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

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

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

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

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

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

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

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.

The origin of the major and minor grooves

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

Three conformations of DNA

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