double-strand break

A double-strand break is a clean cut straight through both rails of the DNA ladder, severing the chromosome into two free ends. It is one of the most dangerous events a cell can face — like snapping a rope that was holding something up — so cells respond to it urgently.

Because both strands of the double helix are cut at the same place, the cell cannot simply copy the intact strand to repair it. Instead it triggers one of two main repair pathways: non-homologous end joining, which quickly glues the ends back but often adds or deletes a few bases, or homology-directed repair, which copies from a matching template to restore the sequence precisely.

Genome editing exploits this. Tools like Cas9 deliberately create a double-strand break at a chosen site to provoke repair; whether the cell uses the sloppy or the precise pathway determines whether you get a gene knockout or a clean, templated edit. The break itself is just the trigger — the repair choice does the real work.