What is the linking number in vivo for a plasmid that is 4200 bp in size, if there is normally 1 negative supercoil in every 20 base pairs of a circular DNA? Assume there are 10.5 bp per turn of the double helix.

190.

Explanation:

The linking number may be defined as the sum of the twist and writhes present in the DNA molecule.

Linking number = twist + writhes.

Writhe = Total size / base pairs in circular DNA

Writhes = 4200 / 20 = 210.

Writhes must be - 210 because the DNA is negatively supercoiled.

Twist number = Total size / 10.4

Twist number = 4200 / 10.5 = 400.

Then, linking number = - 210 + 400

Linking number = 190.

Thus, the answer is 190.