Primer (molecular biology) A primer is a strand of nucleic acid
that serves as a starting point for DNA synthesis. It is
required for DNA replication because the enzymes that catalyze
this process, DNA polymerases, can only add new nucleotides to
an existing strand of DNA. The polymerase starts replication at
the 3'-end of the primer, and copies the opposite strand. Many
of the laboratory techniques of biochemistry and molecular
biology that involve DNA polymerase, such as DNA sequencing and
the polymerase chain reaction (PCR), require DNA primers. These
primers are usually short, chemically synthesized
oligonucleotides, with a length of about twenty bases. They are
hybridized to a target DNA, which is then copied by the
polymerase. Mechanism in vivo  The lagging strand of DNA is that
strand of the DNA double helix that is orientated in a 5' to 3'
manner. Therefore, its complement must be synthesized in a 3'→5'
manner. Because DNA polymerase III cannot synthesize in the
3'→5' direction, the lagging strand is