Question

After how many cycles of polymerase chain reaction (PCR), the amplified product of required length is generated for the first time from a chromosomal DNA template?

• A. 2
• B. 3
• C. 4
• D. 5

Hint:

In PCR, the exponential amplification of target-sized fragments begins from the 3rd cycle onward.

Answer 1

The Correct Option is B

Step 1: Understand PCR mechanism.
PCR consists of three steps: denaturation, annealing, and extension. In the initial cycles, the DNA products are longer than the desired target.
Step 2: Cycle analysis.
- Cycle 1: Produces fragments longer than the target sequence (overhangs remain).
- Cycle 2: Still produces long DNA strands with one end defined.
- Cycle 3: For the first time, double-stranded DNA molecules of the exact required length are generated.
Step 3: Conclusion.
Thus, the required-length product first appears after 3 cycles.
Final Answer: \[ \boxed{\text{After 3 cycles of PCR, the required-length product appears for the first time.}} \]