Describe double helical model of DNA.

Explanation

Watson and Crick in 1953, proposed a model to explain the structure of DNA called double helix model. According to this model,
(a) DNA is composed to two polynucleotide chains coiled spirally around a central axis todorm a double helix.
(b) The two poly nucleotide chains of a double helix are opposite in direction. One chain runs in `5^(I)to3^(I)` direction and the other is `3^(I)to5^(I)` direction.
(c) Hydrogen bonds are formed homaanitroganoushases. The base pairing in DNA consists of one Purine and one primiome. Further Adenine (A) always pairs with Thymine (T) and guanine (G) Galwayspains with glosing. These hasc pairing is said to be complementary base pairing. Two hydrogen bonds are formed between A and T and between G and C three hydrogen bonds are formed.
(d) The total amount of Ais equal to that of T and the total amount of G is equal to that of C.in DNA molecules. This relationship between base pairs is known as Chargoffs's rule. It is represented as A=T, C-G.
(e) Backbone of the DNA helix is made up of sugar and phosphate.
(f) One double helical tum is 'S'shaped and contains one major groove and a minor groove. One complete turn of the helix is `34A^(@)` in length and it consists of 10 base pairs. Thus the distance between two adjacent bases is `3.4A^(@)`. The constant distance between the two strands is `20A^(@)`