Molecular Biology Of The Cell 6th Edition by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter

Exercise 13

Which statements are true? Explain why or why not.
-Cre recombinase is a site-specific enzyme that catalyzes recombination between two LoxP DNA sites. Cre recombinase pairs two LoxP sites in the same orienta- tion, breaks both duplexes at the same point in each LoxP site, and joins the ends with new partners so that each LoxP site is regenerated, as shown schematically in Figure Q5-4 a. Based on this mechanism, predict the arrange- ment of sequences that will be generated by Cre-medi- ated site-specific recombination for each of the two DNAs shown in Figure Q5-4B.
E. coli, where the replication fork travels at 500 nucleotide pairs per second, the DNA ahead of the fork- in the absence of topoisomerase-would have to rotate at nearly 3000 revolutions per minute.