Question

While performing a PCR, the student forgot to add one of the two primers. The number of single-stranded DNA molecules produced after 25 PCR cycles is ______.

Hint:

PCR with one primer gives linear amplification; with two primers → exponential amplification ($2^n$).

Answer 1

Correct Answer: 25

Step 1: Recall the role of primers.
In PCR, two primers (forward and reverse) are needed for exponential amplification of double-stranded DNA.
Step 2: If one primer is missing.
Only one strand will be synthesized in each cycle. The new strand cannot serve as a template for further amplification because the complementary primer is absent.
Step 3: Amplification pattern.
The number of synthesized single strands increases linearly with the number of cycles: \[ \text{After } n \text{ cycles: } (n + 1)~\text{single-stranded products.} \] Step 4: Substitute.
\[ n = 25 \Rightarrow 25 + 1 = 26 \] Step 5: Conclusion.
Hence, total number of single-stranded DNA molecules after 25 cycles = 26.