DNA, on the other hand, is stiff stuff. The sugar phosphate backbone in DNA containes deoxyribose, which constrains the molecule to a more linear configuration. Unlike RNA, DNA is happiest in a double stranded form, base-paired with a complementary strand. The two strands run in antiparallel directions (5’ to 3’ vs. 3’ to 5’) and embrace each other in the famous double helix. Why do they call it a double helix? No, it’s not because there are two strands, each a helix. Check out figure 10. Yes, the two strands are wrapped around each other in paired spirals—that’s just helix number one. But the entire molecule  spirals around its long axis—helix number two.

Figure 10. Double Helix. Animation. Strands wind around each other (helix number 1) and the entire molecule winds around its central axis (helix number 2).