When CRISPR-Cas9 can't save the cell Whenever a phage infection occurs, the CRISPR array gets transcribed. If the bacterium has encountered that specific phage before (or its ancestor cells had), a spacer corresponding to that phage exists in the CRISPR array. Therefore, crRNA targeting the phage DNA will be produced during transcription, and the CrRNA-tracrRNA-Cas machinery will carry out the interference phase of the immune response. If a spacer for that phage doesn't already exist in the CRISPR array, the cell may still fight it off, just more slowly as acquisition has to occur. But bacteria are not always able to fight off phage infections via the CRISPR-Cas immune system. Viruses frequently acquire mutations, which can pose new challenges to a bacterial population. In this exercise, you'll evaluate different scenarios and determine whether the CRISPR-Cas system could effectively fight the phage infection in each one. Sort each scenario according to whether the phage will be able to infect the bacterial cell or whether the bacterial cell will be able to resist phage infection.
a. The phage has undergone a mutation far away from a PAM sequence since the bacterium encountered it last.
b. The phage attacks a bacterial cell whose parent cell encountered the phage before it divided.
c. The phage has undergone a mutation near a PAM sequence since the bacterium encountered it last.
d. The phage's DNA has been engineered to remove all its PAM sequence.
1. PAM Phage could potentially infect bacterium
2. Bacterium could potentially destroy phage DNA