Molecular Biology of the Gene 7th Edition by Richard Losick, James Watson, Michael Levine, Tamara Baker, Alexander Gann

Exercise 6

You isolate a supercoiled 10,000-bp plasmid from E. coli cells. Assume the plasmid is covalently closed circular DNA (cccDNA). You treat the 10,000-bp plasmid with DNase I under conditions such that the DNase I nicks on average once per DNA molecule.
i. Name the bond that DNase I breaks.
ii. How does this treatment change the topological state of the 10,000-bp plasmid?
B. You then inactivate the DNase I and treat the DNA with DNA ligase plus ATP to create a new population of cccDNA. (DNA ligase is an enzyme that seals the nicks and requires ATP for enzymatic activity.) Briefly describe the topological state of the resulting cccDNAs. Explain your reasoning with respect to Lk (linking number) and Lk 0.