Question

Describe the Watson & Crick model of DNA.

Answer 1

Structur of DNA:

DNA is found in all the plants, animals, prokaryotes, DNA viruses (most animal viruses) and bacteriophages. In plants & animals, DNA binds with basic proteins to form nucleoproteins.
Most of the DNA is found in the nucleus but little DNA is also found in mitochondria and chloroplasts. Normally, the DNA is double stranded (ds DNA), but in bacteriophages like Φ X 174, M 13 coliphage etc., the DNA is single stranded (SS DNA). Two British bio-chemists, J.D. Watson and F.H.C.
Crick (1953) propounded double helical structure of DNA. They also prepared a model of DNA. For this discovery, they were awarded Nobel prize of 1962. According to it, the structure to DNA is as follows-

1. DNA consists of two polynucleotide chains which are complementary to each other.
2. Both the chain twist spirally and form a double helix.
3. Both the chains are antiparallel i.e. they show polarity ie. the 5’→ 3’ direction of one strand runs opposite to the 3’→ 5’ direction of toher strand.
4. DNA molecule exhibits right handed coiling (helical sense).
5. The length of one coil is 34A° which includes 10 pairs of nucleotides. Hence, each nucleotide pair has a distance of 3.4A°.
6. In DNA molecule, phosphates remain outside and the nitrogen bases remain in the centre.
7. Both the chains remain held together with the help of hydrogen bonds formed between the nitrogen bases.
8. There is specific pairing between the nitrogen bases. Adenine combines with thymine by two hydrogen bonds (A = T or T = A) and cytosine combines with guanine by three hydrogen bonds (C s G or G=C).
9. According to ChargafFs rue, the base ratio is one. or A=T=C=G=1 Purine (A + G) = Pyrimidines (C + T)
10. The common nuclear DNA is also called as B-DNA.
11. The angle of nucleotides from the axis (base tilt normal to the helix axis) is 6°