Sept 17, 2009
Factors That Promote Z-DNA Formation
1) Sequence of alternating pyrimidines & purines on the same strand
2) High salt concentrations, because the proximity of the phosphate groups in the
helix are close together (creating a negative repulsion) therefore the salt helps to
neutralize this repulsion.
3) Cytosine methylation makes for easier Z-DNA formation so that the genes cannot
4) Negative super-coiling
Size of DNA
-The size of a DNA molecule is measured in the number of base pairs (bp), one
nucleotide residue has an average molecular weight of about 330, and therefore,
one base pair will weigh in at 660.
-Since the average length of a nucleotide residue is about 0.34nm then the length
of a kbp is approximately 340nm (refer to tab. 1.3 Signer & Burk)
-The haploid length of an average mammalian chromosome is approximately 3
billion bps and with an average length of 0.34nm per bp therefore the total length
is around 1m in length.
-The idea that B-DNA is the perfect model for a normal helix is not exactly true.
-The bending of the molecule itself occurs most frequently in long stretches of
A’s. The pro