Salient features of the double helix structure of B-DNA, proposed by Watson and Crick are as follows:
DNA consists of two deoxyribose-polynucleotide chains. The backbone of the DNA is constituted by deoxyribose sugar-phosphate and the base that projects inside. Deoxyribose nucleotides are the basic unit (monomer) of DNA molecule.
The two chains of DNA run in anti-parallel fashion with polarity in one and polarity in another chain.
The bases of the two parallel strands are paired by hydrogen bonds to form the base pairs (bp). Adenine forms two H-bonds with thymine from the opposite parallel strand and vice versa. Similarly, guanine binds with cytosine with three H - bonds from the opposite parallel strand and vice versa. Hence, always a purine comes opposite to a pyrimidine.
The double chain of DNA is helically twisted in a right-handed fashion (for B-DNA).
One complete turn is called pitch or gyre. Each turn of a double helix or the pitch of the helix is 3.4 nm (a nanometer is one-billionth of a meter, that is 10-9 m). It has roughly 10 base pairs in each turn (for B-DNA). The distance between two adjacent base pairs is approximately equal to 0.34 nm.
The diameter of the double helix is 2 nm.
Sugar and phosphate form backbone of the helix, while bases are aligned towards the axis.
Distance between two bases present in opposite strands is 1.1 nm. The bases are H-bonded with each other based on Chargaff’s rule, i.e., Adenine binds to Thymine by 2 H-bonds, while Cytosine binds to Guanine by 3 H-bonds
The plane of one base pair stacks over the other in a double helix that confers stability to the helical structure.
The turn of the helix forms a larger groove called a major groove which is of 2.2 nm and a minor grove which is about 1.2 nm. These grooves are responsible for binding with proteins and enzymes to regulate the functions of DNA and provide stability to the structure of DNA.
