Lesson eLearning

In a PCR reaction, 10 copies of a 200‑bp DNA fragment are amplified with an efficiency of 70 % per cycle. After 20 cycles, approximately how many copies are present? Round to the nearest whole number.

medium 2 marks Class 12 · Biology · Molecular Basis of Inheritance
Answer
405,700 copies

The correct answer is 405,700 copies — Rounded to the nearest whole number gives 405,700 copies.

Step-by-step solution

  1. Let the efficiency per cycle be (e=0.70).
  2. The number of copies after (n) cycles is (N=N_0(1+e)^n).
  3. Here (N_0=10), (n=20).
  4. Compute the factor ((1+e)^n=(1.70)^{20}).
  5. Calculate ((1.70)=0.2304).
  6. Multiply by 20: (20\times0.2304=4.608).
  7. So ((1.70)^{20}= 4.05\times10^4).
  8. Multiply by the initial copies: (10\times4.05\times10^4approx4.05\times10^5).
  9. Rounded to the nearest whole number gives 405,700 copies.
Common mistake: Using the efficiency factor directly as a multiplier instead of adding 1 to it.

Concept tested: PCR efficiency calculation

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