Question

In nature how is crispr cas 9 system used by bacteria

Answer 1

Bacteria use CRISPR Cas 9 system for immunity.

the virus which infects bacteria are called bacteriophage.

Bacteriophage infects and injects the DNA into the bacteria, viral DNA can either replicate in the cell or can be inserted into the bacterial DNA. the viral DNA replicates in the cell and then viral particles are released from the cell, this leads to the lysis of the bacterial cell.

to prevent the infection with the virus as well as the future infections by the same virus, bacteria use CRISPR Cas 9 system.

during the first time infection, the virus injects the DNA into the bacteria. Cas complex in the bacteria cleaves the viral DNA into smaller fragments, one fragment called spacer is inserted into the CRISPR locus, CRISPR locus contains the spacer sequences from different viruses inserted between repeat sequences.

during the further infection, the CRISPR RNA is transcribed from the CRISPR locus. there is another RNA called Tracr RNA, which is required for the Cas 9 function. Cas 9, Tracr RNA, and RNase III, associates with the CRISPR RNA and cleaves it, so that a complex of the crRNA-tracr RNA-Cas9 complex forms, which can associate to the viral DNA, then Cas9 nuclease cleaves the viral DNA. so the further infection of the bacteriophage is prevented.

Bacteria use the CRISPR Cas9 system for adaptive immunity against bacteriophage.