Biotechnology and Bioinformatics (1+1)

1.2.2.3. Variety of DNA structures

The structure for DNA proposed by Watson and Crick represents B form of DNA. B form is present in most DNA in the cell.

A form differs from the B form in several aspects. The plane of a base pair is no longer perpendicular to the helical axis, but tilts 20 degrees away from horizontal. Also, the A helix packs in 11 base pairs per helical turn instead of 10 found in the B form, and turn occurs in 31 angstroms instead of 34.

The distance between base pairs, is only 2.8 nm instead of 3.4 nm, as in B-DNA.

Both the A and B form DNA structures are right handed; the helix turns clockwise.

Alexander Rich and his colleagues discovered in 1979 DNA can exist in an extended left-handed helical form. Because of the zigzag look of this DNA's backbone when viewed from the side, it is often called Z DNA. There is evidence that living cells contain small proportion of Z-DNA. The distance between base pair is 4.5 nm and number of bases per turn is 12.

RNA-DNA hybrid strand assumes the A form.

Normal DNA has 2 grooves (major and minor). Z- DNA has single groove.