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The Watson-Crick model of DNA has been the basis for further work on DNA replication, mutation and recombination. Much evidence exists to establish the validity of this model.
Nevertheless, the model has been scrutinized by other workers, and from time to time other models have been suggested by Wu (1970), Arnott (1968), Wang (1979) and a few others. Wang’s model is based on artificially synthesised small length of DNA of known sequence and is described here.
Wang (1979) crystallized DNA fragments of fixed sequence and analysed their structure by X-ray diffraction. They used a double helical fragment of DNA containing 6 base pairs in alternating sequence CGCGCG. The double helix was revealed to be left handed due to alternating conformations of the C and G residues. It has some new features not present in the Watson-Crick right handed helix of the B form of DNA.
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The fragment has antiparallel strands with Watson- Crick base pairing A-T and G-C. But the two strands have a left-handed helical sense. The sugar phosphate backbone has a zigzag course. The sugar in molecules attached to the guanine bases have the ring O pointing upwards, while those attached to cytosine have ring O pointing downwards.
Because of this alternating conformation of the sugar molecules, the repeat unit in the hexamer fragment is a dinucleotide, not a mononucleotide as in the Watson-Crick model. Furthermore, the helix contains 12 base pairs per turn. Each pair in the complementary chains is related to the next by a rotation of 60°. The bases themselves are tilted 7° from the helix axis.
There are two general positions which a base can occupy relative to the sugar residue called anti and syn. In the left handed helix the cytosine residues are in the anti-conformation as in B DNA, but the guanines are in the syn conformation. The zigzag course followed by the sugar phosphate backbone is due to the alternating conformations of the guanine and cytosine residues. Wang et al., have called this Z-DNA.
The bases are not stacked directly above one another, but are displaced laterally by 7Å. The guanines moreover are stacked upon the oxygen atom of the ribose sugar. The guanine residues in Z-DNA are located away from the centre; in B DNA they are closer to the centre.
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Z-DNA looks like a cylinder and has less of a grooved appearance than B DNA. The minor groove is deep in Z-DNA and forms below the base pairs, in B DNA it is above the base pairs. There is no prominent major groove in Z-DNA as in the Watson-Crick model.
Instead there is a convex surface above the base pairs. Z-DNA has a smaller diameter of 18 Å and one complete turn of the helix has 12 base pairs or 6 dinucleotide pairs. A length of 44. 6° is covered by these 12 pairs; in B DNA the same length is occupied by 13 base pairs.
