1. The two chains in the double helix are antiparallel (one goes 5' to 3' whereas the complementary strand goes 3' to 5').
2. Phosphate groups link together the sugar backbone via phosphodiester bonds.
3. The bases on the two chains pair in a complementary fashion. Adenine (A) pairs with thymine (T) and guanine (G) pairs with cytosine (C).
4. Hydrogen bonds between bases (3 for G-C, 2 for A-T) hold the double helix together.
5. There are ten bases per turn of the helix,
so the helix rotates 36
per base.
6. The rise of the double helix is the distance parallel to the axis of the helix from the level of one base to the level of the adjacent base. The pitch in B-DNA is 3.4 nm because the rise is 0.34 nm and there are ten base pairs per turn of the helix.
7. B-DNA possesses a major and a minor groove, as shown in Figure 4.15b.