How Does CRISPR Gene Editing Work?

CRISPR-Cas9 gene editing works in three steps: (1) A guide RNA (gRNA) — a short RNA sequence of about 20 nucleotides — is designed to match the target DNA sequence. (2) The gRNA forms a complex with the Cas9 protein (a molecular "scissors" originally from Streptococcus bacteria) and guides it to the corresponding location in the genome by base-pairing with the target strand. (3) Cas9 makes a double-strand break in the DNA at that precise location. The cell then repairs the break through one of two mechanisms: non-homologous end joining (NHEJ), which often introduces small insertions or deletions that disrupt the gene ("gene knockout"), or homology-directed repair (HDR), where a provided DNA template is incorporated, allowing precise gene correction or insertion. The system is derived from a natural bacterial immune system: bacteria use CRISPR to store and recognise viral DNA, then use Cas proteins to cut and destroy matching sequences if the virus attacks again. The technology is cheap (a few hundred dollars), fast (days), and works in virtually any organism.