4. The transition from B-DNA to Z-DNA involves converting right-handed twist to left-handed twist; that is, unwinding the helix (the sign of unwinding is negative). Since no phosphodiester bonds are broken (no topoisomerases), the linking number cannot change. Since twist will change, there must be an equal, but opposite, change in writhe. The change in twist per base pair = Z twist - B twist. Z twist = -1/12 and B twist = +1/10.5, so the change in twist per base pair = -1/12 - 1/10.5 = - 0.0833 - 0.0952 = -.1785. Since 12 B-form base pairs will change to 12 Z-form base pairs, the total change in twist = 12 (-0.1785) = - 2.142. The change in writhe = - change in twist = - (- 2.142) = +2.142. This means that negative supercoiling is reduced (that is, the negatively supercoiled molecule is relaxed).