Saturday, February 28, 2009



Most cellular RNA is single stranded, although some viruses have double stranded RNA (Wound tumor plant virus, Reo animal virus). The single RNA strand is folded upon itself, either entirely or in certain regions. In the folded region a majority of the bases are complementary, and are joined by hydrogen bonds. This helps in the stability of the molecule. In the unfolded region the bases have no complements. Because of this RNA does not have the purine-pyrimidine equality that is found in DNA.

RNA also differs from DNA in having ribose sugar instead of deoxyribose. The common nitrogenous bases are adenine, guanine, cytosine and uracil. Thus the pyrimidine uracil substitutes thymine of DNA. In region where purine-pyrimidine pairing takes place, adenine pairs with uracil and guanine with cytosine.

Three types of cellular RNA have been distinguished:
1. messenger RNA (mRNA) or template RNA,
2. ribosomal RNA (rRNA) and
3. transfer RNA (tRNA) or soluble RNA (sRNA).

Ribosomal RNA: Ribosomal RNA, as the name suggests, is found in the ribosomes. Ribosomes contain about 60% ribonucleic acid, which is ribosomal RNA and 40% protein. It comprises of about 80% of the total RNA of the cell. Ribosomal RNA consists of a single strand twisted upon itself in some regions. In these twisted regions the base pairs are complementary. But there is no purine-pyrimidine equality. rRNA is synthesized on the part of the chromosome traversing the nucleolus or the nucleolar organizer. The DNA associated with this is called ribosomal DNA. The 70s ribosome of prokaryotes consists of a 30s sub unit and a 50s sub unit. The30s sub unit contains 23s and 5s rRNA. The 80s eukaryote ribosome consists of a 40s and a 60s sub unit. In vertebrates the 40s sub unit contains 18s rRNA while the 60s sub unit contains 28-29s, 5.8s and 5s rRNA. In plants and invertebrates the 40s sub unit contains 16-18s rRNA, while the 60s sub unit contains 25s and 5s and 5.8s rRNA. Thus there are three types of rRNA, based on sedimentation and molecular weight. Two of these classes are high molecular weight RNAs, while the third is a low molecular weight RNA. The three classes are –
1. high molecular weight rRNA with molecular weight of over a million(21s-29s rRNA)
2. high molecular weight rRNA with molecular weight below a million(12s-18s rRNAs)
3. low molecular weight rRNA (5s rRNA)

The function of rRNA is in protein synthesis, but it’s role is not yet completely clear.

Messenger RNA: Jacob and Monod (1961) proposed the name messenger RNA for the RNA carrying information for protein synthesis from the DNA (genes) to the sites of protein formation (ribosomes). It consists of only 3 to 5% of the total cellular RNA. Messenger RNA is always single stranded. It contains mostly the bases adenine, guanine, cytosine, and uracil. There are few unusual bases. There is no base pairing as base pairing in the mRNA strand destroys its biological activity. mRNA is synthesized on a DNA strand through the enzymatic action of RNA polymerase. The mRNA has the following structural features:
1. Cap: at the 5’ end of the mRNA molecule in most eukaryote cells and animal virus molecules is found a ‘cap’. This is blocked methylated structure. The rate of protein synthesis depends upon the presence of the cap. Without the cap mRNA molecules bind very poorly to the ribosomes.
2. Non-coding region 1: The cap is followed by a region of 10 to 100 nucleotides. This region is rich in A and U residues, and does not translate protein.
3. The initiation codon is AUG in both prokaryotes and eukaryotes.
4. The Coding region consists of about 1500 nucleotides on the average and translates protein.
5. Termination Codon: translation on mRNA is brought about by a termination codon. In eukaryotes the termination codons are UAA, UAG or UGA.
6. Non-coding region 2 consists of 50-100 nucleotides and does not translate proteins. This region contains an AAUAAA sequence in all the examples sequenced.
7. Poly (A) sequence: At the 3’end is a polyadenylate or poly (A) sequence, which initially consists of 200 – 250 nucleotides, but which becomes shorter with age. The poly (A) sequence is added in the nucleus before the mRNA reaches the cytoplasm.

It is now established that when the code has been transcribed from DNA on to mRNA, the later leaves the nucleus passes through the nuclear membrane into the cytoplasm. Here it moves to the ribosomes, the site of protein synthesis, the mRNA molecules attach reversibly to the surface of ribosome always binding to the smaller sub-units.

Transfer RNA (tRNA) or Soluble RNA (sRNA): After rRNA the second most common RNA in the cell is transfer RNA. It is also called sRNA because it is small to be precipitated by ultracentrifugation. It constitutes about 10-20% of the total RNA of the small. Transfer RNA is synthesized in the nucleus on a DNA template. Only 0.025% of DNA codes for tRNA. The function of tRNA is to carry amino acids to mRNA during protein synthesis. A specific tRNA carries each amino acid. Since 20 amino acids are coded to form proteins, it follows that there must be at least 20 types of tRNA. However, it has been proved that there are at least two types of tRNA for each amino acid. Thus there are many tRNA molecules than amino acid types.
Holley et al (1965) first worked out the structure of tRNA. He proposed the cloverleaf model for tRNA. According to the cloverleaf model the single polynucleotide chain of tRNA is folded upon itself to form 5 arms. As a result of the folding the 3’ and the 5’ ends of the chain come near each other. An arm consists of a stem and a loop. In the double helical stems there is internal base pairing which follows the A-U and G-C combinations. There is no base pairing in the loops. One of the arms has stem but not a loop and is called the acceptor stem. The other arms are called the D arm, the anti codon arm, the variable arm and the TjC arm. The variable arm may or may not have a stem. The acceptor stem consists of 7 base pairs and 4 unpaired nucleotide units. The latter include a constant 3’ terminal-CCA sequence and a fourth nucleotide, which is a variable purine (A or G). The amino acid molecule attached to the 3’terminal of the –CCA sequence is known as the amino acid binding site. The 5’ end of tRNA is either guanine (G) or Cytosine (C).
The second arm is called the D arm. It consists of 15-18 nucleotides with 3 – 4 base pairs in the stem and 7 – 11 unpaired nucleotides in the loop. The loop of the D arm is called Loop I or dihydrouridine (DHU) loop or the D loop. The synthetase site that recognizes the amino acid activating enzyme is located on a part of the D loop.
The third arm or anti codon arm consists of an anti codon stem of 5 base pairs and a loop, called Loop II or the anti codon loop. This loop consists of 7 unpaired nucleotides of which the middle three form the anti codon. The anti codon recognizes the 3 complementary bases, which constitute the codon of mRNA.
The variable arm (the lump, the mini loop, Loop III) is of two types. In one type there is a loop containing 4 – 5 bases but no stem. In the other type, the arm consists of 13-21 residues, and both the stem and the loop can be distinguished.
The TjC arm consists of a stem having 5 base pairs and a loop of 7 nucleotides. The outer most of the 5 pairs of the stem is C –G. The TuC loop contains a constant TjC sequence (ribothymine-pseudouracil-cytosine). All tRNAs have a ribosome recognition site on the TuC loop consisting of a G-T-u-C sequence. (Ribothymidine(T), Pseudouridine(j), are unusual base pairs).


Hatchery Technology of Giant Freshwater Prawn
Macrobrachium rosenbergii is the giant freshwater prawn usually cultured in India. It is euryhaline and eurythermal in habit, so it is a suitable candidate species for culture. Macrobrachium rosenbergii matures when they are months old. The female after a premating moult mates with a hard shelled male. The fertilized eggs are carried by the female in a brood pouch in the ventral side of the abdominal region. The fertilized eggs hatch out in 18 to 23 days. The newly hatched larvae require estuarine water but it can survive in fresh water for three to five days. The larvae after passing through many stages in estuarine region become postlarvae which then migrate to freshwater.

Brood stock collection and hatching:
Female prawn carrying fertilized eggs in the abdominal pouch is called berried female. It can be collected either from rivers or culture ponds. While collecting, care should be taken to see that the eggs are not lost due to rough handling. The berried females are transported with oxygen packing. Hatching can be done either in larval tanks or in a separate hatching tank using freshwater or 12 ppt saline water. Usually hatching of egg takes place during night hours. The hatching period lasts for 3 or 4 days. Major quantity of larvae is released on the second day of the hatching. The newly hatched-out larvae can survive for about 5 days in fresh water. After hatching the mother prawn should be removed from the hatching tank. The larvae pass through 11 stages to become post larvae. For collecting the larvae they can be concentrated at one portion of the tank by putting a light at the collection point after covering other parts with a black cloth or black sheet as they are very much attracted by light.

Feed and feeding:
Feeding of larvae starts from second day onwards. Artemia is the most ideal and widely used feed for larval rearing. Artemia is a small crustacean seen in hyper saline water. Its eggs are collected, packed and supplied by several companies. The cysts are hatched in separate tanks and the hatched out naupli are fed to larvae.
Giant Freshwater Prawn Farming

Farming of giant freshwater prawn, Macrobrachium rosenbergii is a growing industry in India. Generally prawn ponds are shallow having depth of 1.5 to 2.00 meters.

Preparation of Pond:
Pond preparation includes checking and repair of sluice gates and eradication of unwanted species, liming, placing of hide outs, manuring and filling of water. In older ponds, pumping out the water and drying helps in elimination of unwanted species. Drying helps mineralization of bottom soil and improves the productivity of the bottom sole. Disease spreading micro organisms and parasites also can be eliminated by drying. Ploughing of bottom soil after drying helps removal of obnoxious gases. In undrainable ponds, eradication can be done by poisoning. Poisons of plant origin, like mahua oil cake, tea seed cake derris root powder are advisable. After eradication, liming and ploughing can be done. Liming helps in disinfection of pond bottom and improves the fertility of the bottom soil. The quantity of lime could be around 200 kg to 1000 kg based on the pH of the soil. After drying, liming and ploughing, the pond bottom should be kept in a moist condition for a week to facilitate bacterial action. Then manure can be applied. Manuring helps the development of worms like tubifex, zooplankton, algae, etc.

Laying of shelters: Freshwater prawn are cannibalistic. They grow by moulting. Moulting is a process by which the old exoskeleton is shed and new exoskeleton is formed over the body. It takes nearly 24 hours for hardening of the new exoskeleton. During that period moulted prawns require protection from other prawns. Tile, bricks, etc., can be provided in the pond as hide outs. Discarded PVC pipes, earthern pipes can also be used as hide outs. Apart from hide outs, it is advisable to keep some floating weeds like Eichornia, Pistia and duck weed.

Stocking the pond: 7 to 10 days old (PL 7 to 10) post larvae is the ideal age group for transportation and stocking the pond. Before stocking the PLs are to acclimatized to freshwater, preferably with the pond water itself. The density of stocking depends on the intensith of management. For semi-intensive culture with a targeted production of 3 to 3.5 tonnes per year per hectatre, 75000 PLs or 60000 juveniles can be stocked.

During the rearing period, artificial diet is given to the growing prawns. Water quality parameters are to be checked regularly. Disease control measures are to be followed by maintaining a healthy environment, avoiding over stocking, etc. Harvesting should be done by netting the pond, after removing the hide outs. Harvesting should be done during early hours when the temperature is less. After harvesting the prawns are cleaned in freshwater and are iced and transported.

Role of DNA and RNA in Protein Synthesis

The central dogma of protein synthesis is that DNA makes RNA, which in turn makes protein. This can be expressed as follows:

Replication Transcription Translation
DNA------------------à DNA-----------------à RNA---------------à PROTEIN

Protein synthesis consists of two main events, transcription and translation. Transcription is the copying of a complementary messenger RNA strand on a DNA strand. The DNA strand unwinds and one of the two strands forms a mRNA strand. This strand is called anti-sense strand or template strand. The nucleotides of mRNA are complementary to those of the DNA strand. However, in mRNA uracil (U) is replaces thymine (T) of DNA. As a result the complementary pairing is A-U and G-C. The mRNA thus formed makes its way through the pores in the nuclear membrane to the cytoplasm. Here it forms a complex with a group of ribosomes.

Transcription requires along with a template strand, ribonucleotide triphosphate (ATP, GTP, UTP, CTP), the enzyme RNA polymerase and divalent metal ions. RNA polymerase consists of a core enzyme (with sub units α, α, β, β and ω) and a sigma (σ)factor. The sigma factor initiates transcription of mRNA on the DNA template and the core enzyme continues transcription. All the RNA chains start with either pppG or pppA and are synthesized in the 5’ – 3’ direction. Chain termination is brought about by a rho factor.

Translation: During translation the genetic information present in mRNA directs the order of specific amino acids to from a polypeptide or protein. The mRNA has a series of triplet bases, each triplet forming a codon. The codons pair with anticodons of the tRNA molecule. Each anticodon consists of three free bases. This pairing follows the A-U and G-C combination. Thus the codon GUC pairs with the anticodon CAG to tRNA. Thus the series of codons on mRNA determines the series of anticodons of the different tRNA molecules, and hence of the amino acids. Since the triplets of mRNA in turn depend upon the series of bases in DNA, it follows that the DNA molecule determines the sequence of amino acids, and thus the structure of the protein molecule.

The translation process consists of –
1. activation of amino acids
2. transfer of the activated amino acid to tRNA
3. initiation of polypeptide chain synthesis
4. chain elongation and
5. chain termination

1. Activation of amino acids: The 20 amino acids (aa) used in protein synthesis are activated by ATP in the present of specific activating enzymes (E) called aminoacyl synthetases to from aminoacyl adenylates (aaa), also called aminoacyl AMP. Pyrophosphates (PPi) are released.

aa + ATP + E --------à E-aa-AMP + PPi

2. Transfer of activated amino acids to tRNA: The activated amino acid is transferred to a specific tRNA with the release of AMP and activating enzyme.

E.aa.AMP + E + tRNA -----------à aa-tRNA + AMP + E

3. Initiation of chain synthesis: This requires the ribosome subunits, mRNA, an energy source (GTP), activated amino acids attached to tRNA (aa-tRNA) and initiation factors (IF). These factors are called IF-1, IF-2, IF-3 in prokaryotes and eIF-2, e-IF2’, eIF-2a1 eIF-2a2 , eIF-2a3 and eIF-3 in eukaryotes.
a. The 30S ribosomal subunit attaches to mRNA to form an mRNA-30S complex. The process requires IF-3 and Mg++
b. the starting amino acid is methionine (Met) in eukaryotes and N-formyl methionine in prokaryotes. The amino acid-tRNA complex (fMEt-tRNA) attaches to the initiation codon, AUG, on mRNA through its anticodon UAC to form the 30S initiation complex. The process requires initiation factors IF-2 and IF-1 as well as GTP.
c. The larger ribosomal subunit (50S in prokaryotes) joins to the 30S initiation complex to form the complete initiation complex (70S).
d. The larger ribosomal subunit has two binding sites for tRNA, an A or acceptor site and a P or peptidyl site. fMet-tRNA binds to the P site.

4. Chain Elongation: Elongation of the polypeptide chain requires elongation factors (EF). These are EF-Tu. EF=Ts and EF-G in prokaryotes and EF-1 and EF-2 in eukaryotes.
a. The second amino acid-tRNA complex (aa2-tRNA) now occupies the A site. There is enzymatic recognition of internal codons. The process requires EF-Tu, GTP and Mg++.
b. Formation of a peptide bond takes place by transfer of fMet to the second amino acid (aa2). Mg++ and K+ are required. The catalyzing enzyme is pepitdyl transferase.
c. Translocation: The aa2-tRNA complex moves from the A site to the P-site. This process is called translocation and requires EF-G, GTP and Mg++. Translocation involves movement of the ribosome relative to mRNA in the 5’--à3’ direction. The third amino acid-tRNA (aa3-tRNA) complex now occupies the vacant A-site.

5. Chain termination: Chain elongation continues until a termination codon (UAA, UAG, UGA) reaches the ribosome. The chain is then terminated and released from the ribosome. This process requires release factors RF-1, RF-2 and RF-3 in prokaryotes and RF in eukaryotes.
a. The termination codon provides signals to the ribosome for the attachment of release factors.
b. the release factors interact with peptidyl transferase causing hydrolysis of the bond between tRNA and the polypeptide chain, and the chain is released from the ribosome.
c. Hydrolysis of GTP results in the dissociation of the release factors from the ribosome. The tRNA is also unloaded. The ribosomal subunits dissociate and mRNA is released, for breakdown to nucleotides.
d. Processing of the polypeptide chain, e.g., cleavage of the formyl residue or of methionine, takes place after release.



One of the most important properties of DNA is that it can make exact copies of itself. This process is called replication. The two stands of DNA double helix are united by hydrogen bonds between the purine and pyrimidine base pairs. When the hydrogen bonds break the two strands separate and unwind. The nucleus contains free nucleotides, which form the nucleotide pool. These free nucleotides pair with the nucleotides of the two separated strands by means of hydrogen bonds. Free adenine nucleotide pairs with thymine nucleotide of the strand and free guanine nucleotide pairs with the cytosine nucleotide of the strand, etc. In this way a new strand is formed around each of the old strand. The result of replication is the formation of tow double helices, each identical to the original.
1. Replication takes place during the interphase between two mitotic cycles.
2. Replication is semi-conservative process in which each of the two double helices formed from the parent double strand have one old and one new strand. Repair replication is non-conservative.
3. DNA replication requires a DNA template, a primer, deoxyribonucleoside triphosphates (dATP, dGTP, dTTP and dCTP), Magnesium ions, DNA unwinding protein, superhelix relaxing protein, a modified RNA polymerase to synthesize RNA primer, the products of dnaA, dnaB, dnaC-D, dnaE and dnaG genes and polynucleotide ligase, a joining enzyme.
4. Replication starts at a specific point called the origin.
5. According to one model replication starts with a ‘nick’ or incision made by an incision enzyme (endonuclease).
6. The two strands of the DNA double helix unwind with the help of a DNA unwinding protein (also called the DNA binding protein) which binds to single DNA strands.
7. The unwinding of the strands imposes strain, which is relieved by the action of a superhelix relaxing protein.
8. Initiation of DNA synthesis requires an RNA primer. The primer is synthesized by the DNA template close to the origin of replication. A special form of RNA polymerase catalyzes the synthesis.
9. Deoxyribose nucleotides are now added to the 3’ end of the RNA primer and the main DNA stand is synthesized on the DNA template. This strand is complementary to the DNA strand and is synthesized by DNA polymerase III.
10. The enzyme DNA Polymerase I now degrades the RNA primer and simultaneously catalyses the synthesis of a short DNA segment to replace the primer. This segment is then joined to the main DNA strand by a DNA ligase.
11. Replication takes place discontinuously and short pieces called Okazaki fragments are synthesized. One strand may synthesize a continuous strand and the other Okazaki fragments, or both strands may synthesize Okazaki fragments. Thus one strand is synthesized forwards and the other backwards.
12. The Okazaki pieces are joined by polynucleotide ligase, a joining enzyme, to form continuous strands.
13. Replication may be in one direction (unidirectional) from the point of origin or in both directions (bidirectional).


Deoxyribose Nucleic Acid

DNA is present in all plant cells, animals, few prokaryotes and in a number of viruses. In eukaryotes it is combined with proteins to form nucleoproteins. In prokaryotes it is without any proteins. The DNA of all plants and animals and many viruses is double stranded. In the bacteriophage phi 174, however it is single stranded.

The widely accepted molecular model of DNA is the double helix structure proposed by Watson and Crick. The DNA molecule consists of two helically twisted strands connected together by ‘steps’. Each strand consists of alternating molecules of deoxyribose (a pentose sugar) and phosphate groups. Each step is made up of a double ring purine base and single ring pyrimidine base. The purine and pyrimidine bases are connected to deoxyribose sugar molecules. The two strands are intertwined in a clockwise direction i.e., in a right hand helix and run in opposite directions. The twisting of the strands results in the formation of deep and shallow spiral grooves.

The DNA molecule is a polymer consisting of several thousand pairs of nucleotide monomers. Each nucleotide consists of the pentose sugar deoxyribose, a phosphate group and a nitrogenous base which may be either a purine or a pyrimidine. Deoxyribose and a nitrogenous base together for a nucleoside and a nucleoside with a phosphate together form a nucleotide.

Deoxyribose is a pentose sugar with five carbon atoms. Four of the five carbon atoms plus a single atom of oxygen form a five-membered ring. The fifth carbon atom is outside the ring and forms a part of a –CH2 group. The four atoms of the ring are numbered 1’, 2’, 3’ and 4’. The carbon atom of the –CH2 is numbered 5’. There are three –OH groups in positions 1’, 3’ and 5’. Hydrogen atoms are attached to carbon atoms 1’, 2’, 3’ and 4’.

Ribose the pentose sugar of RNA, has an identical structure except that there is an –OH group instead of H on carbon atom 2’. All the sugars of one strand are directed to one end, i.e, the strand has polarity. The sugars of the two strands are directed in opposite directions.

Nitrogen bases: there are two types of nitrogenous bases, pyrimidines and purines. The pyrimidines are single ring compounds, with nitrogen in positions 1’ and 3’ of a 6 membered benzene ring. The two most common pyrimidines of DNA are cytosine and thymine. The purines are double ring compounds. A purine molecule consists of a 5-membered imidazole ring joined to a pyrimidine ring at position 4’ and 5’. The two most common purines of DNA are adenine and guanine.

Base Paring: Each step of the DNA ladder is made up of a purine and a pyrimidine pair, i.e., of a double ring and a single ring compound. Two purines would occupy too much space, while two pyrimidines would occupy too little. Because of the purine-pyrimidine pairing the total number of purines in a double stranded DNA is equal to the total number of pyrimidines. Thus A/T = 1 and G/C = 1 or A+G = C+T. The ratio of A+T/G+C, however, rarely equals to 1, and varies with different species from 0.4 to 1.9. The purine and pyrimidine bases pair only in certain combinations. Adenine pairs with thymine and guanine with cytosine. A and T are joined by two hydrogen bonds through atoms attached to positions 6’ and 1’. Cytosine and guanine are joined by three hydrogen bonds through positions 6’, 1’ and 2’. The pyrimidine and purine bases are linked to the deoxyribose sugar molecules. The linkage in pyrimidine nucleosides is between position 1’ of deoxyribose and 3’ of the pyrimidine. In purine nucleosides it is between position 1’ of deoxyribose and position 9’ of the purine.

Phosphate: In the DNA strand the phosphate group alternate with deoxyribose. Each phosphate is joined to carbon atom 3’ of one deoxyribose and to carbon atom 5’ of another. Thus each strand has a 3’end and a 5’ end. The 3’ end of one strand corresponds to the 5’ end of the other. Consequently the oxygen atoms of deoxyribose point in opposite directions in the two strands.

Forms of DNA: DNA can exist in the A. B. C and D forms. Sugar puckering is the most important characteristic for distinguishing the DNA forms. The A form has 3’-endo puckering and one turn of the helix consists of 11 base pairs (11-fold helix). The B form is with 3’-exo puckering and 10 base pairs (basic structure proposed by Watson and Crick). C form is with 2’-endo puckering with a helical symmetry of 91/3 i.e., there are fewer residues per turn than in the B form. The D form is with 3’-exo puckering with 8 base pairs per turn of the helix.

RL helix and Z DNA: According to the Watson and Crick model DNA exists in the form of right handed helix. But G.A. Rodley’s group working in New Zealand and V. Sasisekharan’s group working in India have independently proposed a structure of B-DNA radically different from the Watson and Crick model. According to them, the DNA duplex is formed by alternating right and left handed helices arranged side by side. Each strand of the DNA duplex has 5 base pairs in the right handed helix alternating with 5 base pairs in the left-handed helix.


Chromosomes were first seen by Hofmeister (1848) in the pollen mother cells of Tradescantia in the form of darkly stained bodies. The term chromosomes (Gr: chrom = colour, soma = body) was used by Waldeyer (1888) to designate their great affinity to basic dyes. W. S. Sutton (1900) described their functional significance.

Chromosomes are the most significant components of the cell, particularly they are apparent during mitosis and meiosis. A chromosome can be considered as a nuclear component having special organisation, individuality and function.


The morphology of chromosome can be best studied at the metaphase or anaphase of mitosis when they are present as definite organelles, being most condensed or coiled.
Number: The number of chromosomes in a given species is usually constant containing diploid number (2n) of chromosomes in their somatic cells and haploid (gametic or reduced) number (n) of chromosomes in their sex cell. The number of chromosomes in variable from one to several hundreds among different species. For e.g., in Ascaris megalocephala it is 2, while in certain protozoans (Aulocantha), there are 1600 and in man there are 46.
Size: Chromosomes range, on an average from 0.5 to about 30 microns in length and from 0.2 to 3microns in diameter. The salivary gland chromosomes of Diptera are 2mm long. Usually all the chromosomes in a cell are of the same size. Plant cell normally posses larger chromosomes than animal cells. Among the animals, grasshoppers, crickets, mantids newts and salamanders have large chromosomes. Variation in size of the chromosomes can be induced by a number of environmental agents like temperature, rate of cell division, etc.
Shape: The shape of the chromosomes is changeable from phase to phase in the continuous process of the cell growth and cell division. In the resting phase or interphase stage of the cell, the chromosomes occur in the form of thin, coiled, elastic and contractile, thread like stainable structures, called the chromatin threads.
Structure: In the earlier light microscopic description of chromosome was thought to consist of a coiled thread called the chromonema lying in a matrix. The chromosome was supposed to be covered by a membrane called pellicle. However electron microscopic studies revealed that there is no pellicle. During metaphase a chromosome appears to possess two threads called chromatids.
Chromonema: The chromatids are really spirally coiled chromonemata (singular chromonema). They may be composed of 2, 4, or more fibrils depending on the species. The fibrils of the chromonema are coiled with each other. The coils are of two types:
1. Paranemic coils: When the chromonemal fibrils are easily separable from each other.
2. Plectonemic coils: When the chromonemal fibrils are closely interwined and they cannot be separated easily.

Chromomeres: The chromonema of thin chromosomes of mitotic and meiotic prophases have been found to contain alternating thick and thin regions and thus giving the appearance of a necklace in which several beads occur on a string. The thick or bead like structures of the chromonema are known as the chromomeres and the region in between the chromomeres is termed as the inter-chromomeres. Actually the chromomeres are regions of the super-Imposed coils.
Centromere: (Kinetochore or Primary Constriction) It is indispensable part of the chromosome and forms the primary constriction of metaphase. The position of the centromere is constant for a particular chromosome. The structure and function of the centromere is different form that of the rest of the chromosome. During division the centromere is functional, while the rest of the chromosome is genetically inactive. The position of the centr4omere varies in different chromosomes. Since the spindle fibers are attached to the centromere, the shape of the chromosome during anaphase will depend on the position of the centromere. Four different categories of chromosomes can be recognized, based on the position of the centromere. They are – Metacentric, Submetacentric, Acrocentric and Telocentric.
In Metacentric chromosomes the centromere is near the middle of the chromosome. The two arms of the chromosome are nearly equal and the chromosome appears ‘V’ shaped during anaphase.
In Submetacentric chromosome the centromere is situated some distance away from the middle; one arm of the chromosome is shorter than the other. Such a chromosome will appear L-shaped during anaphase.
In Acrocentric chromosome the centromere is situated near the end of the chromosomes. It appears rod-shaped.
In Telocentric chromosomes the centromere is truly terminal, i.e., situated at the tip of the chromosome.
Secondary constriction: In addition to the primary constriction or centromere the arms of the chromosome may show one or more secondary constrictions called Secondary Constriction II. These differ from nucleolar organizers called Secondary Constriction I. The location of secondary constriction II is constant for a particular chromosome, and is, therefore, useful for in identification of chromosomes.
Nucleolar Organizer: Normally in each diploid set of chromosomes, two homologous chromosomes have additional ‘constrictions’ called nucleolar organizers. These are so called because they are essential for the formation of nucleolus. The nucleolar organizer appears as a ‘constriction’ near one end of chromosome. The part of the chromosome beyond the nucleolar constriction is very short and appears like a sphere or satellite. In man chromosomes 13, 14, 15, 21, 22, and Y have nucleolar organizers and satellites. Chromosomes bearing satellites are called SAT-Chromosomes. The prefix Sat stands for ‘Sine Acid Thymonucleionico’ (without thymonucleic acid or DNA), since the chromosomes on staining shows relative deficiency of DNA in the nucleolar region.
Euchromatin and Heterochromatin: Certain segment of the chromosomes or the entire chromosomes, are more condensed than the rest of the karyotype during various stages of the cell cycle. Such difference in thickening has been called heteropycnosis. Heteropycnosis may be positive, where there is overcondensation or negative, where there is undercondensation. Chromosomes, which remain condensed during interphase, are called heterochromosomes and the non-condensed chromosomes, which extend during interphase, are called euchromosomes.
Chromatin material is of two types- heterochromatin and euchromatin. Chromatin material showing heteropycnosis at any stage is called heterochromatin. Regions of the chromosomes, which never show heteropycnosis, consist of euchromatin. Heterochromatin stains deeply whereas euchromatin stains less deeply. Heterochromatin is found in the condensed region of the chromosome, and is associated with tight folding and coiling of the chromosome fibre. Euchromatin consists of the diffused or less tightly coiled regions. There are two types of heterochromatin, constitutive heterochromatin and facultative heterochromatin. Constitutive heterochromatin shows heteropycnosis in all cell types and throughout the cell cycle. Facultative heterochromatin on the other hand is hyperpycnotic only in some special cell types, or at some particular stages of the life cycle.


Two views have been proposed for ultra structure of chromosome.
1. Multistranded view: This was proposed by Ris (1966). By electron microscope the smallest visible unit of the chromosome is the fibril which is 100Ao in thickness. This fibril contains two DNA double helix molecules. Next largest unit is the half chromatid. The half chromatid consists of four 100Ao fibrils so that it is 400Ao in thickness and contains eight double helixes of DNA. Two half chromatids form a complete chromatid consisting of 16 double DNA helix molecules. As the chromosome consists of two chromatids, thus total number of helixes will be 32 and diameter 1600Ao thick before duplication or synthesis. In summary, the chromosome is composed of numerous micro fibrils, the smallest of which is a single nucleoprotein molecule.
2. Folded - Fibril Model: Dupraw (1965) presented this model for the fine structure of chromosome. According to this model, a chromosome consists of single long chain of DNA and protein forming what is called a fibril. The fibril is folded many times and irregularly entwined to form the chromatid. This measures 250 – 300Ao in thickness. Dupraw’s model of DNA association is now considered unlikely by the discovery that DNA itself looped around histone beads to form nucleosomes.
3. Nucleosome: The chromatin is formed of repeating units called nucleosomes. The term was given by Oudet et al, (1975). The nucleosome is made up of DNA and histone proteins. The proteins form a core particle, which consists of two molecules of each of four histone proteins – H2a, H2b, H3 and H4. The surface of core particle is surrounded by 1.75 turns of DNA (200 base pairs). The DN linking the core particle is called linker DNA. An other histone protein H1, is bounded to the linker DNA.