Primer (molecular biology)

A primer is a nucleic acid strand, or a related molecule that serves as a starting point for DNA replication. A primer is required because most DNA polymerases, enzymes that catalyze the replication of DNA, cannot begin synthesizing a new DNA strand from scratch, but can only add to an existing strand of nucleotides.

In most natural DNA replication, the ultimate primer for DNA synthesis is a short strand of RNA. This RNA is produced by primase, and is later removed and replaced with DNA by a DNA polymerase.

Many laboratory techniques of biochemistry and molecular biology that involve DNA polymerases, such as DNA sequencing and polymerase chain reaction (PCR), require primers. The primers used for these techniques are usually short, chemically synthesized DNA molecules with a length about twenty bases.