double-strand break repair

Of all the injuries DNA can suffer, the most dangerous is a double-strand break — a clean cut straight through both strands of the helix, severing the chromosome into two loose pieces. Most damage spares one strand to act as a template, so the cell can simply rebuild the broken strand by reading its partner. A double-strand break removes that safety net: both copies of the information at that spot are gone at once.

The danger is twofold. First, the cell may fail to find both ends again and lose a whole chromosome arm, or it may glue the wrong ends together and fuse chromosomes that should be separate — the kind of chaos that drives cancer. Double-strand breaks come from ionising radiation, certain chemicals, collapsed replication forks, and, in a controlled and deliberate way, from the cell itself during processes like meiotic recombination and immune-gene shuffling. Because an unrepaired break can be lethal and a misrepaired one can be cancerous, cells treat these breaks as top-priority emergencies.

There are two main routes to fix a double-strand break, and they embody a trade-off between speed and accuracy. Non-homologous end joining simply trims and rejoins the two broken ends directly — fast, available at any time, but error-prone, often losing or adding a few bases at the seam. Homologous recombination is slower but accurate: it uses an intact identical copy of the region, usually the sister chromatid present after replication, as a template to rebuild the break perfectly. Which route a cell chooses depends largely on the stage of the cell cycle and whether a sister copy is at hand.