U3 AOS1 Topic 8: CRISPR Cas9

CRISPR is a sequence of DNA which is found in bacteria to help them fight against viruses. This sequence is transcribed by bacteria to form guide RNA (gRNA).

Cas9 is an endonuclease enzyme which is coded by Cas9 gene in bacteria. It helps in defense system of bacteria by chopping the viral genome.

CRISPR cas9 is a complex of cas9 and guide RNA used to cut targeted viral DNA sequence.

CRISPR cas9 defense system:

Bacteria are infected by viruses called as bacteriophages. Bacteria cope with these viruses by using a defense system called as CRISPR cas9 which works by following three steps:

1.     Exposure: Bacteriophage virus injects its DNA into a bacterium which is recognized and cut at a specific sequence by enzymes cas1 and cas2. The sequence which is cut is typically 30 nucleotides long and is called as protospacer. Protospacer gets incorporated into bacterium CRISPR and becomes spacer. CRISPR also known as clustered regularly interspaced palindromic repeat gets this name as bacterium palindromic repeats are regularly interrupted by spacer sequences from virus. This sequence allows the bacterium to recognize any viral DNA when invaded by viruses in future.

2.     Expression: CRISPR sequence is transcribed into guide RNA which is complementary to viral DNA. gRNA forms a complex with cas9 protein and guides the complex toward any viral DNA which is complementary to gRNA.

3.     Extermination: when CRISPR cas9 complex encounters viral DNA, guide RNA hybridize with viral DNA and cas9 cuts both strands of viral DNA and forms blunt ends.