When plasmids are mixed with histones, they will bind fewer histones than the equivalent length of linear DNA. Upon addition of a topoisomerase, more histones will be bound. Explain this observation.

Topoisomerases relax the positive supercoil of the plasmid when histones bind, thereby allowing for more histones to bind.

Explanation:

Plasmids are closed-circular DNA. Unlike linear DNA which can bind a greater number of histone molecules due to greater freedom of rotation, when plasmids binds to a histone molecule, a negative supercoil is introduced in the plasmid. However, since the plasmid is closed and the DNA is not broken, a compensatory positive supercoil in the unbound region of DNA is introduced. This limits further binding of histone molecules to the plasmids.

However, with the introduction of topoisomerases, they relax the positive supercoils, thereby stabilizing the negative supercoil. Further binding of histone molecules are initiated with the formation of negative supercoils and relaxing of positive supercoils result in more histone molecules bindingd to the plasmids.