Watson and Crick Model of DNA:
Deoxyribonucleic acid (DNA) is the hereditary material and carries genetic information. Therefore, it is called a blueprint of life. In 1953, it were James D. Watson (an American biologist) and Francis H.C. Crick (a British physicist) who gave first the chemical structure of DNA. They also explained the replication of DNA by a semi-conservative method. Watson and Crick along with M. Wilkins were awarded Nobel Prize in 1962. The structure of DNA is popularly known as a double-helical model of Watson and Crick or Watson and Crick model of DNA.
DNA is a long double chain or duplex ‘molecule’ formed of millions of deoxyribonucleotides. The two chains or strands are antiparallel i.e. their 3′, 5′-internucleotide phosphodiester bridges run in the opposite direction. The coiling of two strands in such that they cannot be separated from each other except by unwinding the coils; such coiling is termed plectonemic.
In each chain, the phosphate group is linked to one sugar molecule and the base of each nucleotide (a purine or a pyrimidine) is linked to a sugar, not to a phosphate. Through the phosphate group, each nucleotide is attached to its neighbour in the chain. Since one chain is composed of one nucleotide after another attached through the sugar and phosphate components, a DNA chain may be called a polynucleotide. The two complementary polynucleotide chains are twisted around each other in a helical or spiral form. The bases of the two chains are always bound together in a specific way. Adenine (A) always pairs with Thymine (T) and the other purine base Guanine (G) always pairs with the other Pyrimidine base Cystosine (C). There are always two hydrogen bonds between A and T and three between C and G. Thus, the sequence of nucleotides in one determines the sequence of nucleotides in the other, and hence the two chains of each DNA molecule must be complementary.
The Watson-Crick model and the genetic components of DNA clearly imply that each gene is a segment of DNA along the molecule. A gene contains many nucleotides or rather nucleotide pairs because DNA is a double helix. The double helix is of a constant diameter of 2 nanometres (nm) or 20 Angstroms (Å) and has a major groove about 22 Å wide and a minor groove about 12 Å wide alternately. One complete spiral (turn) of the helix is 3.4 nm (34 Å) long and has 10 base pairs. The bases face the interior of the double helix and are stacked 0.34 nm (3.4 Å) apart. The sugar and phosphate components form backbones on the outside. The helix is generally right-handed i.e. its turns run clockwise looking along the helix axis.
As the sequence of nucleotide pairs in one gene is not exactly the same in the following gene, each gene has its own distinctiveness and by that, it differs from the next one. For each ‘A’ on any one chain, there is a corresponding ‘T’ on the other chain at the same location. Similarly for each ‘G’, there is ‘C’. Therefore, the sequence of the nucleotides on one chain of the pair can be determined from the sequence on the other, if that sequence is known.
Watson-Crick model not only depicts the structure of DNA, but also shows that DNA molecule can replicate itself faithfully, and remains relatively stable from generation to generation, and also explains how information is contained in DNA molecule.