A
primer is a short nucleic acid sequence that provides a starting point for DNA
synthesis. In living organisms, primers are short strands of RNA. A primer must
be synthesized by an enzyme called primase, which is a type of RNA polymerase,
before DNA replication can occur. The synthesis of a primer is necessary
because the enzymes that synthesize DNA, which are called DNA polymerases, can
only attach new DNA nucleotides to an existing strand of nucleotides. The
primer therefore serves to prime and lay a foundation for DNA synthesis. The
primers are removed before DNA replication is complete, and the gaps in the
sequence are filled in with DNA by DNA polymerases. In the laboratory,
scientists can design and synthesize DNA primers with specific sequences that
bind to sequences in a single-stranded DNA molecule. These DNA primers are
commonly used to perform the polymerase chain reaction to copy pieces of DNA or
for DNA sequencing.