Most mammalian cell types do not undergo homologousrecombination (HR), and even mouse ES cells do so at low frequency- about 1/200000 cells. However, if you first introduce a crRNAdesigned to target a mouse genomic sequence next to a PAM site,plus a source of Cas9 protein and a plasmid with homology arms,most mouse cell types will undergo HRwith very high frequency -as high as 1/5 or more of the treatedcells. What does this tell you about the mechanism of HR inmammalian cells?
A) The critical first step in HR is introduction of a double strandbreak at the target gene site.
B) Mouse ES cells have DNA repair systems that are defectivecompared to other cell types.
C) Cas9 protein directly activates expression of HR system genes inmammalian cells.
D) Mouse ES cells contain a larger number of PAM sites than mostcell types.
E) Mouse ES cells already express higher levels of Cas9 than mostcell types.
