Principles of Genetics and Breeding

1.2.3.5 A 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 groove (major and minor). Z- DNA has single groove.