In molecular biology, primers are short lengths of single-stranded RNA (or DNA*) that attach to a longer strand of DNA to initiate DNA synthesis. This priming sequence is necessary because the DNA polymerase*s' responsible for DNA replication in living cells can’t synthesize DNA de novo. Polymerases can only extend existing strands of DNA by adding new nucleotide bases to the 3'-OH end. Primers (RNA in living cells and DNA is PCR) provide the first 3'-OH group needed for the synthesis of double-stranded DNA from a single-stranded template.
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