Which of the following statements about the polymerase chain reaction…

The correct answer is Option (4) → Each cycle of PCR has three steps: denaturation, primer annealing and ligation.

The polymerase chain reaction (PCR) makes many copies of a gene or piece of DNA in vitro. Each cycle has three steps: denaturation (heat separates the two DNA strands), primer annealing (primers bind to the single strands) and extension of primers (DNA polymerase lengthens the primers using the nucleotides supplied).

Option (1) In PCR, multiple copies of the gene (or DNA) of interest are synthesised in vitro. — A correct statement. PCR copies the gene or DNA of interest outside the cell (in vitro), in a tube.

Option (2) PCR uses two sets of primers and the enzyme DNA polymerase. — A correct statement. PCR needs two sets of primers and the enzyme DNA polymerase (a thermostable one, from Thermus aquaticus).

Option (3) The primers are small, chemically synthesised oligonucleotides complementary to regions of the DNA. — A correct statement. Primers are small, chemically made oligonucleotides. They match the two ends of the region to be copied.

Option (4) Each cycle of PCR has three steps: denaturation, primer annealing and ligation. — An incorrect statement. The third step is extension of primers by DNA polymerase, not ligation. Ligation means joining DNA pieces with DNA ligase, which plays no part in a PCR cycle.

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.”

Figure 9.6 Polymerase chain reaction (PCR): Each cycle has three steps: (i) Denaturation; (ii) Primer annealing; and (iii) Extension of primers