Comparison of different helical parameters of A, B, and Z – DNA

A – DNA

• Much wider and flatter.
• Right handed helix.
• major grooves: Narrow and deep
• minor grooves: wide and shallow
• Glycosidic bonds – anti-form
• 11 base pair

B – DNA

• Solid central core.
• Right handed
• Major groove: wide and narrow
• Minor groove: narrow and deep
• Glycosidic bond – anti-form
• 10 base pair

Z – DNA

• Solid core at centre.
• Left handed
• Major groove: More or less flat
• Minor groove: narrow and deep
• Glycosidic bond  is anti-for Pyrimidines and syn-for Purines
• 12 base pairs

Parameter	A-DNA	B-DNA	Z-DNA
Helix sense	Right	Right	Left
Base pairs per turn	11	10	12
Axial rise	0.26	0.34	0.45
Helix pitch	28	34	4.5
Base pair tilt	20	-6	7
Twist angle	33	36	-30
Diameter of helix	2.3	2.0	1.8