Why do a bacterium need CRISPR system?

Question

[Farhat]

I'm learning about CRISPR at my college.
I understand that when viral DNA is inserted into a bacterial cell, the Cas1-Cas2 proteins identify the PAM site in the viral DNA and then cut the protospacer from the viral DNA to make it a spacer in the CRISPR array for the adaptive immunity of the bacteria. I'm confused about why they need the CRISPR system if the Cas1-Cas2 cuts the protospacer, and at that time only the viral DNA gets inactivated.

Answer 1

[Mark]

Cas1-Cas2 is part of CRISPR.

it's not a separate system. the C in Cas is CRISPR.

[Farhat]

Then what is the need for Cas9 in bacteria?

[GenoMax]

It infers adaptive immunity for the bacterium against invading foreign DNA, think phage/viruses.

[Farhat]

But cas1-cas2 is also doing the same thing right?

[GenoMax]

Those are different types of nucleases. They form part of the CRISPR system. See: https://en.wikipedia.org/wiki/Cas12a

Cas12a is distinguished from Cas9 by a its single RuvC endonuclease active site, its 5' protospacer adjacent motif preference, and for creating sticky rather than blunt ends at the cut site.

[Farhat]

Thanks. Now I understood it