J.D Watson and F.H.C.Crick(1935) proposed a double helix model of DNA molecule, this is the widely accepted model.According to this Watson and Crick model both the strands of DNA were connected by hydrogen bonds and are twisted helically.
These strands contain purine which pairs with pyrimidine on the opposite strands. A strand of DNA is a polymer chain which is having four nucleotides ie A, T, G, C. These double strands joined together to form a double-helical structure. A combine with T with the help of two hydrogen bonds and G pairs with C with the help of three hydrogen bonds.
In this,
A=Adenine
T=Thymine
G=Guanine
C=Cytosine.
In RNA only Thymine is replaced by Uracil means A now pairs U with the help of two hydrogen bonds.
These double strands run anti-parallel to each other.
Watson and Crick postulates:-
1.The two strands with hydrogen bonds are about 20A0. The double helix coils in the right-hand direction(clockwise direction) and it completes a turn of 34A0 and it includes 10 nucleotides.
2.The helical turning results in a double helix which forms a major and minor groove.
3.The major groove is the site for the attachment of protein.
4.The backbone of the double-helical structure is made up of a Sugar-phosphate backbone(Nucleoside).
5.It also suggests it copying mechanism of genetic information.
6.The DNA consists of Nucleotides which is made up of -Nucleosides, sugar, and phosphate.
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