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Cellular 1

INTRODUCTION • Specialized intracellular -bound (Fig. 1.2), such as mitochondria, , (ER). This chapter is an overview of eukaryotic cells, addressing • Large size (relative to prokaryotic cells). their intracellular organelles and structural components. A basic appreciation of cellular structure and function is important for an understanding of the following chapters’ information concerning and nutrition. For fur- ther detailed information in this subject area, please refer to EUKARYOTIC ORGANELLES a reference textbook. Nucleus The eukaryotic The nucleus is surrounded by a double membrane (nuclear Humans are multicellular eukaryotic . All eukary- envelope). The envelope has multiple pores to allow tran- otic organisms are composed of eukaryotic cells. Eukaryotic sit of material between the nucleus and the . The cells (Fig. 1.1) are defined by the following features: nucleus contains the cell’s genetic material, DNA, organized • A membrane-limited nucleus (the key feature into linear structures known as . As well as differentiating eukaryotic cells from prokaryotic cells) chromosomes, irregular zones of densely material that contains the cell’s genetic material. are also present. These are the nucleoli, which are responsible­

Inner nuclear Nucleus membrane Inner Outer Outer mitochondrial nuclear mitochondrial membrane membrane membrane Intermembrane space Chromatin Mitochondrial Rough matrix Mitochondrial Nuclear endoplasmic ribosome pore reticulum Mitochondrial mRNA Smooth Vesicle endoplasmic Circular reticulum mitochondrial of the DNA Vesicle budding transport off rough ER Vesicles fusing system with trans face of Cytoplasm Golgi apparatus ‘Cis’ face + discharging / Golgi apparatus ‘Trans’ face Vesicles leaving Golgi with modified protein/lipid cargo

Cell membrane Fig. 1.1 Ultrastructure of a typical eukaryotic cell and structure of important intracellular organelles. ER, Endoplasmic reticulum. 1 Cellular biology

Extracellular Membrane- Rough ER fluid spanning Peripheral protein Hydrophobic integral protein Rough ER is far more abundant than smooth ER. It is dis- (external face) interior tinguished from smooth ER on electron microscopy by the presence of membrane-associated (presenting a ‘rough’ appearance). The ribosomes intermittently attach/ detach to the rough ER. Attachment occurs when ribosomes bind with mRNA strands (see Chapter 9) that encode pro-

Peripheral protein teins destined for . The developing polypeptide is (internal face) extruded into the rough ER interior, where it may remain to Phospholipid hydrophobic hydrophilic complete its development (see Chapter 9). Alternatively, na- Cytoplasm ‘tail’ groups ‘head’ groups scent proteins may be transported via vesicles to the Golgi apparatus or another destination for further posttranslation Fig. 1.2 Cross-section of a typical cell membrane. Note the modifications. phospholipid bilayer membrane and integral and peripheral proteins. Smooth ER Smooth ER differs from rough ER by the absence of for ribosomal RNA (rRNA) synthesis and ribosome assem- ­membrane-bound ribosomes. The main function of bly. Messenger RNA (mRNA) synthesis (translation of ge- smooth ER is lipid synthesis – assembling phospholip- netic material; see Chapter 9) occurs within the nucleus. ids, and other . It is therefore more abun- mRNA can then exit the nucleus via the nuclear pores into dant in cell types with secretory roles. The large surface the cytoplasm. area of the convoluted structure also presents a useful intracellular surface for attachment, for example, Mitochondrion ­-6-phosphatase, a key enzyme in gluconeogenesis (see Chapter 5). Smooth ER also plays an important role in The ultrastructure of a mitochondrion is illustrated in attaching nascent receptors to membrane proteins prior to Fig. 1.2. Mitochondria are present in all human cells except their membrane insertion. mature red blood cells. Mitochondria are semiautonomous, self-replicating organelles. They are separated from the cy- toplasm by a double membrane, the inner membrane being Golgi apparatus highly folded into inward-projecting ‘cristae’. Because of its large size, the Golgi apparatus (Fig. 1.2) was The inner membrane is the location of the electron one of the first identified intracellular organelles. It is a sys- transport chain (see Chapter 4), where oxidative phosphor- tem of 5 to 8 cup-shaped interconnected membranous sacs ylation takes place. This is the main role of mitochondria; that receive vesicles containing lipids and proteins from the oxidative phosphorylation is responsible for the vast ma- smooth and rough ER, respectively. It modifies these mol- jority of (ATP) production. ATP is ecules in various ways and then distributes them to appro- the intracellular energy currency used to ‘power’ nearly all priate areas within the cell, packaged within vesicles. The intracellular endergonic reactions. The tricarboxylic overall structure possesses a ‘cis’ and a ‘trans’ face. The cis (TCA) cycle (see Chapter 3) is another extremely important face is the ‘entry’ portal to the Golgi apparatus, and ­modified metabolic pathway that occurs only in the mitochondrial molecules exit at the trans face. The Golgi apparatus has an- matrix. other important function – it manufactures . Mitochondria contain mitochondrial versions of RNA and ribosomes, and synthesize their own proteins coded for by distinct mitochondrial DNA. This DNA is arranged in Lysosomes and circular form, rather than the structure seen The cytoplasm contains two different types of specialized in the nucleus. single-membrane-bound vesicular structures: lysosomes and peroxisomes. These differ by their enzyme contents. Endoplasmic reticulum The endoplasmic reticulum (ER) is a complex series of in- Lysosomes terconnected, flattened membranous sacs or ‘cisternae’. The Lysosomes are spherical membrane-bound vesicles with an ER possesses a double membrane, the interior of which is acidic (pH 4–5) interior. They are the intracellular spaces contiguous with the intermembrane space of the double- for enzyme-mediated degradation of obsolete intracellular membraned nucleus at distinct points. Intracellular ER is molecules or imported extracellular material. They are de- divided into two types: rough and smooth. Both smooth rived from the trans face of the Golgi. Lysosomes are highly and rough ER are continuous with each other as well as with variable in size and contain multiple pH-sensitive hydro- the intermembrane space of the nuclear membrane. lases. These can degrade most .

2 The cell membrane 1

Peroxisomes environment. However, the cell membrane also participates Peroxisomes are vesicular, ER-derived structures. They are in many important cellular processes, for example: smaller than lysosomes and contain different , • Maintenance of the resting via primarily oxidative enzymes. They participate in the β-­ regulation of entry/exit. oxidation of fatty with very long chains (see Chapter 7) • Interaction with the intracellular . and in the pentose pathway (see Chapter 5). • Transport of , metabolites and . Peroxisomal catalase also detoxifies reactive oxygen species • to external structural elements within the such as hydrogen peroxide. surrounding or to neighbouring cells. Cell are impermeable to most molecules, how- Ribosomes ever specialized membrane-spanning transport proteins Eukaryotic cells contain 80S ribosomes, composed of a permit selective permeability to specific ions/molecules. small 40S and a large 60S subunit. They are composed of Structural and functional modification of these proteins al- rRNA and are manufactured in the nucleus. The two sub- lows regulation of entry and exit of the relevant transported units unite immediately prior to beginning translation (see molecule. Chapter 9). Ribosomes translate information contained in the mRNA into polypeptides by assembling peptides from Membrane components amino acids in the order dictated by the mRNA sequence. Ribosomes within the cytoplasm typically synthesize cy- Cell membranes are composed of a phospholipid bilayer toplasmic proteins, whereas those producing proteins des- studded with membrane proteins and cholesterol (Fig. 1.3). tined for the plasma membrane or vesicles associate with the rough ER. Phospholipids consist of a hydrophilic ‘head’, containing phosphate, and a hydrophobic ‘tail’ of varying length and saturation. The amphiphilic nature of the mol- THE CELL MEMBRANE ecule means that phospholipids spontaneously adopt a bi- layer structure. The hydrophilic ‘heads’ form the surfaces of The cell membrane (Fig. 1.2) is a biological barrier that sep- the membrane, and the hydrophobic ‘tails’ interact with each arates the cellular interior from the external environment. other, forming the interior of the bilayer. In this way, the hy- The main function of the cell membrane is to separate the drophilic components are in contact with the intracellular cell from its surroundings and provide a distinct intracellular­ and extracellular environments.

Properties Functions

Microfilaments Pair of helically intertwined protofilaments (F- fibres) •Strong •Cell shape •Flexible •Cell motility • G-actin subunit 7 nm

Microtubule

•Stiff • •Inflexible transport •Chromosome translocation Hollow Tubulin () interior dimer α β 25 nm

Intermediate filaments •Intercellular Protofilament: keratin, , vimentin adhesion •Nuclear anchorage •Structural support 8 coiled protofilaments 10 nm Fig. 1.3 Cytoskeletal components.

3 Cellular biology

Cholesterol The rate of movement of ions through a membrane via Cholesterol is a , which is an integral part of cell mem- channels depends on: branes. It intercalates between the phospholipids that make • The ion concentration gradient and the charge up the bilayer. The presence of cholesterol affects the mem- difference across the membrane relative to the ion’s brane in different ways, depending on the temperature: charge. The combination of these two features is known • At lower temperatures, cholesterol disrupts the as the ‘electrochemical gradient’. interaction between phospholipids, increasing • The number of open channels. Regardless of the membrane viscosity. electrochemical gradient, an ion cannot cross the • At higher temperatures, cholesterol increases membrane if the specific ion channels are closed. This by raising the bilayer melting point. permits an additional level of regulation; channels may The fluidity of the bilayer is significant because it affects be ‘gated’ such that traffic is possible only in certain the movement of membrane proteins within the bilayer and situations. thus influences local membrane permeability. The role of cholesterol in metabolism is discussed further in Chapter 7. Active transport couples the movement of molecules Membrane proteins against electrochemical gradient with a thermodynamically Protein components of the cell membrane may span the favourable reaction ‘powering’ the energetically unfavour- membrane completely (integral proteins) or be associated able direction of travel. Active transport may be primary or with either the internal or external face of the membrane secondary: (peripheral proteins). These proteins may fulfil various roles • Primary active transport is coupled directly to the including: of ATP. • Receptor function: allowing external messengers (e.g. • Secondary active transport is coupled indirectly to the ) to effect intracellular changes, usually via hydrolysis of ATP. intracellular signalling cascades. Primary active transport • Bidirectional transmembrane transport of a myriad of Primary active transport requires energy to transport mole- ions and molecules. cules across a membrane against their electrochemical gra- • Integral structural functions – acting as anchors for the dient. Sodium and potassium are examples of ions that are internal cytoskeleton (integral or internal peripheral transported across the cell membrane against their electro- proteins). chemical gradient by primary active transport. This trans- • Adhesion to neighbouring cells – intercellular port is via the Na+/K+-dependent adenosine triphosphatase adhesion and adhesion to extracellular structural tissue (ATPase). For every ATP hydrolysed, this transporter pumps components, stabilizing the cell location within a tissue three Na+ ions outward and two K+ ions inward. (integral or external peripheral proteins). • Some of these proteins are enzymes (integral, internal Secondary active transport peripheral and external peripheral proteins). Secondary active transport is not directly coupled to ATP hydrolysis, but exploits a concentration gradient that itself is maintained by primary active transport coupled directly Permeability and transmembrane to ATP hydrolysis. transport The Na+/K+–ATPase establishes an Na+ gradient across the membrane. This renders Na+ influx into the cell thermo- Passive (simple) dynamically favourable, because the ion is following its elec- Simple diffusion describes the free movement of molecules trochemical gradient. This energetically favourable sodium across a membrane down their concentration gradient. influx can be coupled to the movement of another molecule Small nonpolar molecules (e.g. O2 and CO2) and uncharged against its gradient, for example, glucose (see Chapter 5). polar molecules (e.g. urea) may diffuse directly through the in this manner. No energy is required to drive the molecular movement, and diffusion continues until an equilibrium is attained between the intracellular and extra- THE CYTOSKELETON, CELL cellular compartments. MOTILITY AND INTRACELLULAR TRANSPORT Because charged molecules cannot diffuse directly through The cytoskeleton (Figs 1.3 and 1.4) is a dynamic system the lipid bilayer, they rely on specific proteins to traverse of structural proteins that support the cell membrane. It the membrane. Proteins mediating facilitated diffusion are contributes significantly to determining cellular three-­ typically ion channels or carrier proteins. dimensional (3D) shape and cellular motility. Cytoskeletal 4 The cytoskeleton, cell motility and intracellular transport 1

(A) (B)

(C) intermediate filaments (D) a + b + c merge

Fig. 1.4 Fluorescence microscopy of cytoskeletal elements. Each of the different types ((A) microfilaments; (B) microtubules and (C) intermediate filaments) is stained and assigned its own colour in this image (D). (From Omary MB, Ku NO, Tao GZ, Toivola DM, Liao J: “Heads and tails” of intermediate phosphorylation: multiple sites and functional insights, Trends Biochem Sci 31:383–394, 2006 Box 2, Fig. I.) components are also paramount for normal mitotic and cytoskeleton. meiotic cellular division. The major components of the cy- toskeleton are: Actin • Microfilaments (actin polymers) Actin is the most abundant cellular protein. Actin filaments • Microtubules (tubulin polymers) (F-actin) consist of linear polymerized globular subunits • Intermediate filaments (IFs) (G-actin). Bundles of actin filaments are able to form linear structures, as well as two-dimensional and 3D meshwork. Actin polymerization is closely regulated by the cell and Components of the cytoskeleton may be influenced by extracellular signalling by surface receptors. Microfilaments Actin-binding proteins—Protein binding to actin causes Microfilaments, composed of actin, form a parallel lam- changes to the 3D structure of the actin molecules. The ina closely associated with the internal face of the cell most important example of this is myosin, found in contrac- membrane. They also traverse the cytoplasm in multiple tile cells (e.g. muscle or cardiac myocytes). However, myosin directions. Microfilaments are crucial for cell polariza- is by no means the only one; actin participates in a large tion and motility. , number of protein–protein interactions. movement and muscle contraction are prime exam- ples of the dynamism that microfilaments confer to the 5 Cellular biology

Microtubules Polymerized deoxyribonucleotides form the strand of the Microtubules, like microfilaments and intermediate fila- deoxyribonucleic acid. ments (IF) are cytoskeletal components that contribute to maintaining cellular structure. They are present in all cell Nitrogenous bases types except red blood cells. The nitrogenous bases of DNA are heterocyclic molecules As well as structural support, microtubules also enable derived from purines (adenine and guanine) or pyrim- various types of intracellular movement to occur: intracel- idines (cytosine and thymine). Note that RNA does not lular transport and organelle movement during or ­include thymine and instead contains uracil, a ­pyrimidine meiosis. ­derivative. For details of purine and pyrimidine metabo- Microtubules are formed from assembled linear pro- lism, please see Chapter 10. tofilaments, arranged in parallel in a hollow cylindrical ­structure. Protofilaments consist of linearly polymerized Deoxyribonucleotide tubulin heterodimers, with each heterodimer consisting of α- and β-tubulin. Microtubules extend from distinct origins polymerization or -organizing centres. These origins act as a fo- Deoxyribonucleotides polymerize in linear fashion via cus for microtubule development; developing microtubules phosphodiester bond formation between the phosphate radiate outwards from the microtubule-organizing centres. group of one deoxyribonucleotide and the deoxyribose Like the constituent actin, tubulin group of the neighbouring deoxyribonucleotide. These in- interacts with specific proteins. These are known as teractions underpin the assembly of consecutively linked ‘­microtubule-associated proteins’. ATP-reliant molecular deoxyribonucleotides. The (or ‘polymer’) motors such as dynein and kinesin exploit microtubules formed is a nucleic acid. as intracellular roadmaps, allowing cargo such as secretory vesicles or organelles to travel along the microtubule to ­specific locations within the cell. The double helix: paired strands of DNA Intermediate filaments Discovered by Watson and Crick in 1953, the structure of Intermediate filaments (IFs) are the most abundant compo- DNA consists of two intertwined strands, held nents of the cytoskeleton. They are extremely stable, more together in a double helical structure by nitrogenous base so than microfilaments and microtubules, as their subunits pairing. Adenine and thymine pair via two hydrogen bonds do not dissociate under physiological conditions. They between opposing strands, whereas guanine and cytosine pair therefore represent the more permanent structural compo- via three hydrogen bonds. Base pairing results in two ‘com- nents of the cytoskeleton, which is their main function. plementary’ strands of nucleic acid, orientated antiparallel to IF subunits consist of a family of α-helical proteins. The each other (i.e. one runs in the 5′→3′ direction, and the other particular components vary with cell type. These subunits runs in the 3′→5′ direction). The structure is characterized by wind together to form the rope-like structure that character- a ‘core’ of inwardly orientated nitrogenous bases and an outer izes IF. IF are present in both the nucleus and the cytoplasm, ‘shell’ of externally protruding phosphate groups. where they form meshworks of laminae closely associated with the internal leaflet of the enclosing membranes in a similar manner to microfilaments. IF also form supportive HINTS AND TIPS internal frameworks for intracellular spatial organization, DIRECTIONALITY OF NUCLEIC ACIDS for example, by interconnecting the external nuclear mem- brane to the cellular membrane. Nucleic acids have directionality. One end is the five-prime (5′) end, where the phosphate group is GENETICS attached to carbon 5 of the deoxyribose ring. At the three-prime (3′) end, the phosphate group is attached to carbon 3 of the deoxyribose. Deoxyribonucleotides To understand the structure of deoxyribonucleic acid, or ‘DNA’, one must first appreciate the components of the basic Complementary pairing of nucleic acids units (deoxyribonucleotides). The basic ‘unit’ of DNA is the Each strand is described as ‘complementary’ to its partner deoxyribonucleotide (Fig. 1.5). Each deoxyribonucleotide is strand. Complementary polynucleotide strands interact composed of: with each other through hydrogen bonds between the ni- • a deoxyribose sugar trogenous base component of each deoxyribonucleotide as • a phosphate group described earlier. Note that in DNA each single strand is • a nitrogenous base (adenine, cytosine, guanine or thymine) partnered by another single strand. When the term ‘DNA’ is 6 Genetics 1 used, this refers to the pair of helically intertwined polynu- deoxyribonucleotides in the DNA sequence will be cleotide chains. substituted by uracil (U) ribonucleotides in the mRNA sequence. Strand terminology • Noncoding strand = template strand (3′→5′) = antisense strand (contains anticodons). A strand with Genetic information is only carried by one of the two nu- the sequence of complementary to what cleic acid strands making up a length of DNA. This strand is will be the transcribed mRNA. RNA polymerase (RNA known as the ‘coding’ strand. The coding strand is the 5′→3′ pol II) binds to this strand, transcribing it into mRNA. strand, whereas the noncoding template strand is the 3′→5′ strand. Confusingly, the ‘coding’ strand is also known as the ‘’ strand and the ‘nontemplate strand’, and the noncod- The ing (template) strand as the ‘antisense’ strand. Essentially: A gene is the basic functional unit of heredity responsible • Coding strand=nontemplate strand (5′→3′) = for the passage of genetic information from one generation sense strand (contains codons). This is a strand to the next. also play a vital role in protein synthesis, with a nucleotide sequence identical to that which encoding the information needed to assemble the primary will appear in the mRNA transcribed from the structure (see Chapter 9) of any particular protein. A gene template strand, with the exception that thymine (T) (Fig. 1.5) is composed of a length of DNA.

Nucleus

Chromosome

Noncoding strand

Coding strand

P = Promoter PIEEIIES I = Intron E = Exon Gene S = Stop signal

- O DNA

O P O

phosphodiester link O

CH2 O B 5’ position HH H H 3’ position OH H

Fig. 1.5 Representation of the DNA → gene → deoxyribonucleotide relationship, illustrating the molecular structure of a deoxyribonucleotide.

7 Cellular biology

Gene expression is ‘one gene, one polypeptide’, because multiple polypeptides may contribute to a fully formed protein, that is, multiple Each gene encodes its specific polypeptide in the following way: genes would contribute to a single protein. 1. The gene acts as a blueprint determining the order of The process of translation (see Chapter 9) from mRNA ribonucleotide assembly into mRNA, which occurs in to protein is directed by the ribonucleotide sequence, which the nucleus. This process is called ‘’. itself is determined by the original gene sequence (minus 2. The mRNA moves out of the nucleus and into the the introns). Each of the amino acids is represented within cytoplasm. the mRNA sequence by a codon. A codon consists of a trip- 3. In the cytoplasm, mRNA attached to ribosomes let of consecutive ribonucleotides. As there are four different functions as a physical blueprint for ribosomes, bases in DNA, there are 43 (64) possible codons. Of these 64 dictating the order of polypeptide assembly; the possible codons, the genetic code consists of 61 amino acid-­ specific order of the amino acids represents the encoding codons and three termination codons, which arrest primary structure (see Chapter 9) of the new translation. polypeptide. Although there are 61 amino acid-coding codons, only The processes of transcription and translation are discussed 20 amino acids are commonly used in polypeptide synthe- in much greater detail in Chapter 9. sis – thus more than one codon may represent an amino acid. For example, the codons GGU, GGC, GGA and GGG Gene components all encode the amino acid glycine. This feature is described as ‘degenerate’. Exons and introns mRNA transcript codons correspond to (and form Each individual gene consists of exons, which are functional temporary base pair hydrogen bonds with) transfer RNA coding regions. The exons are separated from each other by (tRNA) ‘anticodons’, which define which amino acid noncoding introns. Together, the separate exons make up a is transported by that particular tRNA. Three mRNA coding sequence, which encodes the genetic information ­codons (UAA, UAG and UGA) are not recognized by that ultimately dictates the primary sequence of the en- tRNAs, and these are termed ‘stop codons’. They mark the coded protein. end of a polypeptide and signal to the ribosome to finish Introns from the coding strand do not encode proteins, synthesis. despite being on the coding strand. Their mRNA is removed or ‘spliced’ out of the developing mRNA during its synthesis Mitochondrial DNA and the (see Chapter 9). Introns do not therefore contribute to the ge- genetic code netic code for each polypeptide. Mitochondria contain their own unique DNA, which in hu- mans consists of 16 kb of circular DNA coding for: Noncoding deoxyribonucleotide sequences Between different genes, there are large expanses of non- • 22 mitochondrial (mt) tRNAs coding deoxyribonucleotide sequences. These were once • two variants of mitochondrial rRNAs thought to serve no function, but now are believed to • 13 separate mitochondrial proteins (e.g. subunits of ­undergo transcription, forming noncoding RNA strands. the electron transfer system responsible for oxidative Noncoding play various roles in the regulation of phosphorylation). gene expression. Chromosomes Promoter sequences The human genome (i.e. the entirety of human genes) is A promoter sequence is typically located at the 5′ end of divided into DNA superstructures packaged around DNA- a gene (although promoter sequences can also be located associated proteins such as histones. Each of these super- at completely separate sites within the coding strand). structures represents a chromosome. Each chromosome Promoter sequences function as binding sites for RNA poly- represents a segment of the individual’s genome. merase (see Chapter 9), an enzyme necessary for transcrip- The combination of DNA with DNA-associated pro- tion of the gene. teins is termed ‘chromatin’. Chromatin allows the enormous lengths of DNA comprising the chromosome to occupy a The genetic code relatively tiny volume within the nucleus. The ‘one gene, one polypeptide’ hypothesis states that the base sequence of DNA determines the amino acid sequence Chromosomal inheritance in a single corresponding polypeptide. By convention, gene Every human cell (apart from gametes) contains 23 pairs sequences are described in the 5′→3′ direction, because this of chromosomes stored within the nucleus. Each pair of an is the direction of in vivo nucleic acid synthesis. Do not slip individual’s chromosomes consists of one chromosome de- into the error of stating ‘one gene, one protein;’ the phrase rived from the mother and one from the father.

8 Epigenetics 1

EPIGENETICS methylation of cytosine base in the DNA) may be inherited in their offspring. The significance of this is that these changes can facili- Epigenetics refers to heritable characteristics that are tate or impede expression of a gene. In an epigenetic change, not related to the genetic code sequence of the parents. the inherited gene itself is identical, but its ability to be ex- Environment-induced changes to parental DNA-binding pressed, or ‘switched-on’, may be different after the change. proteins (such as histones) or covalent modifications (e.g.

Chapter Summary

• Eukaryotic cells are defined by the presence of a membrane-bound nucleus, which contains the cell’s genetic material, and several types of membrane-bound organelles, which are responsible for specific intracellular functions. • Cell membranes are composed of phospholipid bilayers, studded with proteins responsible for specific functional roles, and cholesterol. Transport across membrane is governed by presence of transport proteins and electrochemical gradients or direct/ indirect adenosine triphosphate hydrolysis when the direction of transfer is against an electrochemical gradient. • Human DNA consists of a double helix of deoxyribonucleotide polymers. A deoxyribonucleotide consists of a nitrogenous base (purine or pyrimidine), a phosphate group and a deoxyribose sugar group. • A gene consists of discontinuous lengths of DNA. Coding introns are interspersed by noncoding exons. Arrays of genes specific to the individual together comprise the genetic code. A chromosome represents a portion of the total DNA content of the nucleus; in humans there are 46 chromosomes (23 pairs). Each member of a pair is either paternally or maternally derived. • Chromosomal inheritance underpins the transfer of genetic information between parents and offspring. Chromosomes consist of condensed DNA, complexed with specific proteins such as histones, which can also play a part in controlling expression of genes.

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