Deoxyribonucleic Acid  ( DNA):

 DNA is the genetic material and forms molecular acid of heredity in all organisms. In certain viruses,such as tobacco mosaic virus (TMV), RNA is the genetic material. DNA is a right _ handed, double helix of two coiled polynucleotide  chains ( Waston and Crick model), having a major groove and a minor groove alternately. Each chain is composed of deoxyribonuclease of adenine, guanine, cytosine and thymine. The two chains are complementary, antiparallel, and held together by hydrogen bonds ( two between A and T and three  between C and G). High temperature denatures DN molecule.

● Location of DNA:

In prokaryotic cells, DNA occurs in the cytoplasm and is the only component of the prochromosome and plasmids.In eukaryotic cells, DNA is largely confined to th nucleus and is the main component of the chromosomes. It is called nuclear DNA. Each chromosome contains a single DNA molecule extending from one end of the chromosome to the other. The DNA is associated with histone proteins in about 1: 1 ratio to form deoxyribonucleoprotein ( DNP) or chromatin . A small quantity of DN also occurs in some cytoplasmic organelles, such as mitochondria and plastids. This is called extracellular or organeller DNA. DNA is present in the chromosome of some viruses too.


The DNA content is fairly constant in all the cells of a given species.Just before cell division, however, the amount of DNA is doubled. The gametes have half the amount of DNA as they contain half the number of chromosomes. 

● Chemical Composition:

Chemical structure of DNA was explained by P.A.Levene.DNA is the largest macromolecule in the organisms. It is a long double* chain of deoxyribonucleotide, or deoxyribotide, units. The two deoxy_ ribonucleotide chains are twisted around a common axis to form a right handed double helix ( spiral) that enclose a cylindrical space in it. Each deoxyribonucleotide unit, in turn, consists of three molecules:phosphate, ( PO¾-),  a 5_ carbon deoxyribose sugar  ( C5H10O4) and a nitrogenous base. Nitrogen gives the base its basic nature. The nitrogenous base may be a 9_ membered, double _ ringed purine, i.e, adenine ( A) or guanine ( G); or a 6__ membered, single _ ringed pyrimidine, vuz.,thymine ( T,) or cytosine (C).

There are 4 Types of deoxyribonucleotides in DNA, namely, adenine_  deoxyribose _ phosphate, or adenosine monophosphate ( AMP) ; guanine__ deoxyribose __ phosphate, or guanosine monophosphate ( GMP); cytosine __ deoxyribose _ phosphate, or cytidine monophosphate ( CMP); and thymine__ deoxyribose __ phosphate, or thymidine monophosphate ( TML,).In each chain, the phosphate component carried by the carbon atom at position 5' of the sugar in one nucleotide unit is joined by phosphodiester bond to the hydroxyle component of the carbon atom at position 3'  of the sugar in the next nucleotide unit. These 3', 5' _ phopshodiester bonds provide a considerable stiffness of the polynucleotide. The alternating sugar__ phosphate __ sugar components form the " backbones" of the DNA double helix.The phosphate groups provide acidity to the nucleic acid. The nitrogenous base molecules are joined to the sugar molecules at th 1' __ carbon position by glycosidic bonds and project into the space enclosed in the helix at about 90° to the long axis of the helix at about The nitrogen 9 of a purine and the nitrogen 1 of a pyrimidine join the 1' carbon of the deoxyribose sugar. The two deoxyribonucleotide chains are held together by hydrogen bonds between paired bases, and by van  der Waals attraction between the stacked bases. Adenine of one chain is always joined thymine of the other chain by 2 hydrogen bonds ( A= T). Cytosine of one chain is always linked to guanine of the other chain by 3 hydrogen bonds( C=G).There are only Four possible base pairs:
A__ T,T__ A, C__ G and G__ C in the DNA molecule. 

Structure of DNA:

The structure of DNA refers to its double helix shape, which resembles a twisted ladder. The two strands of DNA are made up of nucleotides, which are composed of a sugar molecule (deoxyribose), a phosphate group, and a nitrogenous base (adenine, thymine, cytosine, or guanine). The two strands are held together by hydrogen bonds between the bases, with adenine always pairing with thymine on one strand and cytosine always pairing with guanine on the other strand.

● Level of Structure:

A DNA molecule shows primary, secondary and tertiary structure like a protein molecule, being coiled in three orders for accommodation in a small space.

● Polarity: 

The polynucleotide chains show polarity ( direction). One end of each DNA strand is called roman fat ' 5 end. The last deoxyribose unit at this end has the carbon at position 5 free. The other end of the strand is termed 3' end. The last deoxyribose unit at this end has the carbon at position 3 free.


Although only four types of base pairs are involved in the formation of DNA molecule, these base pairs may occur in any sequence, and there may be any number of sequences in a molecule. This given an infinite variety to the DNA molecules. Each individual has a specific sequence of base pairs in its DNA.

Complementarity of Base Pairs:

The two nucleotide chains of DNA molecule are not identical,but complementary to each other with respect to base pairs. This is due to restriction on base pairing. A base pair must consist of a purine and a pyrimidine.There are two reasons for this limitation.

(i) The space available between the two sugar _ phosphate chain of DNA, i.e, 20A° ( 2nm), can accommodate one purine and one pyrimidine, but not two purine, which would be too large, and not two Pyrimidines, which would not be close enough to form proper hydrogen bonds. It may be noted that purine bases, with two carbon rings, are about twice as wide as pyrimidine bases.

(ii) A specific purine pairs with a specific pyrimidine because of a perfect match between hydrogen donor and acceptor sites on the two bases. Only adenine and thymine and also guanine and cytosine have the proper spatial arrangement to form correct hydrogen binding. Adenine and cytosine, and also guanine and thymine do not form proper hydrogen bonds and cannot pair. 

The concept of complementary base pairing provide that an adenine in one chain must be matched with a thymine  in the other chain; and a guanine in one chain must have cytosine opposite to it in the other chain. The two chains are complementary to each other. The base sequence in one DNA chain, the base sequence in its complementary chain can be easily predicted. 

For example, if the base sequence in one chain is A, T,C,G,G,T that in the other chain will be T, A, G, C, C, G, A, T.

Base Pair:

The term " base pair" refers to two bases, one in each chain of DNA molecules, joined together by hydrogen bonds.Each base pair consists of one 2 _ ringed purine and one 1_ ringed pyrimidine. All the base pairs have equal width, and the DNA helix has a constant diameter. 

● Antiparallel Directions:

The two chains of DNA molecule run in opposite or antiparallel directions. This means that the carbon atom at position 5 in the sugar components is in one direction in one chain and in the opposite direction in the other chain between paired bases, and by van der Waals attractions between the stacked bases. The two chains are parallel but their 5 prime ~ symbol ➡️~ roman 3' directions are opposite. This is analogous to a 2_ lane road, where the lanes run parallel but carry traffic in opposite directions. 

Basic for DNA Replication:

The complementarity of bases in DNA molecule and antiparallel directions of the two chains of DNA molecule provide the basis for  the precise replication of  DNA.

● Chargaff's Rules: In 1950, E.E.Chargaff formulated important generalizations about DNA structure. These generalizations are called Chargaff's rules in his honour. They are summarized below__ 

 (i) The DNA molecule, irrespective of its source, always has the A__ T base pairs equal in number to the  G__ C base pairs.

(ii) The purines and Pyrimidines are always in equal amounts i.e., A+ G= T+C

(iii) The amount of adenine is always equal to that of thymine, and the amount of guanine is always equal to that of cytosine, i.e., A= T and G= C.

(iv) The base ratio A+ T/G+C may vary from one species to  another, but is constant for a given species. This ratio can be used to identify the source of DNA, and can help in classification. 

(v) The deoxyribose sugar and phosphate components occur in equal proportions. 

● Denaturation and Renaturation:

If DNA molecule is exposed to help temperature or titration with an acid or an alkali, the two strands unwind and separate by breakdown of hydrogen bonds between the base pairs.This process is called denaturation¹,or melting. When denatured DNA is incubated at a low temperature, the two separated strands reassociate to form a DNA duplex. This process is termed renaturation ².

Physical Structure:

Astbury, Wilkins and Franklin have suggested 3_ dimensional, helical configuration for DNA molecule by X__ ray diffraction studies. The purines and Pyrimidines are flat rings stacked one above the other, and arranged at right angles to the sugar_ phosphate backbone.The adenine __ thymine and guanine _ cytosin base pairs are precisely of the same size and shape. The double helix has a constant diameter of 20A° along its entire length. Its one complete turn is 34A° long and has 10  base pairs. The successive base pairs are 3.4 A° apart. These investigations helped Watson and Crick to design a model of DNA molecule. 

 Function of DNA:

The function of DNA is to store and transmit genetic information. It contains the instructions that are necessary for the development and functioning of all living organisms. DNA is responsible for encoding genes, which are segments of DNA that carry the instructions for making proteins. These proteins then play various roles in the body, such as enzymes that catalyze chemical reactions, structural components of cells, or hormones that regulate bodily processes.

DNA replication is another important function of DNA. Prior to cell division, DNA is replicated so that each new cell receives an identical copy of the genetic information. This replication process ensures the continuity of genetic traits from one generation to the next.

Overall, the structure of DNA allows for its function in storing and transmitting genetic information, which is essential for the survival and reproduction of living organisms.

Types of DNA:

DNA molecules are of 2 types :

1): Linear Linear DNA is found in the nucleic of Eukaryotic cells. It is associated with proteins in about 1: 1 ration.

2): Circular Circular DNA is found in many viruses, all prokaryotic cells and in the mitochondria and chloroplasts ( plastids) of eukaryotic cells. It is associated with very little protein.

☆Watson Cricl Model:

J.D. Watson, an American biologist, and F.H.C.Crick, an English chemist, in 1953 suggested a model of DNA molecule to explain its structure. Their paper consisted of just 2 pages but it became a cornerstone of the molecular biology. Their model got them the 1962 Nobel Prize. According to Watson _model, the DNA molecule consists of two long, parallel chains (strands) which are joined together by short crossbars at regular intervals. The two chains are spirally coiled around a common axis in a regular manner to form a double helix. The double helix is of a constant diameter and has a major groove about 22 A° wide and a minor groove about 12 A° wide alternately. The bases face the interior of the double helix whereas the sugar and phosphate components form backbone on the outside. The helix is generally right _ handed, that is, the turns run clockwise looking along the helical axis.

   In other words, the DNA molecule has the form of a twisted ladder. The vertical bars of the ladder are formed of alternating phosphate and deoxyribose sugar components. The horizontal rungs of the ladder are joined to the sugar components of the vertical bars, and are composed of purines and Pyrimidines linked by hydrogen atoms.

● DNA Forms:

Five forms of DNA have been reported : A,B,C D and Z. The A,B,C and D forms are right __ handed double helices. The B_ DNA occurs under the physiological conditions prevailing in the living cells. It has the specification given in the foregoing description of DNA. The A__ DNA has 11 base pairs per turn of the helix. The C_ DNA has 9 base pairs per turn of the helix and D__ DNA has only 8  base pairs. The Z__ DNA is left__ handed, contains 12 base pairs per turn of the helix. The last feature gives the Z__ DNA its name.


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