Concept:Polymerase Chain Reaction (PCR) is a molecular biology technique that amplifies a specific DNA segment through repeated cycles of controlled temperature changes.
Key Fact:Each PCR cycle consists of three mandatory steps: denaturation (at
94∘C), annealing (at
50−65∘C), and extension (at
72∘C), in that fixed order.
Explanation:The double-stranded DNA template must first be denatured by heating to separate it into two single strands.
This separation exposes the primer binding sites on each strand.
Then, during annealing, the temperature is lowered to allow primers to hybridize specifically to their complementary sequences on the single-stranded DNA.
Finally, during extension, DNA polymerase adds nucleotides to the primers in the
5′ to
3′ direction, synthesizing new complementary strands.
This exact order ensures that each cycle doubles the amount of target DNA, leading to exponential amplification.
Answer:Denaturation → Annealing → Extension (Option D).
