Explain the following salient features of a DNA molecule:

(a) Complimentarity of the base pairs of its two chains.

(b) Anti-parallel polarity of the two chains.

(a) Complimentarity of the base pairs of its two chains:

• In general terms complimentarity is defined as the state of working usefully together.

• Similarly, the nitrogenous bases of the DNA work together and form complimentary pairs with each other.

• DNA has four nitrogenous bases: Adenine, Guanine, Thymine and Cytosine.

• Adenine and Guanine are referred to as Purines and Thymine and Cytosine as Pyrimidines.

• When base pairs are formed in the two strands of DNA, purine always forms a pair with pyrimidine. In this way Adenine forms base pair with Thymine and Guanine forms base pair with Cytosine.

• The base pairs are formed due to Hydrogen bonds. Adenine always forms two hydrogen bonds with Thymine and vice versa. Similarly Guanine always forms three hydrogen bonds with Cytosine and vice versa.

(b) Anti-parallel polarity of the two chains:

• The DNA is a double stranded structure.

• When the two strands come together they have anti parallel polarity.

• If one strand of DNA has polarity of 3’ (to) 5’, then the polarity of the other strand that forms complimentary pair with it, will be 5’ (to) 3’.