 Both the Sanger chain termination method and the “Next-Gen” method of DNA sequencing require primers to initiate the reaction.The polymerase chain reaction (PCR) uses a pair of custom primers to direct DNA elongation toward each-other at opposite ends of the sequence being amplified.These primers are typically between 18 and 24 bases in length, and must code for only the specific upstream and downstream sites of the sequence being amplified.