Problem: Cas9 can likely bind (but not cleave) target sites that contain only an 11 nt match to the 3' end of the spacer sequence. A 5'-NGG-3' PAM is also required for binding.
a. For a given spacer, on average, how many off-target binding sites are present in a 3 Mb bacterial genome? Assume that all bases occur in this genome at an equal frequency.
b. How many off-target sites are present in a 3 Gb human genome? Again, assume equal frequencies of all bases.