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

Step 1: Understanding the Question:
PCR (polymerase chain reaction) copies a target DNA sequence again and again using repeated heating and cooling cycles. At 100% efficiency, every single strand present at the start of a cycle is copied exactly once during that cycle. The question asks how many times the starting amount has multiplied after 3 such cycles.

Step 2: Key Formula or Approach:
At 100% efficiency, the amount of target DNA doubles with every cycle, because each strand present makes one new complementary copy. After \(n\) cycles, the amplification factor is
\[ 2^n \]
This is the standard formula used to describe exponential amplification in PCR.

Step 3: Detailed Explanation:
Start with 1 unit of target DNA before cycling begins.
After cycle 1, the amount doubles to
\[ 2^1 = 2 \]
After cycle 2, the amount doubles again to
\[ 2^2 = 4 \]
After cycle 3, the amount doubles a third time to
\[ 2^3 = 8 \]
So after 3 cycles, the DNA amount is 8 times what it was at the start.

Step 4: Final Answer:
At 100% efficiency, 3 cycles give \(2^3 = 8\) times the initial amount of target DNA.