Q. 41 A polymerase reaction is carried out for 10 cycles in a volume of 1 ml with 5 molecules of template
DNA. Assuming that the efficiency of the reaction is 𝟏𝟎𝟎%, the number of molecules of DNA present in
𝟏𝟎𝟎𝝁𝐥 at the end of the reaction is ____ (correct to integer number).

PCR reactions exponentially amplify DNA templates under ideal conditions. With 100% efficiency over 10 cycles and 5 initial molecules, the final count in 100 µl follows precise math.​

PCR Amplification Formula

Each cycle doubles the DNA molecules at 100% efficiency, so the total after $n$ cycles is initial molecules times 2n2n. Here, 5 templates undergo 10 cycles: 5×210=5×1024=51205×210=5×1024=5120 molecules in 1 ml. The 1 ml volume holds all amplified product before aliquoting.​

Volume Adjustment

The query specifies molecules in 100 µl, or 0.1 ml, from the 1 ml total. Proportion: $5120\times (0.1/1)=512$ molecules. This integer matches exam expectations for perfect doubling without losses.​

Correct Answer

The number of DNA molecules in 100 µl at reaction end is 512.​

Common Misconceptions

• Wrong: 5120 – Total in 1 ml, ignores 100 µl aliquot.​

• Wrong: 1024 – Assumes 1 template, not 5.​

• Wrong: 500 – Forgets exponential growth, assumes linear.​
  Amplification assumes no primer limits or inhibition, valid for 100% efficiency. Real PCR may vary slightly beyond early cycles.​