Genetics: Analysis and Principles 4th Edition by Robert Brooker

Exercise 1

Two circular DNA molecules, which we will call molecule A and molecule B, are topoisomers of each other. When viewed under the electron microscope, molecule A appears more compact than molecule B. The level of gene transcription is much lower for molecule A. Which of the following three possibilities could account for these observations
First possibility: Molecule A has three positive supercoils, and molecule B has three negative supercoils.
Second possibility: Molecule A has four positive supercoils, and molecule B has one negative supercoil.
Third possibility: Molecule A has zero supercoils, and molecule B has three negative supercoils.