proofreading, the end-replication problem, and telomerase

Picture two safeguards for copying an important book. First, a good scribe rereads each line as soon as it is written and corrects any slip on the spot. Second, there is a structural problem at the very end of every page that keeps shaving a little off the margin each time the book is recopied, so the book needs a special way to top up that margin. In DNA copying, the first safeguard is proofreading, and the margin problem — plus its fix — is the end-replication problem and the enzyme telomerase.

Proofreading is built right into DNA polymerase: just after it adds a base, the enzyme checks whether the new base is correctly paired, and if not, it backs up, snips out the wrong base, and replaces it. This single feature improves copying accuracy roughly a hundredfold. The end-replication problem is separate and unavoidable: because the lagging strand is built from primed fragments, the very last primer at the end of a linear chromosome cannot be replaced with DNA after it is removed, so each copy leaves the chromosome ends a little shorter. Telomerase is an enzyme that carries its own short RNA template and uses it to extend the chromosome ends — the telomeres — adding back the repeating sequence that would otherwise be lost.

These ideas matter because together they explain both the fidelity and the limits of replication. Proofreading is why mutations are rare; the end-replication problem is why most of your body's cells, which make little or no telomerase, gradually shorten their telomeres and eventually stop dividing — a built-in counter linked to aging. Telomerase is reactivated in egg and stem cells, and, troublingly, in most cancers, where it helps cells divide without limit. A common misconception is that telomerase 'reverses aging'; in reality it maintains chromosome ends, and switching it on indiscriminately would risk cancer rather than guarantee youth.