Chapter 6, Problem 40P

Cre is a recombinase enzyme encoded by a gene in bacteriophage P₁. The Cre enzyme promotes site-specific recombination between two copies of a 34 bp long DNA sequence called loxP that is derived from the same bacteriophage.

Researchers use the Cre/loxP site-specific recombination system in order to make mice homozygous for a deletion of any particular gene and only in a specific tissue. You will see in Chapter 18 that scientists first make mice where a pair of loxP sites are configured in a particular way with respect to the gene to be deleted.

a.	Draw a diagram that shows the configuration of the loxP sites that would enable deletion of a gene by site-specific recombination.
b.	What else would the researchers need to introduce into the mouse genome in order to generate mice where only one tissue is homozygous for the gene deletion?
c.	Why do you think that scientists would want to generate mice like this?
d.	Unlike the DNA rearrangements at attP and attB sites catalyzed by ΦC31 integrase (described in Problem 39), the DNA rearrangements caused by Cre recombinase are reversible. Why?
e.	Why does the reversibility of Cre/loxP-mediated recombination not interfere with the use of the Cre/loxP system to generate mouse tissues with deletions?