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.

Step 1. Count the primers needed: PCR amplifies a double-stranded target, and since the two strands run in opposite (antiparallel) directions, two different short primers are required in every cycle — one complementary to each of the two separated single strands, flanking the target region from either side. Step 2. Explain the role of the primers: after the double-stranded DNA is heat-denatured into single strands, each primer anneals (base-pairs) to its complementary sequence on one strand, and crucially provides the free 3'-OH end that DNA polymerase needs before it can begin adding nucleotides — polymerase cannot start synthesis on a bare template without this primer in place. Step 3. Explain the role of DNA polymerase: once annealed, Taq DNA polymerase extends each primer along its template strand, reading the template and adding complementary nucleotides to synthesise a new DNA strand, so that by the end of each cycle both original strands have been copied into new double-stranded DNA. Step 4. Name the source organism: the DNA polymerase used in PCR is Taq polymerase, iso …

Work through PCR's mechanism strand by strand to justify the primer count, then state each compone …

• Stating only one primer is needed, forgetting that both antiparallel strands must each be primed separately. …