Practice question

Precision of genome editing methods varies drastically correlating with targeting mechanism. Random mutagenesis approaches such as TILLING based on EMS alkylation of guanine causing G to A transitions genome-wide at ~1 per 150 kb requires screening massive populations via CEL1 nuclease assay and mapping; transposon mutagenesis with Ac/Ds or Mutator inserts at TA dinucleotides semi-randomly within genes, excision leaves footprint and instability; T-DNA insertion via Agrobacterium integrates preferentially into euchromatic gene rich regions at 5'-UTRs causing large deletions and chromosomal rearrangements. All three generate unpredictable background mutations complicating breeding. CRISPR/Cas9 provides defined precision at predetermined site because single guide RNA 20 nt dictates cleavage 3 bp upstream of PAM NGG via Watson-Crick base pairing, enabling base editors fusing deactivated Cas9 to deaminases for A to G or C to T changes without DSB, prime editors for insertions. Multiplexing with several guides edits multiple loci simultaneously without crossing. Whole genome sequencing confirms minimal off-targets when high-fidelity Cas9 variants like SpCas9-HF1 employed. This site-specific precision revolutionized crop trait stacking and therapeutic editing for sickle cell disease correcting point mutation.