Suppose you had a circular B-DNA double helix that contained a 96-bp section with alternating
purine-pyrimidine bases. How would delta(L), delta(T) and delta(W) change if you moved this DNA to a high-saltsolution? (delta = change in)
The Answer:
Need changes, not absolute values. Backbone not broken, so L = 0.
96-bp section changes from B-DNA (RH, 10 bp/turn) to Z-DNA (LH, 12 bp/turn).
delta(T) = –96/10 + 96/(–12) = –9.6 – 8 = –17.6
Because delta(L) = 0, delta(W) = –delta(T) so delta(W) = +17.6
I don't understand the value obtained by the change in twist. I understand that twist is the number of base pairs divided by the relaxed/favoured # of twist per turn. However, the calculation doesn't make sense to me as to why the change in twist are negative.
Any help would be appreciated.