What are PCR primers?

PCR primers are custom designed single-stranded DNA oligos with around 20 nucleotides in length. They are the starting point of copying or amplifying in PCR. Two primers are used in each PCR reaction to flank the target region of copying. They bind to the opposite strands of the template DNA and provide a ‘free’ 3’-OH group to which the DNA polymerase can add dNTPs.