1 THE DNA  DNA is a long polymer of deoxyribonucleotides.  The length of DNA is usually defined as number of nucleotides or a pair of nucelotide referred to as base pairs present in it.  This also is the characteristic of an organism.  Few examples are sited below : Organism Genetic material Number of nucleotides or bp  × 174 Virus Lambda () virus Escherichia Coli ss DNA, Linear ds DNA, Linear ds DNA, Circular 5386 nucleotides 48502 bp 4.6 × 106 bp Human ds DNA, Linear 3.3 × 109 bp (n) Chemical structure of DNA polynucleotide :  The basic unit of DNA is a nucleotide which has three components -a nitrogenous base, a pentose sugar (deoxyribose), and a phosphate group.  There are two types of nitrogenous bases - Purines (Adenine and Guanine), and Pyrimidines (Cytosine and Thymine). Cytosine is common for both DNA and RNA and Thymine is present in DNA. Uracil is present in RNA at the place of Thymine.    A nitrogenous base is linked to the pentose sugar through a N-glycosidic linkage to form a nucleoside, such as adenosine or deoxyadenosine, guanosine or deoxyguanosine, cytidine or deoxycytidine and deoxythymidine. Double Stranded Polynucleotide Chain  When a phosphate group is linked to 5'-OH of a nucleoside through phosphoester linkage, a corresponding nucleotide (or deoxynucleotide depending upon the type of sugar present) is formed.         Two nucleotides are linked through 3'-5' phosphodiester linkage to form a dinucleotide. More nucleotides can be joined in such a manner to form a polynucleotide chain. A polymer thus formed, has a free phosphate moiety at 5'-end of sugar, which is referred to as 5'-end of polynucleotide chain. Similarly, at the other end of the polymer the sugar has a free 3'-OH group which is referred to as 3'-end of the polynucleotide chain. The backbone in a polynucleotide chain is formed due to sugar and phosphates (phosphodiester bond) nitrogenous bases linked to sugar moiet projects from the backbone. In RNA, every nucleotide residue has an additional -OH group present at 2' -position in the ribose. Also, in RNA the uracil is found at the place of thymine (S-methyl uracil, another chemical name for thymine). In 1953, James Watson and Francis Crick based on the X-ray diffraction data produced by Maurice Wilkins and Rosalind Franklin, proposed Double Helix model for the structure of DNA.  One of the hallmarks of their proposition was base pairing between the two strands of polynucleotide chain.  However, this proposition was also based on the observation of Erwin Chargaff –that for a double stranded DNA, the ratios between Adenine and Thymine and Guanine and Cytosine are constant and equal.  Chargaff (1950) made observations on the bases and other contents of DNA. These observations or generalizations are called Chargaff's rules. (i) Purine and pyrimidine base pairs are in equal amount, i.e., adenine + guanine = thymine + cytosine. (ii) Molar amount of purine (adenine) is always equal to the molar amount of pyrimidine (thymine). Similarly, guanine is equalled by cytosine. (iii) Sugar deoxyribose and phosphate occur in equimolar proportions. (iv) The ratio of A + T/G + C is constant for a species (Base ratio, e.g., 1.52 for human and 0.93 for E.coli.  The base pairing is a very unique property of the polynucleotide chains.  They are said to be complementary to each other, and therefore if the sequence of bases in one strand is known then the sequence in other strand can be predicted.  Thus, if one DNA strand has A, the other would have T and if one has G, the other, would have C.  Therefore, if the base sequence of one strand is CAT TAG GAC, the base sequence of other strand would be GTA ATC CTG.  Hence, the two polynucleotide strands are called complementary to one another.  Also, if each strand from a DNA or parental DNA acts as a template for synthesis of a new strand, the two double stranded DNA or daughter DNA produced would be identical to the parental DNA molecule. Salient Features Of DNA Double Helix (i) It is made of two polynucleotide chains, where the backbone is constituted by sugarphosphate, and the bases project inside. (ii) The two chains have anti-parallel polarity. It means, if one chain has the polarity 5P  3OH, the other has 3OH  5P. (iii) The bases in two strands are paired through hydrogen bond (H-bonds), forming base pairs(bp). Adenine forms two hydrogen bonds with Thymine from opposite strand and vice-versa. Guanine is bonded with Cytosine with three H-bonds. As a result, always a purine comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of the helix. (iv) The plane of one base pair stacks over the other in double helix. This, in addition to Hbonds, confers stability to the helical structure (v) The two chains are coiled in a right-handed fashion. The pitch of the helix is 3.4 nm and there are roughly 10 bp in each turn. Consequently, the distance between base pairs in a helix is approximately equal to 0.34 nm. It is because of specific base pairing with a purine lying opposite to pyrimidine which makes two chains 2 nm thick. Types of DNA and their comparison DNA Types A B C Z Conditions Base pairs Rotation Vertical rise Helical per turn (n) per bp diameter bp (h) 75% rel. humidity, 11 Right 2.56 Å 23 Å Na+, K+, Cs+ ions handed 92% rel. humidity, 10 Right 3.4 Å 20 Å low ionic strength handed 66% rel. humidity, 9.33 Right 3.3 Å 19 Å Li+ ions handed Very high salt 12 Left 3.8 Å 18.4 Å concentration handed