DNA is a polymer of deoxyribonucleotides. It is composed of monomeric units namely deoxyadenylate (dAMP), deoxyguanylate (dGMP), deoxycytidylate (dCMP) and deoxythymidylate (dTMP). DNA does not contain uracil. The monomeric deoxynucleotides in DNA are held together by 3', 5'-phosphodiester bridges. Double helix structure of DNA
The double helical structure of DNA was ß by James Watson and Francis Crick in 1953 (Nobel Prize, 1962).It is summarized as follows The DNA is a right handed double helix comparable to a twisted ladder.It consists of two polydeoxyribonucleotide chains twisted around each other on a common axis
Two strands are antiparallel, i.e., one strand runs in the 5' to 3' direction while the other in 3 to 5 direction. The width (or diameter) of a double helix is 20A°
(2 nm). Each turn (pitch) of the helix is 34 A°(3.4 nm) with 10 pairs of nucleotide, each pair placed at a distance of about 3.4 A (0.34 nm)Each strand of DNA has a hydrophilic deoxyribose phosphate backbone (3'-5' phosphodiester bonds) on the outside (periphery) of the molecule while the hydrophobic bases are stacked inside .The two polynucleotide chains are complementary to each other due to base pairing. The two strands are held together by hydrogen bonds formed by complementary base pairs. The A-T
pair has 2 hydrogen bonds while G-C pair has 3 hydrogen bonds. The hydrogen bonds are formed between a purine and a pyrimidine only The genetic information resides on one of the two
strands known as template strand or sense strand. The opposite strand is antisense strand. The double helix has major grooves and minor grooves along the phosphodiester backbone.
