Though different authors had described the structure of DNA differently but the most acceptable and convincing structure of DNA molecule had been illustrated by J.D. Watson and H.C. Crick (1953) by using X-ray diffraction photograph of DNA. The model as illustrated them is known as Watson and Crick’s structural model of DNA. It provided an idea of how a various constituents of DNA are attached to one another.
The model has the structure of a double helix formed bv polynucleotides. The helix has a diameter of about 20 A and each helix has one complete turn every 34A along its length. Each turn consists of a stack of ten nucleotides and so distance between each nucleotide is 3.4 A. In DNA molecule the adjacent deoxyribonucleotides are joined in a strand by phosphodiester bridges or bonds which link the ugar of one nucleotide to the sugar of the adjacent nucleotide.
The molecule consists of two such polynucleotide chains remain twisted around each other to form a helical structure. The sugar and phosphate chain remain on the outside and thus form the back bone of each polynucleotide chain. The purines and pyrimidines remain on the inner side of the helix and face each other.
The two polynucleotide chains are held together by hydrogen bonds. The pairing of the nitrogenous bases of the two strands is very peculiar. The dimension of the purine rings is greater (due to double ring atoms in their molecules) than that of the pyrimidine ring (due to the single ring atoms in their molecules). So, to maintain constant diameter of the two-chains helix molecule of DNA, the purines always pair with pyrimidines with hydrogen bonds.
Because of the shape and chemical nature of the purine and pyrimidine bases, adenine can be attached only to thymine by two hydrogen bonds. Similarly guanine can be attached to cytosine by three hydrogen bonds. Therefore, four combinations of purine pyrimidine such as A-T,T – A, G-C and CG could only be found.
nucleotide of the other chain. Thus whatever the sequence of bases is present in one strand of the DNA molecule, a complementary sequence is present in other strand. Due to this type of base pairing, the two strands of DNA are not identical but complementary to each other. This is called complementary base pairing. The four base pairs may occur in infinite ways and thus provide infinite varieties of DNA molecules.
One end of the DNA strand is called 5′ end and the other end is called 3′ end. This is beçause the deoxyribose attaches itself to a phosphoric acid at 5 carbon position and the same deoxyribose attaches itself to the next phosphoric acid at 3 carbon position. This is called polarity of DNA.
The two nucleotide chains of DNA run in opposite or antiparallel direction. It means that in one chain the 5 carbon position in sugar molecule is in upward direction and in other chain its carbon position is in lower direction. This implies that if in one chain the direction is 5′-3′ and in other the direction is 3′-5′. This is known as antipara- llel direction of DNA molecules.
