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CHAPTER-2: Prepared by: Dr. Md. Ariful Alam, Associate Professor A chromosome is the thread-like, -carrying bodies in the nucleus of a . Chromosomes are composed primarily of DNA and protein. They are visible under light microscope in a cell during metaphase stage of mitosis. Strasburger was the pioneer man who discovered chromosomes in 1875, and the term chromosome was first coined by Waldeyer in 1888. Chromosomes  Darkly staining nucleoprotein bodies that are observed in the cells during are called chromosome  Each chromosome carries a linear array of

CATAGORIES Chromosomes are three catagories 1. Viral Chromosomes 2. Prokaryotic Chromosomes 3. Eukaryotic Chromosomes

Viral Chromosome The chromosomes of are called Viral Chromosomes. They occur singly in a viral species and chemically may contain either DNA or RNA. eg. Bacteriophase , influenza virus

Prokaryotic Chromosomes  The Prokaryotic Chromosomes usually consists of a single, giant and circular, double- stranded DNA molecule like .  Different prokaryotic species have different size of chromosome. eg. Escherichia coli has 100 micrometer long chromosome, Mycoplasma has 265 micrometer long chromosome

The main features of eukaryotic chromosomes are given below:  Chromosomes bear genes and thus are concerned with transmission of characters from generation to generation.  Chromosomes of eukaryotes are enclosed by a nuclear membrane while in ; they remain without such envelope, free in the cytoplasm.  Chromosomes vary in shape, size and number in different species of and .  Chromosomes have property of self duplication, segregation and mutation.

1 CHAPTER-2: Chromosome Prepared by: Dr. Md. Ariful Alam, Associate Professor

 Chromosomes are composed of DNA, RNA and . DNA is the major genetic constituent of chromosomes.

Chromosome shape The shape of the chromosomes is determined by the position of . Chromosomes have generally three different shapes, viz., rod shape, J shape and V shape. These shapes are observed when the centromere occupies terminal, sub terminal and median position on the chromosomes respectively.

Chromosomes size Chromosome size is measured with the help of micrometer at mitotic metaphase. It is measured in two ways, viz., in length and in diameter. Animals usually have shorter chromosomes than the cell. Moreover, a species or individuals which have fewer chromosome numbers have larger chromosomes. The maximum length of chromosome is observed during interphase and minimum during anaphase. Thus chromosome size varies from species to species.

Chromosome number The number of chromosomes varies from species to species but generally the number is constant within a species. In most fish, chromosomes occur in pairs, and in which they do are called diploids (2N). There are two types of chromosome number, viz., haploid, diploid Diploid in diploid each somatic cell of them contains one set of chromosomes which are inherited from the maternal parent and a comparable set of chromosomes from the paternal parent. The number of chromosomes in a dual set of a diploid somatic cell is called the diploid number and represented as (2n). Since diploid chromosome number is found in zygotic or somatic cells it is also referred to as zygotic or somatic number. Human beings (except for their gametes), most animals and many plants are diploid. Haploid The sex cells (sperms and ova) of a diploid cell contain half the number of chromosomal sets found in the somatic cells and are known as haploid and represented as (n). Since haploid chromosome number is usually found in the gametes, it is also known as gametic number. Chromosome Morphology The morphology of the chromosome is suited and found following seven parts in a chromosome:

2 CHAPTER-2: Chromosome Prepared by: Dr. Md. Ariful Alam, Associate Professor

(1) Centromere, (2) (3) secondary constriction and (4) (5) chromomere (6) chromonema and (7) matrix. The above mentioned seven part of a chromosome are shown in the following figure:

Fig: A simplified structure of chromosome

A brief description of these above mentioned parts are given below: (1) Centromere: The region of chromosome with which spindle fibres are attached during metaphase is known as centromere or primary constriction or kinetochore. Centromere has four important functions as i. Orientation of chromosomes at metaphase ii. Movement of chromosomes during metaphase iii. Formation of chromatid and iv. In chromosome shape Centromere may occupy various positions on the chromosome and may have different in number. Depending upon the position and number of centromere chromosomes are given following names:

3 CHAPTER-2: Chromosome Prepared by: Dr. Md. Ariful Alam, Associate Professor

Position of Chromosome type Numberof Chromosome type chromosome centromere Median Metacentric Nil Acentric Sub-median Sub metacentric One Monocentric Sub terminal Acrocentric Two Dicentric Terminal Telocentric Three Tricentric Diffused Holokinetic Many Polycentic

Fig. The four morphologic types of chromosomes according to the position of centromere

2) Chromatid: One of the two distinct longitudinal subunits of a chromosome is calls chromatid. These subunits of a chromosome get separated during anaphase. are of two types viz., sister chromatids and non sister chromatic. Sister chromatids are derived from one and the same chromosome, while non sister chromatid; originate from. Homologous chromosomes, Chromatid are formed due to chromosome and DNA replications during interphase.

Fig: A simplified structure of chromatid

4 CHAPTER-2: Chromosome Prepared by: Dr. Md. Ariful Alam, Associate Professor

3) Secondary Constriction: The constricted or narrow region other than that of centromere is called secondary constriction. It has constant position and, therefore, can be used a useful marker. It is generally found a the short arm of a chromosome, away from the centromere.

4) Telomere The terminal region of chromosome on either side is known as telomere. These are not visible in the light or electron microscope- they are rather conceptual structures. Each chromosome has two telomere The telomere of one chromosome cannot unite with other chromosome due to polarity effect.

5) Chromomeres The linearly arranged bead like structures found on the chromosomes are known as Chromomeres. These are clearly visible in the polytene chromosomes. Available evidence indicate that chromosome represents unit of DNA replication, chromosome coiling, RNA synthesis and RNA processing.

6) Chromonema The thread like coiled structures which are observed under light microscope in the chromosomes and chromatids are called chromonema (plural chromonemata). Chromonema are associated with three main functions viz., it controls size of chromosomes, resulting in duplication of chromosomes and is the gene bearing portion of chromosomes.

7) Matrix A mass of acromatic material in which chromonemata are embeded is called matrix. Matrix is enclosed in a sheath which is known as pellicle. Both matrix and pellicle are non-genetic materials.

5 CHAPTER-2: Chromosome Prepared by: Dr. Md. Ariful Alam, Associate Professor

Chemical structure of chromosome Chemically the eukaryotic chromosomes are composed of 1. Genetic component

• DNA (deoxyribonucleic acid) • RNA (ribonucleic acid) 2. Non genetic component  Protein

a. Protein

b. non-histone protein  Lipid  Metallic ions Ca+ and Mg+  Polysacharides The histone proteins have basic properties and have significant role in controlling or regulating the functions of chromosomal DNA. The non-histone proteins are mostly acidic and have been considered more important than the histone as regulatory molecules. Some non-histone proteins also have enzymatic activities. The most important proteins of chromosomes are- • Phosphoproteins • DNA polymerase • RNA polymerase • DPN pyrophosphorylase. The metal ions as Ca+ and Mg+ are supposed to maintain the organization of chromosomes intact.

Molecular structure of Chromosomes According to recent and widely accepted theory of Dupraw and Hans Ris called as unistranded theory state that eukaryotic chromosome is composed of a single, greatly o elongated and highly folded nucleoprotein fibre of 100A thick. This nucleoproteins fibre in its turn is composed of single, linear, double-stranded DNA molecules which remains wrapped in equal amounts of histones and non-histone proteins and variable amounts of different kinds RNA. Dupraw produced a “folded-fibre model” to show the infrastructure of chromosome. This model shows a highly folded nucleoprotein fibre in a chromosome and also suggests that how the nucleoprotein fibre of a chromosome replicates during cell division

6 CHAPTER-2: Chromosome Prepared by: Dr. Md. Ariful Alam, Associate Professor and how the nucleoprotein fibre of a chromosome replicates during cell division and how the nucleoprotein fibre of both chromatids remain held at the centromere by a unreplicated fibre segment to DNA until anaphase.

Fig. The folded fibre model of Dupraw for chromosome. A, B in interphase and C in metaphase stage

Classification of chromosome Chromosome can be classified in different ways. The various criteria which are usually used for the classification of chromosomes include the followings: 1. Position of centromere 2. Number of centromere 3. Shape at anaphase 4. Structure and appearance 5. Role in heredity (essentiality) 6. Role in sex determination and 7. Structure and function A brief classification on the bases of these criteria is presented below: 1. Position of centromere Depending of the position of centromere, chromosomes are divides into the followings:

7 CHAPTER-2: Chromosome Prepared by: Dr. Md. Ariful Alam, Associate Professor

Metacentric Chromosome A chromosome in which centromere is located in the middle portion is called as the metacentric chromosome. Such chromosomes assume V shape at anaphase. Sub-metacentic A chromosome in which centromere is located slightly away from the centre point or has sub median position. Such chromosomes assume J shape at anaphase. Acrocentric Chromosome A chromosome in which centremere is located very near to one end or has sub-terminal position. They are also called as sub-terminal chromosome. Such .chromosome assumes J shape or rod shape during anaphase. Telocentric Chromosome A chromosome in which centromere is located at one end. Such chromosome assumes rod shape during anaphase. 2. Number of Centromere Acentric Chromosome A chromosome without centromere is termed as acentric chromosome. Such chromosome remains as laggard during cell division and is eventually lost. Monocentric Chromosome A chromosome with one centromere is termed as monocentric chromosome. It represents normal type of chromosomes. Dicentric Chromosome A chromosome having two is temed as dicentric chromosome. Such chromosome makes dicentric bridge at anaphase and is produced due to inversion and translocations. Polycentric Chromosome A chromosome having more than two centromeres

3. Shape at anaphase V shape Chromosome A chromosome which assumes V shape at the anaphase stage is termed as V shape chromosome. It includes metacentric chromosome. J shape Chromosome A chromosome which assumes J shape at anaphase stage is termed as J shape chromosome. It includes sub-metacentric and subterminal chromosomes. Rod shape Chromosome A chromosome which assumes rod like shape during anaphase stage is termed as rod shape chromosome. It includes telocentric chromosome.

4. Structure and appearance Linear Chromosome A chromosome with linear structure or having both the ends free is termed as linear chromosome. Such chromosomes are found in eukaryotes. A chromosome with circular shape and structure is termed as circular chromosome. They are found in and viruses.

5. Essentiality A-Chromosome Normal members of chromosome complements of a species which are essential for normal growth and development.

8 CHAPTER-2: Chromosome Prepared by: Dr. Md. Ariful Alam, Associate Professor

B-Chromosomes Chromosome which are found in addition to normal chromosome complements of a species is called as B-chromosome. They are also called as accessory, supernumerary or extra chromosomes and are not essential for normal growth and development.

6. Role in sex determination Allosomes Chromosomes which differ in morphology and number in male and female sex and contain sex determining genes. They are generally of two types, viz., X and Y or Z and W types. Chromosomes which do not differ in morphology and number in male and female sex and rarely contain sex determining genes is termed as autosomes.

7. Structure and Function Normal chromosome Chromosome with normal structure (shape and size) and function are termed as normal chromosome. Special Chromosome Chromosomes which significantly differ in structure and function from the normal chromosomes are known as special chromosomes. Special chromosomes include lampbrush chromsome, polytene chromosome and B-chromosome.

Function of centromere: During cell divisions, the microtubules of the spindle are get attached with the chromosomal centromeres and move them towards the opposite points of cells

Karyotype and Idiogram: When the chromosomes of a species are arranged according to their shape, size and structure that is called of that species; and when these karyotype of a species are represented by the diagram then such diagrams are called idiograms.

Genetic significance of chromosomes: The chromosomes are considered as the organs of heredity because of following reasons-  They form the only link between two generations  A diploid chromosome set consists of two morphologically similar (except X and Y chromosomes) sets, one is derived from the mother and another from the father at fertilization 9 CHAPTER-2: Chromosome Prepared by: Dr. Md. Ariful Alam, Associate Professor

 The genetic material, DNA or RNA is localized in the chromosome and its contents are relatively constant from one generation to the next  The chromosomes maintain and replicate the genetic information’s contained in their DNA molecule and this information is transcribed at the right time in proper sequence into the specific types of RNA molecules which directs synthesis of different types of proteins to form a body from like the parents.

Materials of chromosomes The material of the eukaryotic chromosomes according to its percentage of DNA, RNA, proteins and consequently due to its staining property, has been classified into following kinds- 1. :  The euchromatin is the extended form of chromatin and it forms the major portion of chromosomes  It has special affinity for basic stains and is genetically active because its component DNA molecule synthesizes RNA molecules only in the extended form of chromatin.

2. :  The heterochromatin is a condensed inter-coiled state of chromatin.  It contains 2-3 times more DNA than euchromatin.  It is genetically inert as it does not directly synthesis of RNA () and protein and is often replicated at a different time from the rest of the DNA.

Recently three kinds of heterochromatins have been identified, such as- -constitutive -facultative -condensed heterochromatin

Constitutive  The constitutive heterochromatin is present at all times and in the nuclei of virtually all the cells of an . In a inter-phase nucleus, it tends to clump together to form chromo-centre or false nucleoli.

 Constitutive heterochromatin contain highly repititive satellite DNA which is late replicating, it fails to replicate until late in the S-phase and is then replicated during a brief period just before the G2.

10 CHAPTER-2: Chromosome Prepared by: Dr. Md. Ariful Alam, Associate Professor

Facultative:  The facultative heterochromatin reflects the existence of a regulatory device designed to adjust the “dosages” of certain genes in the nucleus.

 It is organized during the process (facultative heterochromatin) in which a chromosome or a set of chromosome become heterochromatic in the cells of one sex, while remaining become euchromatic in the cells of opposite sex. It remains indirectly related to sexual differentiation.

Condensed heterochromatin: It is deeply staining tightly coiled chromatin which does not resemble with two other kinds of chromatin, has some specific role in gene regulation and is found in many inter-phase nuclei.

Special types of Chromosome

Other than usual type of chromosomes (autosomes and sex chromosomes), the eukaryotes possess some unusual and special types of chromosomes in some body cells or at some particular stage of their cycle. These special types of eukaryotic chromosomes are- 1. Polytene . Lampbrush . B- . Holokinetic Chromosome

Polytene Chromosome  These nuclei of the salivary gland cells of the larvae of dipterian like Drosophila have unusually long and wide chromosomes which are called Polytene Chromosome.  These chromosomes are about 100 to 200 times in size of the chromosomes in and mitosis of the same species  Polytene Chromosomes become exceptionally giant-sized by a process, called “endomitosis” in which, the salivary gland cells do not divide after the glands are formed, but their chromosomes replicate in several times. It is also called multistranded chromosomes.

11 CHAPTER-2: Chromosome Prepared by: Dr. Md. Ariful Alam, Associate Professor

 The endomitosis process result in the production of 2X chromosomes where X gives the number of multiplication.

Polytene Chromosomes of D. melanogaster It contains 1000 to 2000 chromosomes, formed by 9 to 10 consecutive multiplication cycle. It contains alternating dark and light bands along their length.

The dark bands are comparable with chromomeres of a simple chromosomes and are disc-shaped structures occupying the whole diameter of chromosome. They contain euchromatin. The light bands or inter bands are fibrillar and composed of heterochromatin. This chromosome also bear a enlarge area called “puff” which change its location according to the developmental stages of larvae and indicates the changes in gene activity and involves several processes eg. Formation of chromosomal loops, synthesis of m-RNA etc.

Lampbrush chromosome:  In diplotene stage of meiosis, the yolk-rich oocytes of vertebrates contain the nuclei with many lampbrush-shaped chromosomes of exceptionally large sizes.  The lampbrush chromosomes are formed during the active synthesis of m-RNA molecules for the future use by the egg during cleavage when no synthesis of m-RNA molecules is possible due to involvement of chromosomes in the mitotic cell division.  A lampbrush chromosome contains a main axis, whose chromosomal fibre (DNA molecule) gives out lateral loops throughout its length.  The loop produces m-RNA molecules of different kinds.  In a mature egg, as the chromosome contracts the lateral loops disappear.

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B-chromosome:  Many plants and species possess special type of chromosome without obvious genetic function, called B- chromosome.  It is also known as Supernumerary chromosomes, accessory chromosomes, accessory fragments etc.  They can be predominantly heterochromatic (insects, maize etc) or predominantly euchromatic.  In animals, B- chromosomes disappear from the non-reproductive (somatic) tissue and are maintained only in the cell-lines that lead to the reproductive organs.  B-chromosomes have negative consequences for the organism, as they have deleterious effect because of abnormal crossing over during meiosis of animals and abnormal nucleus divisions of the gametophyte plants.  The origin of the B- chromosomes is uncertain. In some animals, they may be the derivatives of sex- chromosomes, but it is not the rule.  It has no pairing affinity with A-chromosomes.

Holokinetic Chromosome  The chromosomes of most plants and animals have centromeres that are situated at one specific position in each chromosomes. But in a number of animals (eg. Insects, and a few plants, the kinetic activity is distributed over the entire chromosome and such chromosomes are called Holokinetic Chromosomes.

 Achromosome with diffused centromere. Centromere does not occupy a specific position, but is diffused throughout the body of chromosome. Whole body of such chromosome exhibits centromeric activity. They are also called as holocentric chromosome. 13 CHAPTER-2: Chromosome Prepared by: Dr. Md. Ariful Alam, Associate Professor

Homologous chromosomes: Chromosomes that are paired during the production of sex cells in meiosis is called as the homologous chromosome. Such chromosomes are alike with regard to size and also position of the centromere. They also have the same genes, but not necessarily the same alleles, at the same locus.

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