1. Why does DNA form a double helix
Why does DNA form a double helix? What forces are responsible for the structure of DNA?

2. Conditions that “melt” double-stranded DNA
Solvents
Heat, urea, and alkali all break hydrogen bonds
Hydrophobic solvents disrupt stacking interactions

3. The double stranded nature of DNA is due to properties of the base
Glycosidic bond allows free rotation
Phosphate group gives DNA its negative surface charge
Replaced with an H in deoxyribose

4. Five different bases in RNA and DNA
All bases are planar
All bases have edges that are hydrophyllic
All bases have surfaces that are hydrophobic

5. If pairing is complementary, then each basepair is of equal width
Pairing like-with-like caused the backbone to buckle in and out

6. The strands must be antiparallel for the bases to have the geometry necessary for H bonds to form

7. The grooves formed by the sugar phosphate backbones are not spaced symmetrically around the axis of the helix

8. Why is there a major and a minor groove in DNA?
Two grooves of equal size are created by a regular helical staircase
Two grooves of unequal size are created because the bases are asymmetrically attached to the sugar-phosphate backbone.

9. The origin of the major and minor grooves

10. Each basepair exposes a different constellation of chemical groups in the major and minor grooves

11. Three conformations of DNA

13. In vivo, DNA in chromosomes exists in a supercoiled form Supercoils can be induced by either undertwisting or overtwisting
In circular DNA molecules, the ends are covalently linked Supercoils are generated in eukaryotic linear chromosomes by constraining the free ends by attachment to proteins or to the nuclear membrane