CRISPR-Cas9 is short for clustered regularly interspaced short palindromic repeats and CRISPR-associated protein 9. CRISPR-Cas9 is adapted from a naturally occurring genome editing system in bacteria. The bacteria capture snippets of DNA from invading viruses or plasmids and use them to create DNA segments known as CRISPR arrays. The CRISPR arrays allow the bacteria to "remember" the viruses (or closely related ones). If the viruses attack again, the bacteria produce RNA segments from the CRISPR arrays to target the viruses' DNA. The bacteria then use Cas9 or a similar enzyme to cut the DNA apart, which disables the virus.
The CRISPR-Cas9 type II system consists of the Cas9 nuclease and a single guide RNA (sgRNA or gRNA), which is a fusion of a CRISPR RNA (crRNA) and a trans-activating crRNA (tracrRNA) that binds Cas9 nuclease and directs it to a target sequence based on a complementary base-pairing rule. The target sequence must be adjacent to a protospacer-adjacent motif (PAM) consisting of a canonical NGG or NAG sequence. At the recognition site, a double-strand break (DSB) is generated that can be repaired by non-homologous end joining (NHEJ), resulting in small insertions or deletions (indels) usually associated with loss of function (knock-down/knock-out).
The genetically engineered CRISPR-cas9 is from Type II CRISPR-Cas systems which is the simplest for genome editing technology among the types of CRISPR systems and is also the most studied. CRISPR-Cas9 enables geneticists and medical researchers to edit parts of the genome by removing, adding or altering sections of the DNA sequence. The CRISPR-Cas9 system has caused a buzz in the scientific community and it is currently the most versatile and precise method of genetic manipulation.
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