primer
DNA polymerase, the enzyme that builds new DNA strands, cannot start from scratch on a bare template — it can only extend an existing strand. A primer is the short starter sequence that gives it that foothold: a brief piece of nucleic acid that base-pairs with the template and provides the free end from which synthesis begins.
In PCR, primers are short, synthetic, single strands of DNA, usually around twenty bases long, designed to be complementary to the sequences flanking the target region. Because the polymerase will only copy DNA downstream of where a primer binds, the choice of primers is what makes PCR specific: they define exactly which stretch of DNA gets amplified. A typical reaction uses a pair, one for each strand, framing the segment between them.
Primers are not only a laboratory tool. During normal DNA replication, cells make short RNA primers to start each new strand, which are later removed and replaced with DNA. In the lab, the ability to design and synthesize a primer to match any sequence is what gives PCR, sequencing, and many other methods their precise, programmable targeting.
To amplify one gene out of an entire genome, a scientist designs a pair of primers that bind only at that gene's borders, so PCR copies that gene and nothing else.
Well-chosen primers turn a whole genome into a single target.
PCR primers are made of DNA, whereas the primers a cell uses during replication are made of RNA and laid down by a specialized enzyme called primase before being replaced with DNA.