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.
Primers in PCR means -
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Chemically synthesised oligonucleotides
Primers are short, chemically synthesised pieces of DNA, that is, oligonucleotides, which are complementary to the regions of DNA at the two ends of the gene. DNA polymerase cannot start a new strand on its own; it can only extend a primer.
(1) Chemically synthesised oligopeptides — Incorrect. Oligopeptides are short chains of amino acids, that is, small proteins. They cannot pair with DNA.
(2) Chemically synthesised oligonucleotides — Correct. A primer is a short chain of nucleotides made in the laboratory.
(3) Biologically synthesised oligonucleotides — Incorrect. The primers used in PCR are made chemically, not by living cells.
(4) Chemically synthesised oligosaccharides — Incorrect. Oligosaccharides are short chains of sugars.
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.”
Figure 9.6 Polymerase chain reaction (PCR) : Each cycle has three steps: (i) Denaturation; (ii) Primer annealing; and (iii) Extension of primers