly, through its isomer allolactose. • Catabolite activator protein (CAP) acts as a glucose sensor. It activates transcription of the operon, but only when glucose levels are low. CAP senses glucose indirectly, through the "hunger signal" molecule cAMP.
Introduction
Lactose: it's what's for dinner! While that may not sound delicious to us (lactose is the main sugar in milk, and you probably don't want to eat it plain), lactose can be an excellent meal for E. coli bacteria. However, they'll only gobble up lactose when other, better sugars – like glucose – are unavailable.
With that for context, what exactly is the lac operon? The lac operon is an operon, or group of genes with a single promoter (transcribed as a single mRNA). The genes in the operon encode proteins that allow the bacteria to use lactose as an energy source.
What makes the lac operon turn on?
E. coli bacteria can break down lactose, but it's not their favorite fuel. If glucose is around, they would much rather use that. Glucose requires fewer steps and less energy to break down than lactose. However, if lactose is the only sugar available, the E. coli will go right ahead and use it as an energy source.To use lactose, the bacteria must express the lac operon genes, which encode key enzymes for lactose uptake and metabolism. To be as efficient as possible, E. coli should express the lac operon only when two conditions are met:Lactose is available, and
Glucose is not available
How are levels of lactose and glucose detected, and how how do changes in levels affect lac operon transcription? Two regulatory proteins are involved:
One, the lac repressor, acts as a lactose sensor.
The other, catabolite activator protein (CAP), acts as a glucose sensor.
TheseThese proteins bind to the DNA of the lac operon and regulate its transcription based on lactose and glucose levels. Let's take a look at how this works.
Structure of the lac operon The lac operon contains three genes: lacZ, lacY, and lacA. These genes are transcribed as a single mRNA, under control of one promoter.
Genes in the lac operon specify proteins that help the cell utilize lactose. lacZ encodes an enzyme that splits lactose into monosaccharides (single-unit sugars) that can be fed into glycolysis. Similarly, lacY encodes a membrane-embedded transporter that helps bring lactose into the cell.
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The lacZ gene encodes an enzyme called β-galactosidase, which is responsible for splitting lactose (a disaccharide) into readily usable glucose and galactose (monosaccharides).
The lacY gene encodes a membrane protein called lactose permease, which is a transmembrane "pump" that allows the cell to import lactose.
The lacA gene encodes an enzyme known as a transacetylase that attaches a particular chemical group to target molecules. It's not clear if this enzyme actually plays any role in lactose breakdown. (Weird but true!)
In addition to the three genes, the lac operon also contains a number of regulatory DNA sequences. These are regions of DNA to which particular regulatory proteins can bind, controlling transcription of the operon.
Structure of the lac operon. The DNA of the lac operon contains (in order from left to right): CAP binding site, promoter (RNA polymerase binding site), operator (which overlaps with promoter), lacZ gene, lacY gene, and lacA gene. The activator protein CAP, when bound to a molecule called cAMP (discussed later), binds to the CAP binding site and promotes RNA polymerase binding to the promoter. The lac repressor protein binds to the operator and blocks RNA polymerase from binding to the promoter and transcribing the operon.
The promoter is the binding site for RNA polymerase, the enzyme that performs transcription. The operator is a negative regulatory site bound by the lac repressor protein. The operator overlaps with the promoter, and when the lac repressor is bound, RNA polymerase cannot bind to the promoter and start transcription.
The CAP binding site is a positive regulatory site that is bound by catabolite activator protein (CAP). When CAP is bound to this site, it promotes transcription by helping RNA polymerase bind to the promoter.
Let's take a closer look at the lac repressor and CAP and their roles in regulation of the lac operon. The lac repressor
The lac repressor is a protein that represses (inhibits) transcription of the lac operon. It does this by binding to the operator, which partially overlaps with the promoter. When bound, the lac repressor gets in RNA polymerase's way and keeps it from transcribing the operon. Where does the lac repressor come from?
When lactose is not available, the lac repressor binds tightly to the operator, preventing transcription by RNA polymerase. However, when lactose is present, the lac repressor loses its ability to bind DNA. It floats off the operator, clearing the way for RNA polymerase to transcribe the operon.
Upper panel: No lactose. When lactose is absent, the lac repressor binds tightly to the operator. It gets in RNA polymerase's way, preventing transcription. LowerLowerLowerLowerLowerLowerLowerLower panel: With lactose. Allolactose (rearranged lactose) binds to the lac repressor and makes it let go of the operator. RNA polymerase can now transcribe the operon.
This change in the lac repressor is caused by the small molecule allolactose, an isomer (rearranged version) of lactose. When lactose is available, some molecules will be converted to allolactose inside the cell. Allolactose binds to the lac repressor and makes it change shape so it can no longer bind DNA.
Allolactose is an example of an inducer, a small molecule that triggers expression of a gene or operon. The lac operon is considered an inducible operon because it is usually turned off (repressed), but can be turned on in the presence of the inducer allolactose. Catabolite activator protein (CAP)
When lactose is present, the lac repressor loses its DNA-binding ability. This clears the way for RNA polymerase to bind to the promoter and transcribe the lac operon. That sounds like the end of the story, right? Well...not quite. As it turns out, RNA polymerase alone does not bind very well to the lac operon promoter. It might make a few transcripts, but it won't do much more unless it gets extra help from catabolite activator protein (CAP). CAP binds to a region of DNA just before the lac operon promoter and helps RNA polymerase attach to the promoter, driving high levels of transcription.
Where does CAP come from?
Upper panel: Low glucose. When glucose levels are low, cAMP is produced. The cAMP attaches to CAP, allowing it to bind DNA. CAP helps RNA polymerase bind to the promoter, resulting in high levels of transcription.
Lower panel: High glucose. When glucose levels are high, no cAMP is made. CAP cannot bind DNA without cAMP, so transcription occurs only at a low level.
CAP isn't always active (able to bind DNA). Instead, it's regulated by a small molecule called cyclic AMP (cAMP). cAMP is a "hunger signal" made by E. coli when glucose levels are low. cAMP binds to CAP, changing its shape and making it able to bind DNA and promote transcription. Without cAMP, CAP cannot bind DNA and is inactive. How is cAMP made, and how does it report glucose levels?
CAP is only active when glucose levels are low (cAMP levels are high). Thus, the lac operon can only be transcribed at high levels when glucose is absent. This strategy ensures that bacteria only turn on the lac operon and start using lactose after they have used up all of the preferred energy source (glucose).
So, when does the lac operon really turn on?
The lac operon will be expressed at high levels if two conditions are met:
Glucose must be unavailable: When glucose is unavailable, cAMP binds to CAP, making CAP able to bind DNA. Bound CAP helps RNA polymerase attach to the lac operon promoter.
Lactose must be available: If lactose is available, the lac repressor will be released from the operator (by binding of allolactose). This allows RNA polymerase to move forward on the DNA and transcribe the operon.
These two events in combination – the binding of the activator and the release of the repressor – allow RNA polymerase to bind strongly to the promoter and give it a clear path for transcription. They lead to strong transcription of the lac operon and production of enzymes needed for lactose utilization. Putting it all together
Now that we’ve seen all the moving parts of the lac operon, let’s put what we’ve learned together to see how the operon reacts to a variety of different conditions (presence or absence of glucose and lactose). Glucose present, lactose absent: No transcription of the lac operon occurs. That's because the lac repressor remains bound to the operator and prevents transcription by RNA polymerase. Also, cAMP levels are low because glucose levels are high, so CAP is inactive and cannot bind DNA.
_ Glucose present, lactose present: Low-level transcription of the lac operon occurs. The lac repressor is released from the operator because the inducer (allolactose) is present. cAMP levels, however, are low because glucose is present. Thus, CAP remains inactive and cannot bind to DNA, so transcription only occurs at a low, leaky level.
Glucose absent, lactose absent: No transcription of the lac operon occurs. cAMP levels are high because glucose levels are low, so CAP is active and will be bound to the DNA. However, the lac repressor will also be bound to the operator (due to the absence of allolactose), acting as a roadblock to RNA polymerase and preventing transcription.
Glucose absent, lactose present: Strong transcription of the lac operon occurs. The lac repressor is released from the operator because the inducer (allolactose) is present. cAMP levels are high because glucose is absent, so CAP is active and bound to the DNA. CAP helps RNA polymerase bind to the promoter, permitting high levels of transcription.
The trp operon
Key points:
• The trp operon, found in E. coli bacteria, is a group of genes that encode biosynthetic enzymes for the amino acid tryptophan. • The trp operon is expressed (turned "on") when tryptophan levels are low and repressed (turned "off") when they are high. • The trp operon is regulated by the trp repressor. When bound to tryptophan, the trp repressor blocks expression of the operon. • Tryptophan biosynthesis is also regulated by attenuation (a mechanism based on coupling of transcription and translation).
What is the trp operon? Bacteria such as Escherichia coli (a friendly inhabitant of our gut) need amino acids to survive—because, like us, they need to build proteins. One of the amino acids they need is tryptophan. If tryptophan is available in the environment, E. coli will take it up and use it to build proteins. However, E. coli can also make their own tryptophan using enzymes that are encoded by five genes. These five genes are located next to each other in what is called the trp operon. What's an operon? If tryptophan is present in the environment, then E. coli bacteria don't need to synthesize it, so transcription of the genes in the trp operon is switched "off." When tryptophan availability is low, on the other hand, the operon is switched "on," the genes are transcribed, biosynthetic enzymes are made, and more tryptophan is produced. Structure of the trp operon The trp operon includes five genes that encode enzymes needed for tryptophan biosynthesis, along with a promoter (RNA polymerase binding site) and an operator (binding site for a repressor protein). The genes of the trp operon are transcribed as a single mRNA.
Diagram of the trp operon. First, we see an E. coli bacterium with a circular chromosome. We zoom in on a small portion of the chromosome and see that the DNA is that of the trp operon. From left to right, the operon contains a promoter (where RNA polymerase binds), and within the right end of the promoter, an operator (where a repressor binds). There are some additional regulatory sequences, not labeled in this diagram, and then five coding sequences: trpE, _trp_D, trpC, trpB, and trpA. The operon is transcribed to produce a single mRNA that contains the coding sequences of all five of the genes. The coding sequences in the mRNA are translated separately, each one producing a protein. These proteins are enzymes (or enzyme subunits) needed for tryptophan biosynthesis.
Turning the operon "on" and "off"
What does the operator do? This stretch of DNA is recognized by a regulatory protein known as the trp repressor. When the repressor binds to the DNA of the operator, it keeps the operon from being transcribed by physically getting in the way of RNA polymerase, the transcription enzyme.
Where does the trp repressor come from?
The trp repressor does not always bind to DNA. Instead, it binds and blocks transcription only when tryptophan is present. When tryptophan is around, it attaches to the repressor molecules and changes their shape so they become active. A small molecule like trytophan, which switches a repressor into its active state, is called a corepressor.
High tryptophan: The tryptophan binds to the trp repressor and causes it to change shape, converting into its active (DNA-binding) form. The trp repressor with the bound tryptophan attaches to the operator, blocking RNA polymerase from binding to the promoter and preventing transcription of the operon.
When there is little tryptophan in the cell, on the other hand, the trp repressor is inactive (because no tryptophan is available to bind to and activate it). It does not attach to the DNA or block transcription, and this allows the trp operon to be transcribed by RNA polymerase.
Low tryptophan: trp repressor is not bound to tryptophan (since there is no tryptophan) and is thus in its inactive state (does not bind to the DNA of the operator). This allows RNA polymerase to bind to the promoter and transcribe the operon.
In this system, the trp repressor acts as both a sensor and a switch. It senses whether tryptophan is already present at high levels, and if so, it switches the operon to the "off" position, preventing unnecessary biosynthetic enzymes from being made. More trp operon regulation: Attenuation
Depending on the class you're taking, or on your own interests, you may also have heard about another form of trp operon regulation called attenuation.
Like regulation by the trp repressor, attenuation is a mechanism for reducing expression of the trp operon when levels of tryptophan are high. However, rather than blocking initiation of transcription, attenuation prevents completion of transcription.
When levels of tryptophan are high, attenuation causes RNA polymerase to stop prematurely when it's transcribing the trp operon. Only a short, stubby mRNA is made, one that does not encode any tryptophan biosynthesis enzymes. Attenuation works through a mechanism that depends on coupling (the translation of an mRNA that is still in the process of being transcribed).
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To understand attenuation, let's zoom in on a region of the trp operon that we skimmed over in the sections above. This section lies between the operator and the first gene of the operon and is called the leader. The leader encodes a short polypeptide and also contains an attenuator sequence. The attenuator does not encode a polypeptide, but when transcribed into mRNA, it has self-complementary sections and can form various hairpin structures.
Image showing the location of the leader. The leader comes after the promoter and operator, but before the trpE gene. From left to right, the leader DNA contains four marked segments labeled 1-4. Segment 1 encodes the leader polypeptide. Segments 2-4 are part of the attenuator.
Once RNA polymerase has started transcribing the operon, a ribosome can attach to the still-forming transcript and begin translating the leader region. The polypeptide encoded by the leader is short, just 141414 amino acids long, and it includes two tryptophan (Trp) residues^11start superscript, 1, end superscript. The tryptophans are important because:
If there is plenty of tryptophan, the ribosome won't have to wait long for a tryptophan-carrying tRNA, and will rapidly finish the leader polypeptide.
If there is little tryptophan, the ribosome will stall at the Trp codons (waiting for a Trp-carrying tRNA) and will be slow to finish translation of the leader.
Why does it matter if the ribosome translates the leader quickly or slowly? As mentioned above, the leader is followed by an attenuator region, which (in its mRNA form) can stick to itself to form different hairpin structures. One structure includes a transcription termination signal, while the other does not end termination (and in fact, prevents formation of the terminator hairpin)^22squared.
If the ribosome translates slowly, it will pause, and its pausing causes formation of the antiterminator (non-terminating hairpin). This hairpin prevents formation of the terminator and allows transcription to continue.
Low Trp levels: When there is not much tryptophan available in the cell, the ribosome will stall at the Trp codons while translating the short polypeptide at the start of the leader. This stalling causes regions 2 and 3 to associate with one another in a hairpin. This hairpin, called an antiterminator hairpin, prevents the terminator hairpin (regions 3 and 4 paired up) from forming. Termination does not occur and RNA polymerase continues transcribing, producing a transcript that includes the trpE-trpA genes.
If the ribosome translates quickly, it will fall off the mRNA after translating the leader peptide. This allows the terminator hairpin and an associated hairpin to form, making RNA polymerase detach and ending transcription.
High Trp levels: The ribosome does not stall at the Trp codons while synthesizing the leader polypeptide, because Trp is abundant (and there are thus plenty of Trp-carrying tRNAs around). Instead, the ribosome quickly synthesizes the leader polypeptide, reaches the stop codon, and detaches from the mRNA. This leaves regions 1 and 2 free to pair up, at which point regions 3 and 4 will also pair up and form a terminator hairpin. The terminator hairpin causes RNA polymerase to detach from the DNA and from the transcript, ending termination. A short mRNA consisting of the leader region is all that gets produced; the trpE- trpA genes are never transcribed.
This mechanism may be complex, but the result is pretty straightforward. When tryptophan is abundant, the ribosome moves quickly along the leader, the terminator hairpin forms, and transcription of the trp operon ends. When tryptophan is scarce, the ribosome moves slowly along the leader, the non- terminator hairpin forms, and transcription of the trp operon continues.
In other words, the logic of attenuation is the same as that of regulation by the trp repressor. In both cases, high levels of tryptophan in the cell shut down the expression of the operon. This makes sense, since high levels of tryptophan mean that the cell does not need to make more biosynthetic enzymes to produce additional tryptophan.
RNA ribonucleic acid, complex compound of high molecular weight that functions in cellular protein synthesis and replaces DNA (deoxyribonucleic acid) as a carrier of genetic codes in some viruses. RNA consists of ribose nucleotides (nitrogenous bases appended to a ribose sugar) attached by phosphodiester bonds, forming strands of varying lengths. The nitrogenous bases in RNA are adenine, guanine, cytosine, and uracil, which replaces thymine in DNA.
RNA Structure
RNA is typically single stranded and is made of ribonucleotides that are linked by phosphodiester bonds. A ribonucleotide in the RNA chain contains ribose (the pentose sugar), one of the four nitrogenous bases (A, U, G, and C), and a phosphate group. The subtle structural difference between the sugars gives DNA added stability, making DNA more suitable for storage of genetic information, whereas the relative instability of RNA makes it more suitable for its more short- term functions. The RNA-specific pyrimidine uracil forms a complementary base
Fig. RNA structure pair with adenine and is used instead of the thymine used in DNA. Even though RNA is single stranded, most types of RNA molecules show extensive intramolecular base pairing between complementary sequences within the RNA strand, creating a predictable three-dimensional structure essential for their function
Types of RNA Genetic and Non genetic RNA
GENETIC RNA – 1. It acts as genetic material. 2. It is found in most of the plant viruses It is found in all the higher organisms some animal viruses and certain bacteriophages. 3. It has self-replication property. 4. It is found in the absence of DNA. 5. It is only of one type. 6. Its main function is regulation. 7. It may be either single or double.
NON GENETIC RNA:- 1.It does not act as a genetic material. 2. It has self replication property, including plants, animals and bacteria. 3. It is always synthesized from DNA. 4. It is found in association with DNA 5. It is of three types, viz., messenger, ribosomal and transfer. 6. Its main function is protein synthesis of gene action. 7. It is always single stranded.
Depending upon the biosynthesis of proteins RNA'S are classified into three main categories rRNA, mRNA and tRNA. RIBOSOMAL RNA (rRNA) - Ribosomal ribonucleic acid (rRNA) is the RNA component of ribosomes, the molecular machines that catalyze protein synthesis. Ribosomal RNA constitute over sixty percent of the ribosome by weight and are crucial for all its functions – from binding to mRNA and recruiting tRNA to catalyzing the formation of a peptide bond between two amino acids. Even the structure of a ribosome is determined by the three-dimensional shape of its rRNA core. Proteins present in the ribosome serve to stabilize this structure through interactions with the core.Ribosomal RNA are transcribed in the nucleus, at specific structures called nucleoli. These are dense, spherical shapes that form around genetic loci coding for rRNA. Nucleoli are also crucial for the eventual biogenesis of ribosomes, through sequestration of ribosomal proteins. MESSENGER RNA (mRNA)is a single-stranded RNA molecule that corresponds to the genetic sequence of a gene and is read by the ribosome in the process of producing a protein. mRNA is created during the process of transcription, where the enzyme RNA polymerase converts genes into primary transcript mRNA (also known as pre-mRNA). This pre-mRNA usually still contains introns, regions that will not go on to code for the final amino acid sequence. These are removed in the process of RNA splicing, leaving only exons, regions that will encode the protein. This exon sequence constitutes mature mRNA. Mature mRNA is then read by the ribosome, and, utilising amino acids carried by transfer RNA (tRNA), the ribosome creates the protein. This process is known as translation. All of these processes form part of the central dogma of molecular biology, which describes the flow of genetic information in a biological system.
TRANSFER RNA (t RNA) A transfer RNA molecule is used in translation and consists of a single RNA strand that is only about 80 nucleotides long, containing an anticodon on the other end; the anticodon base-pairs with a complementary codon on mRNA and transfer RNA (abbreviated tRNA and formerly referred to as sRNA, for soluble RNA[1]) is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length,[2] that serves as the physical link between the mRNA and the amino acid sequence of proteins. tRNA does this by carrying an amino acid to the protein synthetic machinery of a cell (ribosome) as directed by a 3- nucleotide sequence (codon) in a messenger RNA (mRNA). As such, tRNAs are a necessary component of translation, the biological synthesis of new proteins in accordance with the genetic code. FUNCTION OF RNA 1.Serves as intermediary between DNA and protein; used by ribosome to direct synthesis of protein it encodes 2.Ensures the proper alignment of mRNA, tRNA, and ribosome during protein synthesis; catalyzes peptide bond formation between amino acids 3.Carries the correct amino acid to the site of protein synthesis in the ribosome 4. The rRNA forms ribosomes 5. RNA primer is essential for DNA replication. 6. Primary function of RNA. Is protein synthesis.
RIBOSOME The ribosome is a complex made of protein and RNA and which adds up to numerous million Daltons in size and assumes an important part in the course of decoding the genetic message reserved in the genome into protein.
STRUCTURE
Ribosomes are made of proteins and ribonucleic acid (abbreviated as RNA), in almost equal amounts. It comprises of two sections, known as subunits. The tinier subunit is the place the mRNA binds and it decodes, whereas the bigger subunit is the place the amino acids are included.
Both subunits comprise of both ribonucleic acid and protein components and are linked to each other by interactions between the proteins in one subunit and the rRNAs in the other subunit. The ribonucleic acid is obtained from the nucleolus, at the point where ribosomes are arranged in a cell.
The structures of ribosomes include:
• Situated in two areas of the cytoplasm. • They are seen scattered in the cytoplasm and a few are connected to the endoplasmic reticulum. • Whenever joined to the ER they are called the rough endoplasmic reticulum. • The free and the bound ribosomes are very much alike in structure and are associated with protein synthesis. • Around 37 to 62% of RNA is comprised of RNA and the rest is proteins. • Prokaryotes have 70S ribosomes respectively subunits comprising the little subunit of 30S and the bigger subunit of 50S. Eukaryotes have 80S ribosomes respectively comprising of little (40S) and substantial (60S) subunits. • The ribosomes seen in the chloroplasts of mitochondria of eukaryotes are comprised of big and little subunits composed of proteins inside a 70S particle. • Share a center structure which is very much alike to all ribosomes in spite of changes in its size. • The RNA is arranged in different tertiary structures. The RNA in the bigger ribosomes is into numerous continuous infusions as they create loops out of the center of the structure without disturbing or altering it. • The contrast between those of eukaryotic and bacteria are utilized to make antibiotics that can crush bacterial disease without damaging human cells.
Fig Structure of ribosome Transforming Principle: Frederick Griffith in 1928, carried out a series of experiments with Streptococcus pneumoniae (a bacterium that cause pneumonia). He observed that when these bacteria (Streptococcus pneumonia) are grown on a culture plate, some of them produce smooth, shiny colonies (S- type), whereas, the others produce rough colonies (R-type). This difference in character (smooth/rough) is due to a mucous (polysaccharide) coat present in the S-strain bacteria, which is not present in the R-strain.
In his experiments, he first infected two separate groups of mice. The mice that were infected with the S-strain die from pneumonia.:
Note: ‘S’ strains are the virulent strains causing pneumonia.
The mice that were infected with the R-strain do not develop pneumonia and they live.
S-strain (virulent strain) → Inject into mice → Mice die
R-strain (non-virulent strain) → Inject into mice → Mice live
In the next set of experiments, Griffith killed the bacteria by heating them. The mice that were injected heat-killed S-strain bacteria did not die and lived, whereas the mice that were injected a mixture of heat-killed S-strain and live R-strain bacteria, died due to unexpected symptoms of pneumonia.
S-strain (heat killed) → Inject into mice → Mice live
S-strain (heat killed) + R-strain (live) → Inject into mice → Mice die
Griffith concluded that the live R-strain bacteria, were transformed by the heat-killed S-strain bacteria.
He proved that there was some ‘transforming principle’ that was transferred from the heat- killed S-strain, which helped the R-strain bacteria to synthesise a smooth polysaccharide coat and thus, become virulent. That was due to the transfer of the genetic material.
However, he was not able to define the biochemical nature of genetic material from his experiments. Biochemical Characterisation of Transforming Principle: Oswald Avery, Colin MacLeod and Maclyn McCarty (1933-44) worked to determine the biochemical nature of ‘transforming principle’ in Griffith’s experiment in an in vitro system.
From the heat-killed S-cells, they purified biochemicals (proteins, DNA, RNA, etc.) to observe, that which biochemicals could transform live R-cells into S-cells.
Therefore, they discovered that DNA alone from heat-killed S-type bacteria caused the transformation of non-virulent R-type bacteria into S-type virulent bacteria.
Protein-digesting enzymes (proteases) and RNA-digesting enzymes (RNases) did not cause this transformation. This proved that the ‘transforming substance’ was neither the protein no RNA.
DNA-digesting enzyme (DNase) caused inhibition of transformation, which suggests that the DNA caused the transformation. Thus, these scientists came to the conclusion that DNA is the hereditary material.
Hershey and Chase Experiment: The proof for DNA as a genetic material came from the experiment. Alfred Hershey and Martha Chase (1952) carried out some experiments with the viruses that infect bacteria. These viruses are called bacteriophages.
The genetic material of bacteriophage enters the bacterial cell after the bacteriophage gets attached to the bacteria. The bacterial cell treats the genetic material of the virus (bacteriophage) like its own genetic material and then produces more virus particles. Hershey and Chase experimented to find out whether it was protein or DNA from the virus that had entered into the bacteria.
For this, they took two separate media for growing these bacteriophages: (i) Out of two, one medium contained radioactive phosphorus and the other medium contained radioactive sulphur. Viruses (bacteriophage) were then grown on each medium.
(a) The viruses grown in the presence of radioactive phosphorus (32P) contained radioactive DNA (but not radioactive protein). This is because DNA contains phosphorus not protein. (b) In the same way, the viruses grown in the medium containing radioactive sulphur (35S) now contained radioactive protein (not radioactive DNA). This is because DNA does not contain sulphur. (ii) These radioactive viruses (bacteriophages) were then allowed to attach to bacteria (E. colt). As the process of infection with virus continued, the bacteria were agitated in a blender and the viral coats of the bacteria were removed.
(iii) When they were spinned in a centrifuge, the virus particles were separated from the bacteria.
(iv)They observed that the bacteria that were infected with virus containing radioactive DNA were radioactive, whereas the bacteria that were infected with radioactive proteins were not radioactive.
(v) This indicates that only DNA not the protein coat entered the bacterial cell.
(vi) Thus, the genetic material that is passed from virus to bacteria is DNA.
Properties of Genetic Material: From the Hershey and Chase experiment, the fact was established that DNA acts as a genetic material. But later, studies revealed that in some viruses (e.g., Tobacco Mosaic Viruses, QB bacteriophage, etc.) RNA is the genetic material.
Following are the criteria that a molecule must fulfil to act as a genetic material: (i) It should be able to replicate.
(ii) It should be chemically and structurally stable.
(iii) It should provide the scope for slow changes
(iv) It should be able to express itself in the form of ‘Mendelian characters’. According to these criteria, both DNA and RNA have the ability to direct their duplications (because of the rule of base pairing and complementarity). Both the nucleic acids (DNA and RNA) have the ability to direct their duplications, whereas the other molecules in the living system, fail to duplicate, e.g., protein.
The Genetic Material
DNA is the genetic material that carries information from generation to generation. Structure of DNA The genetic material in most organisms is DNA or Deoxyribonucleic acid; whereas in some viruses, it is RNA or Ribonucleic acid. A DNA molecule consists of two polynucleotide chains i.e. chains with multiple nucleotides. Let’s understand the structure of this chain in detail. Structure Of Polynucleotide Chain 1. A nucleotide is made of the following components: 2. Pentose sugar – A pentose sugar is a 5-carbon sugar. In case of DNA, this sugar is deoxyribose whereas, in RNA, it is ribose. 3. Phosphate group 4. Nitrogenous base – These can be of two types – Purines and Pyrimidines. Purines include Adenine and Guanine whereas pyrimidines include Cytosine and Thymine. In RNA, thymine is replaced by Uracil. 5. Nitrogenous base + pentose sugar (via N-glycosidic linkage) = Nucleoside. 6. Nucleoside + phosphate group (via phosphoester linkage) = Nucleotide. 7. Nucleotide + Nucleotide (via 3′-5′ phosphodiester linkage) = Dinucleotide. 8. Many nucleotides linked together = Polynucleotide. 9. A polynucleotide has a free phosphate group at the 5′ end of the sugar and this is called the 5′ end. Similarly, the sugar also has a free 3′-OH group at the other end of the polynucleotide which is called the 3′ end. The backbone of a polynucleotide chain consists of pentose sugars and phosphate groups; whereas the nitrogenous bases project out of this backbone.
Polynucleotide chains of DNA and its components
Double Helix Structure DNA is a long polymer and therefore, difficult to isolate from cells in an intact form. This is why it is difficult to study its structure. However, in 1953, James Watson and Francis revealed the ‘double helix’ model of the structure of DNA, based on X-ray diffraction data from Maurice Wilkins and Rosalind Franklin. This model also reveals a unique property of polynucleotide chains – Base pairing. It refers to the hydrogen bonds that connect the nitrogen bases on opposite DNA strands. This pairing gives rise to complementary strands i.e. if you know the sequence of bases on one strand, you can predict the bases on the other strand. Additionally, if each DNA strand acts as a template for synthesis (parent) of a new strand, then the new double-stranded DNA (daughters) produced are identical to the parental DNA strand. Salient Features of DNA Double-Helix
1. It consists of two polynucleotide chains where the sugar and phosphate group form the backbone and the nitrogenous bases project inside the helix. 2. The two polynucleotide chains have anti-parallel polarity i.e. if one strand has 5′ → 3′ polarity, the other strand has 3′ → 5′ polarity. 3. The bases on the opposite strands are connected through hydrogen bonds forming base pairs (bp). Adenine always forms two hydrogen bonds with thymine from the opposite strand and vice-versa. Guanine forms three hydrogen bonds with cytosine from the opposite strand and vice-versa. Therefore, a purine always pairs with a pyrimidine on the other strand, giving rise to a uniform distance between the two strands of the helix. 4. The two strands coil in a right-handed fashion. Each turn of the helix is 3.4nm (or 34 Angstrom units) consisting of 10 nucleotides. These nucleotides are at a distance of 0.34nm (or 3.4 Angstrom units). 5. The helix is stable because of the base pairs that stack over one another and hydrogen bonds that hold the bases together.
DNA double helix Packaging of DNA Helix
1) If you calculate the length of DNA in a typical mammalian cell, it is approximately 2.2 meters. The dimension of a typical nucleus is only about 10-6 meters! Then, how does such a long polymer fit in the nucleus of a cell? 2) Prokaryotes like E. coli, do not have a defined nucleus. Here, the negatively-charged DNA is held together in large loops by positively-charged proteins in a structure called ‘nucleoid’. In Eukaryotes, however, the organization of DNA in the nucleus is much more complex and is as follows: 3) The negatively-charged DNA is wrapped around a positively-charged histone octamer i.e. a unit of 8 histone molecules. This forms a ‘Nucleosome‘. Histones are positively-charged proteins that are rich in basic amino acids – arginines and lysines. A typical nucleosome has 200bp of DNA helix. 4) Many nucleosomes join together to form a thread-like structure – Chromatin in the nucleus. The nucleosomes in chromatin appear as ‘beads-on-string’ under the electron microscope. 5) The chromatin is packaged to form chromatin fibres which are further coiled and condensed to form chromosomes. The higher level packaging of chromatin requires another set of proteins – Non-histone Chromosomal (NHC) proteins.
Nucleosome Structure Note: Euchromatin is the region of chromatin that is loosely packed and therefore stains lightly; whereas Heterochromatin is the densely packed region and therefore stains dark.
Transcription
Key points:
• Transcription is the first step in gene expression. It involves copying a gene's DNA sequence to make an RNA molecule. • Transcription is performed by enzymes called RNA polymerases, which link nucleotides to form an RNA strand (using a DNA strand as a template). • Transcription has three stages: initiation, elongation, and termination. • In eukaryotes, RNA molecules must be processed after transcription: they are spliced and have a 5' cap and poly-A tail put on their ends. • Transcription is controlled separately for each gene in your genome.
Introduction Have you ever had to transcribe something? Maybe someone left a message on your voicemail, and you had to write it down on paper. Or maybe you took notes in class, then rewrote them neatly to help you review. As these examples show, transcription is a process in which information is rewritten. Transcription is something we do in our everyday lives, and it's also something our cells must do, in a more specialized and narrowly defined way. In biology, transcription is the process of copying out the DNA sequence of a gene in the similar alphabet of RNA.
Overview of transcription Transcription is the first step in gene expression, in which information from a gene is used to construct a functional product such as a protein. The goal of transcription is to make a RNA copy of a gene's DNA sequence. For a protein-coding gene, the RNA copy, or transcript, carries the information needed to build a polypeptide (protein or protein subunit). Eukaryotic transcripts need to go through some processing steps before translation into proteins.
RNA polymerase The main enzyme involved in transcription is RNA polymerase, which uses a single-stranded DNA template to synthesize a complementary strand of RNA. Specifically, RNA polymerase builds an RNA strand in the 5' to 3' direction, adding each new nucleotide to the 3' end of the strand.
RNA polymerase synthesizes an RNA strand complementary to a template DNA strand. It synthesizes the RNA strand in the 5' to 3' direction, while reading the template DNA strand in the 3' to 5' direction. The template DNA strand and RNA strand are antiparallel. Stages of transcription Transcription of a gene takes place in three stages: initiation, elongation, and termination. Initiation. RNA polymerase binds to a sequence of DNA called the promoter, found near the beginning of a gene. Each gene (or group of co-transcribed genes, in bacteria) has its own promoter. Once bound, RNA polymerase separates the DNA strands, providing the single- stranded template needed for transcription.
The promoter region comes before (and slightly overlaps with) the transcribed region whose transcription it specifies. It contains recognition sites for RNA polymerase or its helper proteins to bind to. The DNA opens up in the promoter region so that RNA polymerase can begin transcription. Elongation. One strand of DNA, the template strand, acts as a template for RNA polymerase. As it "reads" this template one base at a time, the polymerase builds an RNA molecule out of complementary nucleotides, making a chain that grows from 5' to 3'. The RNA transcript carries the same information as the non-template (coding) strand of DNA, but it contains the base uracil (U) instead o What do 5' and 3' mean?
RNA polymerase synthesizes an RNA transcript complementary to the DNA template strand in the 5' to 3' direction. It moves forward along the template strand in the 3' to 5' direction, opening the DNA double helix as it goes. The synthesized RNA only remains bound to the template strand for a short while, then exits the polymerase as a dangling string, allowing the DNA to close back up and form a double helix. Termination. Sequences called terminators signal that the RNA transcript is complete. Once they are transcribed, they cause the transcript to be released from the RNA polymerase. An example of a termination mechanism involving formation of a hairpin in the RNA is shown below.
The terminator DNA encodes a region of RNA that forms a hairpin structure followed by a string of U nucleotides. The hairpin structure in the transcript causes the RNA polymerase to stall. The U nucleotides that come after the hairpin form weak bonds with the A nucleotides of the DNA template, allowing the transcript to separate from the template and ending transcription. Eukaryotic RNA modifications In bacteria, RNA transcripts can act as messenger RNAs (mRNAs) right away. In eukaryotes, the transcript of a protein-coding gene is called a pre-mRNA and must go through extra processing before it can direct translation. Eukaryotic pre-mRNAs must have their ends modified, by addition of a 5' cap (at the beginning) and 3' poly-A tail (at the end). Many eukaryotic pre-mRNAs undergo splicing. In this process, parts of the pre-mRNA (called introns) are chopped out, and the remaining pieces (called exons) are stuck back together.
End modifications increase the stability of the mRNA, while splicing gives the mRNA its correct sequence. (If the introns are not removed, they'll be translated along with the exons, producing a "gibberish" polypeptide.)
translation.
In the process of translation, a cell reads information from a molecule called a messenger RNA (mRNA) and uses this information to build a protein. Translation involves “decoding” a messenger RNA (mRNA) and using its information to build a polypeptide, or chain of amino acids. For most purposes, a polypeptide is basically just a protein (with the technical difference being that some large proteins are made up of several polypeptide chains). The genetic code
• In an mRNA, the instructions for building a polypeptide come in groups of three nucleotides called codons. Here are some key features of codons to keep in mind as we move forward: • There are 616161 different codons for amino acids • Three “stop” codons mark the polypeptide as finished • One codon, AUG, is a “start” signal to kick off translation (it also specifies the amino acid methionine) Codons to amino acids In translation, the codons of an mRNA are read in order (from the 5' end to the 3' end) by molecules called transfer RNAs, or tRNAs.Each tRNA has an anticodon, a set of three nucleotides that binds to a matching mRNA codon through base pairing. The other end of the tRNA carries the amino acid that's specified by the codon.tRNAs bind to mRNAs inside of a protein-and-RNA structure called the ribosome. As tRNAs enter slots in the ribosome and bind to codons, their amino acids are linked to the growing polypeptide chain in a chemical reaction. The end result is a polypeptide whose amino acid sequence mirrors the sequence of codons in the mRNA.
Translation stages Initiation ("beginning"): in this stage, the ribosome gets together with the mRNA and the first tRNA so translation can begin. Elongation ("middle"): in this stage, amino acids are brought to the ribosome by tRNAs and linked together to form a chain. Termination ("end"): in the last stage, the finished polypeptide is released to go and do its job in the cell. Initiation
• In order for translation to start, we need a few key ingredients. These include: • A ribosome (which comes in two pieces, large and small) • An mRNA with instructions for the protein we'll build • An "initiator" tRNA carrying the first amino acid in the protein, which is almost always methionine (Met) • During initiation, these pieces must come together in just the right way. Together, they form the initiation complex, the molecular setup needed to start making a new protein.
Elongation methionine-carrying tRNA starts out in the middle slot of the ribosome, called the P site. Next to it, a fresh codon is exposed in another slot, called the A site. The A site will be the "landing site" for the next tRNA, one whose anticodon is a perfect (complementary) match for the exposed codon. Once the matching tRNA has landed in the A site, it's time for the action: that is, the formation of the peptide bond that connects one amino acid to another. This step transfers the methionine from the first tRNA onto the amino acid of the second tRNA in the A site.The methionine forms the N-terminus of the polypeptide, and the other amino acid is the C-terminus. Termination Polypeptides, like all good things, must eventually come to an end. Translation ends in a process called termination. Termination happens when a stop codon in the mRNA (UAA, UAG, or UGA) enters the A site. Stop codons are recognized by proteins called release factors, which fit neatly into the P site (though they aren't tRNAs). Release factors mess with the enzyme that normally forms peptide bonds: they make it add a water molecule to the last amino acid of the chain. This reaction separates the chain from the tRNA, and the newly made protein is released. What next? Luckily, translation "equipment" is very reusable. After the small and large ribosomal subunits separate from the mRNA and from each other, each element can (and usually quickly does) take part in another round of translation.
Proteins: Large molecules composed of one or more chains of amino acids in a specific order determined by the base sequence of nucleotides in the DNA coding for the protein.
Proteins are required for the structure, function, and regulation of the body's cells, tissues, and organs. Each protein has unique functions. Proteins are essential components of muscles, skin, bones and the body as a whole. Examples of proteins include whole classes of important molecules, among them enzymes, hormones, and antibodies. Proteins are one of the three types of nutrients used as energy sources by the body, the other two being carbohydrate and fat. Proteins and carbohydrates each provide 4 calories of energy per gram, while fats produce 9 calories per gram. Primary structure
The simplest level of protein structure, primary structure, is simply the sequence of amino acids in a polypeptide chain. For example, the hormone insulin has two polypeptide chains, A and B, shown in diagram below. (The insulin molecule shown here is cow insulin, although its structure is similar to that of human insulin.) Each chain has its own set of amino acids, assembled in a particular order. For instance, the sequence of the A chain starts with glycine at the N-terminus and ends with asparagine at the C-terminus, and is different from the sequence of the B chain.
Image of insulin. Insulin consists of an A chain and a B chain. They are connected to one another by disulfide bonds (sulfur-sulfur bonds between cysteines). The A chain also contains an internal disulfide bond. The amino acids that make up each chain of insulin are represented as connected circles, each with the three-letter abbreviation of the amino acid's name. The sequence of a protein is determined by the DNA of the gene that encodes the protein (or that encodes a portion of the protein, for multi-subunit proteins). A change in the gene's DNA sequence may lead to a change in the amino acid sequence of the protein. Even changing just one amino acid in a protein’s sequence can affect the protein’s overall structure and function. For instance, a single amino acid change is associated with sickle cell anemia, an inherited disease that affects red blood cells. In sickle cell anemia, one of the polypeptide chains that make up hemoglobin, the protein that carries oxygen in the blood, has a slight sequence change. The glutamic acid that is normally the sixth amino acid of the hemoglobin β chain (one of two types of protein chains that make up hemoglobin) is replaced by a valine. This substitution is shown for a fragment of the β chain in the diagram below. What is most remarkable to consider is that a hemoglobin molecule is made up of two α chains and two β chains, each consisting of about 150 amino acids, for a total of about 600 amino acids in the whole protein. The difference between a normal hemoglobin molecule and a sickle cell molecule is just 2 amino acids out of the approximately 600. A person whose body makes only sickle cell hemoglobin will suffer symptoms of sickle cell anemia. These occur because the glutamic acid-to-valine amino acid change makes the hemoglobin molecules assemble into long fibers. The fibers distort disc-shaped red blood cells into crescent shapes. Examples of “sickled” cells can be seen mixed with normal, disc-like cells in the blood sample bel kled cells get stuck as they try to pass through blood vessels. The stuck cells impair blood flow and can cause serious health problems for people with sickle cell anemia, including breathlessness, dizziness, headaches, and abdominal pain. Secondary structure
The next level of protein structure, secondary structure, refers to local folded structures that form within a polypeptide due to interactions between atoms of the backbone. (The backbone just refers to the polypeptide chain apart from the R groups – so all we mean here is that secondary structure does not involve R group atoms.) The most common types of secondary structures are the α helix and the β pleated sheet. Both structures are held in shape by hydrogen bonds, which form between the carbonyl O of one amino acid and the amino H of another.
Images showing hydrogen bonding patterns in beta pleated sheets and alpha helices. In an α helix, the carbonyl (C=O) of one amino acid is hydrogen bonded to the amino H (N-H) of an amino acid that is four down the chain. (E.g., the carbonyl of amino acid 1 would form a hydrogen bond to the N-H of amino acid 5.) This pattern of bonding pulls the polypeptide chain into a helical structure that resembles a curled ribbon, with each turn of the helix containing 3.6 amino acids. The R groups of the amino acids stick outward from the α helix, where they are free to interact^33cubed. In a β pleated sheet, two or more segments of a polypeptide chain line up next to each other, forming a sheet-like structure held together by hydrogen bonds. The hydrogen bonds form between carbonyl and amino groups of backbone, while the R groups extend above and below the plane of the sheet^33cubed. The strands of a β pleated sheet may be parallel, pointing in the same direction (meaning that their N- and C-termini match up), or antiparallel, pointing in opposite directions (meaning that the N-terminus of one strand is positioned next to the C- terminus of the other). Certain amino acids are more or less likely to be found in α-helices or β pleated sheets. For instance, the amino acid proline is sometimes called a “helix breaker” because its unusual R group (which bonds to the amino group to form a ring) creates a bend in the chain and is not compatible with helix formation^44start superscript, 4, end superscript. Proline is typically found in bends, unstructured regions between secondary structures. Similarly, amino acids such as tryptophan, tyrosine, and phenylalanine, which have large ring structures in their R groups, are often found in β pleated sheets, perhaps because the β pleated sheet structure provides plenty of space for the side chains^44start superscript, 4, end superscript. Many proteins contain both α helices and β pleated sheets, though some contain just one type of secondary structure (or do not form either type). Tertiary structure
The overall three-dimensional structure of a polypeptide is called its tertiary structure. The tertiary structure is primarily due to interactions between the R groups of the amino acids that make up the protein. R group interactions that contribute to tertiary structure include hydrogen bonding, ionic bonding, dipole-dipole interactions, and London dispersion forces – basically, the whole gamut of non-covalent bonds. For example, R groups with like charges repel one another, while those with opposite charges can form an ionic bond. Similarly, polar R groups can form hydrogen bonds and other dipole-dipole interactions. Also important to tertiary structure are hydrophobic interactions, in which amino acids with nonpolar, hydrophobic R groups cluster together on the inside of the protein, leaving hydrophilic amino acids on the outside to interact with surrounding water molecules. Finally, there’s one special type of covalent bond that can contribute to tertiary structure: the disulfide bond. Disulfide bonds, covalent linkages between the sulfur-containing side chains of cysteines, are much stronger than the other types of bonds that contribute to tertiary structure. They act like molecular "safety pins," keeping parts of the polypeptide firmly attached to one another.
Image of a hypothetical polypeptide chain, depicting different types of side chain interactions that can contribute to tertiary structure. These include hydrophobic interactions, ionic bonds, hydrogen bonds, and disulfide bridge formation.
Quaternary structure Many proteins are made up of a single polypeptide chain and have only three levels of structure (the ones we’ve just discussed). However, some proteins are made up of multiple polypeptide chains, also known as subunits. When these subunits come together, they give the protein its quaternary structure. We’ve already encountered one example of a protein with quaternary structure: hemoglobin. As mentioned earlier, hemoglobin carries oxygen in the blood and is made up of four subunits, two each of the α and β types. Another example is DNA polymerase, an enzyme that synthesizes new strands of DNA and is composed of ten subunits^55start superscript, 5, end superscript. In general, the same types of interactions that contribute to tertiary structure (mostly weak interactions, such as hydrogen bonding and London dispersion forces) also hold the subunits together to give quaternary structure.
Flowchart depicting the four orders of protein structure.
Class: Pteropsida Eg.Fern (Dryopteris) Pteris Character: 1. Perennial in habit, worldwide in distribution, present mainly in the high altitude and plains, cool, shady and moist places. 2. Structure: they are sporophytic. Root, stems and leaves are present. Stem is called the rhizome-horizontally creeping. They are perennial. Roots help in the absorption food, mineral, water. 3. Morphology: the plant body is sporophytic. This is green and can be differentiated root, stem and leaves. The stem is also known as rhizome, which is horizontally creeping. These rhizomes are covered by the small brown scales called ramenta. The roots arise from the rhizome. The leaf is petiolate and green feather like. The upper portion of petiole is called rachis, whichhas several leaflets or pinnae that bears many pinnules. The upper green part is called frond. The young leaves are coiled from the top and circinate. The phenomenon is called circinate venation. Reproduction: In the fern reproduction takes place by vegetative and spore formation. Vegetative reproduction: the fern multiply by fragmentation of rhizome and development of adventitious buds. Reproduction by spores: 1. Sporophyllus: the leaves consisting of sorii are called sporophylls. 2. Trophophylls: The leaves without sorii called trophophylls. 3. Placenta: Internally fach sorus consists of parenchymatous cushion shaped papilla called placenta. Sexual reproduction: The sex organs of ferns are of two types. The sperm-producing organ, the antheridium, consists of a jacket of sterile cells with sperm-producing cells inside. Antheridia may be sunken (as in the family Ophioglossaceae and Marattiaceae) or protruding. They vary in size from those with hundreds of sperm to those with only 12 or so. The egg-producing organ, the archegonium contains one gamete (sex cell), which is always located in the lower, more or less dilated portion of,the venter. The upper part of the archegonium, the neck, consists of four rows of cells containing central neck cells. The uppermost of the neck cells are the neck canal cells; the lowest cell is the ventral canal cell, which is situated just above the egg. Fertilization: Fertilization is attained by the ejection of sperm from antheridia. The sperm swim through free water toward simple organic acids released at the opening of the archegonium, the neck of which spreads apart at the apex, permitting the neck cells to be extruded and the sperm to swim in and penetrate the egg. The sperm are made up almost entirely of nuclear material, but their surface is provided with spiral bands of cilia—hairlike organs that effect locomotion. When the egg is fertilized, the base of the neck closes, and the embryo develops within the expanding. PROTHALLUS ;It is a green, heart shaped, thin, flat shaped multicellular struture. It contain the deep notch at its anterior endbelow which lies the growing apex. Thus the prothallus is independent and its mode of nutrition is autotrophic. The prothallus of Dyopteris is monoecius. The sex organ and the rhizoids aresituated on the ventral surface of prothallus. The ventral side is in contact with the moist soil. The archegonia develop near the apical notch and the antheridia develop below the archegonia or near the posterior end while the unicellular brown tubular rhizoid is present. The antheridia appears earlier than the archegonia. Such condition of prothallus is called protandrouS Alternation of generation: Alternation of generation is distinct seen in fern; the sporophytic generation is dominant over the gametophytic generation. In ferns, the different generations exist as distinct individuals. The graceful fronds, or leaves, that we see adorn the sporophytes. If you look under the fronds of a mature plant, you’ll see structures where the spores are produced. The spores are cast from these structures onto the ground, where they develop into gametophytes. The gametophytes are tiny heart-shaped structures that are nearly invisible to the naked eye. They require a moist environment to develop and, once mature, produce sperm and egg. Like the mosses, the sperm require water to swim to the eggs, with each fertilized egg developing into the familiar, frond-bearing sporophyte.
Questions
1. Describe the characteristics of bryophytes and give an outline of their classification 2. Give economic importance of bryophytes 3. Describe alternation of generation in bryophytes with suitable example. 4. Write shirt notes on a) Poet moss b) Bigg succession 5. Describe important characters of class Hepaticopsida . 6. Give diagrammatic representation of life cycle of marchantia 7. Describe sexual and asexual reproduction in MARCHANTIA 8. Give an illustrated account of development of spotogonium of marchantia 9. Write shirt notes on a) Elaters b) Gemma cups c) Rhizoid and scales d) Air pore and air chamber e) Function of foot of sporogonium in marchantia 10. Discuss the salient features of Anthocerotopsida 11. Describe the method of sexual reproduction in ANTHOCEROSE 12. Give an account of vegetagive reproduction in ANTHOCEROSE 13. Write shirt note on following a) Pseudolators b) Antheridia c) Dehiscence of capsule 14. Give an account of development and structure of mature sporogonium of ANTHOCEROSE with the help of suitable diagrams. 15. Give the structure and development of sporophyte in funaria 16. Write short notes on a) Gametophytes of moss b) Spore dispersal in funaria c) Protonema of funaira d) Peristome of funaria e) Sporophyte of funaria 17. Describe the structure of funaria capsule and mention function of each part. 18. Give a well illustrated account of the life cycle of Funaria 19. What do you mean by Vascular cryptogams 20. Discuss general characteristics of Pteridophytes 21. Describe stellar system in Pteridophytes 22. Give classification and economic importance of Pteridophytes 23. Give a detailed account of Rhynia 24. Give the structure of sporophyte and sporangia of Rhynia 25. What are resurrection plants? 26. Write short notes on following in selaginella a. Ligule b. Male gametophytes c. Structure of sporogonium in selaginella d. Heterospory and seed habit 27. Give diagrammatic life cycle of Equisetum 28. What is alternation of generation in Equisetum 29. Give an illustrated account of the life cycle of Pteris 30. Write short notes on following in Pteris a. Sporophyll b. Development of antheridia and archagonia c. Stellar system d. Anatomy of root e. Spore dispersal 31. Give general characteristics of Pteris 32. Disscus DNA is a genetic material and it’s helical structure given by Watson and Crick. 33. .Write a note on a. Nucleosome b. Nucleotide and Nucleoside c. Circular DNA d. Satellite DNA e. Genetic material 34. Describe physical properties of DNA and it’s functions. 35. Difference between DNA and RNA 36. What is genetic code? Explain its characteristics features in breif. 37. Write short notes on following a. Ambiguity b. Universality c. Degeneracy 38. What are Mendel's three laws of genetics ? Explain with the help of suitable cross. 39. What do you understand by a dihybrid cross ? State and explain Mendel’s second law. 40. What is monohybrid cross? Define independent assortment. 41. Define linkage and give its significance. 42. Compare complete and incomplete linkage. 43. Explain single gene disorder in human 44. Define epistasis and it’s types 45. What are biomolecules? Define their monomeric and polymeric structure. 46. Write short notes on a. Protein b. Enzymes c. Amino acid d. Sugar and cellulose e. Starch 47. What are Proteins? Define their structure and functions in detail. 48. Protein as enzymes. Explain! 49. What is biology? What is its significance? Why we study biology? 50. What are microorganisms And classify them in detail. 51. What are single celled organisms ? Classify and give their identification features also. 52. Difference between prokaryotes and eukaryotes 53. What are enzymes? Give their classification and mechanism of action in detail. 54. Exothermic and endothermic verses exorganic and endorganic reactions. Explain. 55. Write Keq And it’s relation to standard free energy. 56. Explain. how ATP is a rich energy source why it is called as an energy currency? 57. What is glycolysis? Explain its process in detail. 58. What is photosynthesis? Explain. 59.