You have isolated a 10,500 bp plasmid from bacteria. This plasmid is a closed ci

Question

You have isolated a 10,500 bp plasmid from bacteria. This plasmid is a closed circular double stranded DNA molecule characterized by a superhelical density = -0.05. The superhelical density is defined by the following formula: = Lk/Lk0.

1) Calculate the twist number LK0 for this plasmid assuming that there are 10.5 bp per turn of the helix.

2) The negative supercoiling of the isolated plasmid was due to the action of topoisomerases in the bacteria. Note that, in the bacteria, the topoisomerases added negative supercoils to the relaxed plasmid without changing the twist number .

How many negative supercoils does the plasmid have?

Hint: use the formula describing the relationships between Lk, Tw, Lk0 and LK.

Explanation / Answer

1. LKO for this plasmid assuming that there are 10.5 BP per turn of the helix is 1000

This has been calculated using the fomula LKO = BP / 10.5

2. Negatively supercoiling:

T = 0, W = 0, then L = 0

T = -3, W = 0, then L = -3

T = -2, W = -1, then L = -3

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