Question

Order the steps in one cycle of a PCR reaction, putting the first step at the top.

a) Annealing - Denaturation - Extension
b) Extension - Denaturation - Annealing
c) Denaturation - Annealing - Extension
d) Denaturation - Extension - Annealing

Answer 1

The correct ordering is c) Denaturation - Annealing - Extension. In the PCR process, DNA is first denatured, then primers anneal to single strands, and finally, new DNA strands are synthesized during the extension step.

Step-by-step explanation:

The correct order of the steps in one cycle of a PCR reaction, with the first step at the top, is:

1. Denaturation
2. Annealing
3. Extension

Therefore, the correct option is c) Denaturation - Annealing - Extension. Here is a brief explanation of each step:

• Denaturation: This is the first step where the double-stranded DNA is heated to around 95 °C, causing the strands to separate.
• Annealing: In this step, the temperature is lowered to around 55 °C, allowing primers to bind or anneal to their complementary sequences on the single-stranded DNA.
• Extension: Finally, the temperature is raised to about 72 °C, optimal for Taq polymerase to add nucleotides to the primers, thereby creating new strands of DNA.

Each cycle doubles the number of targeted DNA segments, and multiple cycles result in a significant amplification of the target DNA sequence.