Read the following passage carefully and answer the given questions.
Polymerase Chain Reaction
PCR stands for Polymerase Chain Reaction. In this reaction, multiple copies of the gene of interest is synthesised in vitro using two sets of primers 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. Such repeated amplification is achieved by the use of a thermostable DNA polymerase, which remain active during the high temperature induced denaturation of double stranded DNA. The amplified fragment if desired can now be used to ligate with a vector for further cloning.
In PCR, after 30 cycles, a DNA molecule will be amplified approximately -
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 1 billion times
In PCR, each cycle doubles the number of DNA molecules. After n cycles, one molecule becomes 2n molecules.
\( \displaystyle 2^{30} = 2^{10} \times 2^{10} \times 2^{10} = 1024 \times 1024 \times 1024 \approx 1.07 \times 10^{9} \)
That is about one billion (109) copies. The passage says the same: the DNA can be amplified to approximately billion times, i.e., 1 billion copies are made.
(1) 1 trillion times — Incorrect. A trillion is 1012, about a thousand times too many; it would need about 40 cycles.
(2) 1 million times — Incorrect. A million (106) copies are reached after only about 20 cycles.
(3) 1 billion times — Correct. Thirty doublings give 230, which is about 109, so the DNA segment is amplified about a billion times.
(4) 3 million times — Incorrect. This is far below 230.
Note: This passage is taken from NCERT as it is. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.