Cellular and Molecular Biology Topics
Virtually all double stranded DNA in cells is in the B
form: a right-handed helix, a very regular molecule about the same width all the
way around. The base pairs are roughly perpendicular to the axis of the helix,
packed very tightly together. G-C and A-T base pairs take up essentially the
same amount of space. The sugar phosphate backbone wraps around the outside
of the molecule. This forms major and minor groves in the helical structure,
where base pairs are exposed. Proteins recognize double-stranded DNA by reaching
into specific base sequences inside the grooves, particularly major groves.
RNA cannot exist in the B form because the bulky hydroxyl
groups of ribose prevent the backbone from taking a straight conformation.
Instead it takes an A form, a shorter and wider cylinder were the bases are not
perpendicular to the molecular axis. Groves are much shallower than in the B form.
RNA-DNA hybrids and double-stranded RNA usually exist in the A form.
Double stranded DNA may exist in the A form under special conditions.
DNA can exist in a rare Z form: a left handed helix longer
and thinner than the B form, with a zigzag backbone due to a type of
purine-pyrimidine sequence.
Continue to "DNA Melting
and Renaturing" or take a
quiz: [Q1]
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