DNA polymerase

Think of DNA polymerase as the diligent scribe of replication: it reads along an existing DNA strand and adds matching letters one at a time to build the new strand. It is the enzyme that actually does the copying, choosing each incoming nucleotide so it base-pairs correctly with the template.

It has two strict habits. First, it can only extend an existing strand, never start one from scratch, so it needs a short starter sequence called a primer to begin. Second, it builds in only one direction (adding new nucleotides to the 3′ end), which is why one strand is copied smoothly and the other in short fragments.

Many DNA polymerases also proofread: if they add the wrong nucleotide, they can back up, snip it out, and try again. This built-in error correction is a major reason replication is so accurate. The same enzyme family is harnessed in the lab — for example, a heat-stable DNA polymerase is what makes PCR possible.