Polymerase Chain Reaction (PCR): A single PCR cycle consists of which correctly ordered steps?

Introduction / Context:PCR exponentially amplifies a specific DNA region through repeated thermal cycles. Knowing the names and order of steps is essential for troubleshooting and optimization in any molecular biology lab.

Given Data / Assumptions:

• Template DNA, primers, thermostable DNA polymerase, and dNTPs are present.
• Thermal cycler controls temperature precisely.
• We consider the canonical three-step PCR.

Concept / Approach:Each cycle has: (1) Denaturation to separate strands; (2) Primer annealing to allow primers to bind complementary sequences; (3) Extension (elongation) where DNA polymerase synthesizes new DNA from the primers. Cycling repeats 25–35 times, doubling target copies per cycle (theoretical maximum).

Step-by-Step Solution:Identify standard order: denaturation → annealing → extension.Relate each temperature to function: high for denaturation, lower for annealing, intermediate/optimal for extension.Match to the option that lists these three in order.Select: denaturation, primer annealing and elongation.

Verification / Alternative check:Protocol sheets for Taq polymerase or high-fidelity enzymes universally use this sequence (with optional two-step PCR when annealing and extension are combined).

Why Other Options Are Wrong:

• “Initiation” and “termination” are not standard PCR step names.
• Annealing cannot precede denaturation in a thermal cycle.

Common Pitfalls:Using an annealing temperature too low (nonspecific products) or too high (reduced yield). Always calculate based on primer Tm and validate empirically.

Final Answer:three steps, denaturation, primer annealing and elongation