Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): In the polymerase chain reaction (PCR), a thermostable DNA polymerase isolated from the bacterium Thermus aquaticus is used.
Reason (R): In PCR, the enzyme DNA polymerase extends the primers using the nucleotides provided in the reaction and the genomic DNA as template.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).
Assertion (A): In the polymerase chain reaction (PCR), a thermostable DNA polymerase isolated from the bacterium Thermus aquaticus is used. — True. A is about which enzyme is used in PCR. The DNA polymerase used is thermostable; it is isolated from the bacterium Thermus aquaticus and stays active at high temperature.
Reason (R): In PCR, the enzyme DNA polymerase extends the primers using the nucleotides provided in the reaction and the genomic DNA as template. — True. R is about what the enzyme does in each cycle. DNA polymerase extends the primers, using the nucleotides added to the reaction and the genomic DNA as template.
Why R does not explain A? Every DNA polymerase extends primers on a template, so R does not tell why a heat-stable polymerase is needed. The real reason is that each PCR cycle starts with denaturation at a high temperature, and the cycle is repeated many times; an ordinary DNA polymerase would lose its activity at that temperature, while the enzyme from Thermus aquaticus remains active. So both statements are correct, but R is not the explanation of A.
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. Such repeated amplification is achieved by the use of a thermostable DNA polymerase (isolated from a bacterium, Thermus aquaticus), which remain active during the high temperature induced denaturation of double stranded DNA.”