Describe double helical model of DNA.

Explanation

Watson and Crick in 1953, proposed a model to explain the structure of DNA called the double helix model. According to this model,
(a) DNA is composed of two polynucleotide chains coiled spirally around a central axis to form a double helix.
  (b) The two polynucleotide chains of a double helix are opposite in direction. One chain runs in `5^(1)` to `3^(1)` direction and "the other is `3^(1)` to` `5^(1). direction.
(c) Hydrogen bonds are formed between nitrogenous bases. The base-pairing in DNA consists of one Purine and one pyrimidine. Further Adenine (A) always pairs with Thymine (T) and guanine (G) always pairs with cytosine. These base pairing is said to be complementary base pairing. Two hydrogen bonds are formed between A and T and between and hydrogen bonds are formed (d) The team is equal to that of T and the amount of G is equal to that of A molecules. This relationship of base pairs is known as Chan el. It is represented as A=T,
(e) Backbone DNA helix is made up of sugar and phosphate.
(f) One double click turn is .Shaped and contains a major groove and a minor groove. One complete turn of the helix is 34Ao in length and it consists of 10 base pairs. Thus the distance between two adjacent bases is 3:4 Ao The constant distance between the two strands is 20A".