How many cycles in a PCR will be required for amplifying the genetic…

The correct answer is Option (3) → 30 cycles

In PCR each cycle of denaturation, annealing and extension doubles the number of copies of the DNA segment. After n cycles, one copy becomes 2ⁿ copies.

• 2¹⁰ = 1024, about a thousand

• 2³⁰ = 2¹⁰ × 2¹⁰ × 2¹⁰ ≈ 1000 × 1000 × 1000

• = about one billion (10⁹)

So about 30 cycles amplify the DNA one billion times, as the book’s PCR figure shows. 10 cycles give only about a thousand copies, 15 about 33,000, and 35 cycles about 34 billion.

NCERT Extract:
Amplification of Gene of Interest Using PCR – “… PCR stands for Polymerase Chain Reaction. In this reaction, multiple copies of the gene (or DNA) of interest is synthesised in vitro using two sets of primers (small chemically synthesised oligonucleotides that are complementary to the regions of DNA) and the enzyme DNA polymerase. The enzyme extends the primers using the nucleotides provided in the reaction and the genomic DNA as template. If the process of replication of DNA is repeated many times, the segment of DNA can be amplified to approximately billion times, i.e., 1 billion copies are made.”