Q.Mention the number of primers required in each cycle of PCR. Write the role of primers and DNA Polymerase in PCR. Name the source organism of the DNA Polymerase used in PCR.

Each PCR cycle uses two primers (forward and reverse) that flank and define the target DNA region, and Taq polymerase — a heat-stable enzyme from Thermus aquaticus — extends them to synthesise new DNA strands.

The Polymerase Chain Reaction (PCR) amplifies a specific segment of DNA in vitro through repeated cycles of denaturation, annealing and extension. In every single cycle, two short synthetic single-stranded DNA primers are used: a forward primer, complementary to one strand at one end of the target sequence, and a reverse primer, complementary to the other strand at the opposite end. During the annealing step, these primers base-pair (hybridise) with the denatured, single-stranded template DNA at their respective complementary sites, flanking the region to be amplified. Because DNA polymerase can only add nucleotides to an existing free 3'-OH end and cannot start synthesis on a completely single strand, the primers are essential — they provide this starting point and precisely determine which stretch of DNA will be copied in each cycle. The DNA polymerase used in PCR must remain active despite the repeated ~95 degC denaturation steps that would destroy an ordinary enzyme; this heat-stable ('thermostable') polymerase is Taq polymerase, originally isolated from Thermus …