superhelical DNA from cells and virions. The energy of supercoilingis a quadratic function of the density
of supercoils as described by the equation
where Ris the gas constant, Tthe absolute temperature and Nthe number of base pairs.
B–Z transitionsare especially important for supercoiling since the conversion of one right-handed B-
turn into a left-handed Z-turn causes a change in Twof –2. This must be complemented by Wr 2
through the formation of one left-handed superturn.
G^RT
N
s^1050 Lk^21 kJ molDNA and RNA Structure 47
Figure 2.30 Supercoil formation in closed circular DNA. (a) Closed circle of 20 duplex turns (alternate turns
in colour). (b) Circle nicked, under-wound two turns, and re-sealed. (c) Base pairing and stacking
forces result in the formation of B-helix with two new right-handed helix turns and one compensating
right-handed supercoil
Figure 2.29 Sedimentation velocity for SV40 DNA as a function of bound ethidium (a) for closed circular DNA
(––––) and (b) for nicked circular DNA (––––) showing the transition from a negative
supercoil (left) through a relaxed circle (centre) to a positive supercoil (right)
(Adapted from W. Bauer and J. Vinograd,J. Mol. Biol., 1968, 33 , 141. © (1968), with permission from
Elsevier)