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

Two grooves of equal size are created by a regular helical staircase

Why is there a major and a minor groove in DNA?

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 linkedSupercoils are generated in eukaryotic linear chromosomes by constraining the

free ends by attachment to proteins or to the nuclear membrane