Question

If a PCR reaction is working at 100% efficiency, how many copies of the target are produced after 3 cycles, relative to the initial amount?

• A. Double the initial amount
• B. Thrice the initial amount
• C. Four times the initial amount
• D. Eight times the initial amount

Hint:

At 100% efficiency, PCR doubles the target every cycle, so after n cycles the amount is 2 to the power n.

Answer 1

The Correct Option is D

This question checks whether you know that PCR amplification is exponential, not linear, when the reaction runs at 100% efficiency.

1. Double the initial amount: This would be true only after a single cycle ($2^1=2$), not after three cycles, so it undercounts the amplification.
2. Thrice the initial amount: This treats the growth as additive (adding one starting amount per cycle), but PCR growth is multiplicative, so this option is wrong.
3. Four times the initial amount: This is the result after two cycles ($2^2=4$), one cycle short of what the question asks.
4. Eight times the initial amount: Each cycle at 100% efficiency doubles whatever DNA is present. Starting at 1 unit, cycle 1 gives 2, cycle 2 gives 4, and cycle 3 gives 8, matching $2^3=8$.

Since DNA doubles every cycle, three cycles multiply the starting amount by $2\times2\times2=8$, so the amount after 3 cycles is eight times the initial amount.

Let's summarize:

• 100% efficient PCR doubles the target with every single cycle.
• After n cycles the multiplication factor is $2^n$.
• For 3 cycles, $2^3=8$, so the DNA is eight times the starting amount.

After 3 cycles at 100% efficiency, the amount is eight times the initial amount.