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.
Annealing in PCR means -
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Addition of primer to single strand of DNA
PCR makes many copies of a chosen piece of DNA in vitro. Each cycle has three steps: denaturation, primer annealing and extension.
(1) Separation of DNA strands — Incorrect. This is denaturation. High temperature separates the two strands of the DNA.
(2) DNA polymerase adds nucleotides — Incorrect. This is extension. The thermostable DNA polymerase extends the primers using the nucleotides.
(3) Addition of primer to single strand of DNA — Correct. In annealing, the mixture is cooled, and the primers bind to the complementary regions of the single-stranded DNA.
(4) Amplification of DNA — Incorrect. Amplification is the result of repeating the whole cycle many times, not one step of it.
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.
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.”
