Z Form of DNA, Right Handed Form DNA, Poly GC, Left Handed Helical Form, Z DNA Structures, Eukaryotes

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	Z Form of DNA - Alexander Rich and his colleagues discovered in 1979 that DNA does not always have the right-handed form, it can be left handed also. They showed that double stranded DNA containing strands of alternating purines and pyrimidines (e.g. poly GC) can exist in an extended left handed helical form. Because of the zigzag look of this DNA backbone when viewed from the side, it is often called as Z- DNA. The major groove is flat and the minor groove is narrow and very deep. The diameter of the DNA is 18 A.
Twelve base pairs are present per turn with a distance of 3.7 A between them. Z-DNA structures tend to form in torsionally stressed DNA and are stabilized by dehydration. Originally, it was thought that Z- DNA would not prove of interest to biologists because it required very high salt concentration to become stable. However, it was found that Z-DNA can be stabilized in physiologically normal conditions, if methyl groups are added to the cytosine. Z- DNA may be involved in regulating given expression in eukaryotes.