Question

In a negatively supercoiled circular plasmid (covalently closed), the transition of a small region of B-DNA to Z-DNA can be induced. If this region involves a 12 base pair alternating d(CG) sequence in the closed plasmid, determine changes to W, T, and L after the induced B-to-Z transition. (Remember: ~10 base pairs per right-handed turn for B-DNA and -12 base pairs per left-handed turn for Z-DNA, i.e. Z-DNA twists in the opposite direction).

Answer 1

For Covalently closed circular DNA, Writhe (W) = 0

Linking number (L) is equal to the sum of Twist (T) and writhe (W) of DNA

L= T + W----------------------------------------------------1

Since writhe (W) = 0

Substituting the value of writhe (W) in equation 1, we get

L= T + W

L= T + 0

L=T

Therefore Linking number (L) is equal to the twist (T)

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Twist changes of Z-DNA (T) = Number of base pairs/No. of base pairs per turn

Number of base pairs = 12 base pair sequence

No. of base pairs per turn= -12 base pairs for Z-DNA

Therefore

Twist changes of Z-DNA (T) = 12/-12

Twist changes of Z-DNA (T) = -1

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Since

Linking number (L) is equal to the twist (T) for Covalently closed circular DNA

L=T

Therefore

Linking number (L)= -1