RL Helix and Z-DNA

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RL Helix and Z-DNA

	RL Helix and Z-DNA - According to the Watson-Crick model DNA exists in the form of a right handed double helix. Recently G. A. Rodley's groups working in New Zealand and V. Sasisekharans group- in India have independently proposed a structure of B-DNA radically different from the Watson- Crick Model. According to them the DNA duplex is formed by alternating right and left handed helices arranged side by side.
This structure has been called the right-left handed helix (RL helix), or the side by side helix. The repeating unit of the helix is of 10 base pairs. Each strand of the DNA duplex has 5 base pairs in the right-handed helix alternating with 5 base pairs in the left-handed helix. The RL helix model can more easily explain unwinding of the DNA duplex during replication than the Watson-Crick model. In the latter model torque would be generated in the DNA strands during unwinding, because of twisting.
In the RL helix model mere breaking of the hydrogen bonds between the two strands would cause the separate without twining around each other. The model can also explain super coiling of DNA in eukaryote chromosomes without the presence of the proposed kinks in the regular helix.
Sasisekharan and Gupta have recently proposed that there are two topologically distinct types of DNA duplexes, the uniform helix and the zigzag helix. In the uniform helix the twisting of the helix is uniform and the phosphate groups go around the helix in a zigzag manner and not uniformly.
	Right handed helices are produced by purines in the anti conformation and left handed helices (uniform or zigzag) by purines in the synconformation. Rich and his group at the MIT, USA, have studied the molecular structure of a DNA fragment at atomic resolution.

They have proposed a left-.handed double helix model with a zigzag sugar-phosphate backbone in antiparallel organisation.

This DNA has been called Z-DNA. The purines in the zigzag left-handed helix are in the syn-conformation as predicted theoretically by Sasisekharan and Gupta.

They have considered the possibility that the Z-DNA duplex occur along with the right-handed B-DNA duplex.

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Single Stranded DNA Localization of Nucleic Acids Forms of DNA Characteristics of Different Forms of DNA RL Helix and Z-DNA Palindromic DNA Repetitive DNA (Satellite DNA) Satellite DNA of Drosophila Virilis

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