Shape

Broadest

Intermediate

Narrowest

Rise per base pair

2.3 Å

3.4 Å

3.8 Å

Helix diameter

~26 Å

~20 Å

~18 Å

Screw sense

Right-handed

Right-handed

Left-handed

Glycosidic bond*

anti

anti

Alternating anti and syn

Base pairs per turn of helix

11

10.4

12

Pitch per turn of helix

25.3 Å

35.4 Å

45.6 Å

Tilt of base pairs from perpendicular to helix axis

19 degrees

1 degree

9 degrees

*Syn and anti refer to the orientation of the N-glycosidic bond between the base and deoxyribose. In the anti orientation, the base extends away from the deoxyribose. In the syn orientation, the base is above the deoxyribose. Pyrimidines can be in anti orientations only, whereas purines can be anti or syn.