hybrid structures in RNA-DNA

his helical structure is more common in RNA-DNA hybrid structures. A further unusual structure is left handed Z-DNA, which forms when alternate purine-pyrimidine dinucleotide repeats are present developing into a zigzag structure with 12 base pairs per turn, twist of 60’ per dimer and a helix pitch of 45 A. Although hydrogen bond formation between bases is essential for double helix formation, major stability of the double helical structure of DNA is provided by hydrophobic interactions. The bases of DNA are hydrophobic in nature. For this nature, energetically favorable

stacking of bases in the inside core of double helix and presence of charged phosphate group on the outer surface provides the real stability of the double helix. In case of single stranded DNA or RNA also the bases have a tendency to be grouped or stacked together. One of the important features of double helix that helped

Watson and Crick to provide an explanation of mutation in DNA is the tautomeric shifts in the base composition. The two bases, guanine and thymine exist in ‘keto’ form in normal pH, while adenine and cytosine remain in ‘amino’ form. In this condition, Watson-Crick base pairing between (A=T; GC) is observed. However, under higher pH, the keto form shows a reversible tautomeric shift towards enol form. At this condition GT and A=C base pairing are observed. If replication occurs during such changes, wrong nucleotide will be incorporated in the replicated DNA strand leading to mutation. Denatu ration of DNA double helix by alkaline treatment also works on the same principle. On treatment with alkali, pH of the solution increases, destabilizing the A=T and GC base pair and separations of two strands.