Practice question

Cas9 induced double strand break activates cellular repair pathways. Homology directed repair uses donor template for precise correction but operates mainly in S phase. Predominant pathway in mammalian somatic cells is non-homologous end joining which directly ligates broken ends without template, often introducing small errors. Ku proteins and DNA ligase IV mediated NHEJ results in insertion or deletion of few nucleotides at cut site, causing frameshift or premature stop codon. This error-prone repair underlies knockout generation by CRISPR through disruptive indels.