1. Watson and Crick postulated a 3 – dimensional model of RNA structure which consisted of two antiparallel helical DNA chains wound around the same axis to form a right handed double helix.
2. Th e hydrophilic backbones of alternating deoxyribose and phosphate groups are on the outside of the double helix, facing the surrounding water. Th e purine and pyrimidine bases of both strands are stacked inside the double helix, with their hydrophobic and ring structures very close together and perpendicular to the long axis, thereby reducing the repulsions between the charged phosphate groups. The offset pairing of the two strands creates a major groove and minor groove on the surface of the duplex.
3. The model revealed that there are 10.5 pairs (36A°) per turn of the helix and 3.4A° between the stacked bases. They also found that each base is hydrogen bonded to a base in opposite strand to form a planar base pair.
4. Two hydrogen bonds are formed between adenine and thymine and three hydrogen bonds are formed between guanine and cytosine. Other pairing tends to destablize the double helical structure. This specific association of the two chains of the double helix is known as complementary base pairing.
5. The DNA double or duplex is held together by two forces.
- Hydrogen bonding between complementary base pairs.
- Base – stacking interactions.
The complementary between the DNA strands is attributable to the hydrogen bonding between base pairs but the base stacking interactions are largely non-specific, make the major contribution to the stability of the double helix.
