Which one of the following steps is not involved in the PCR?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Transformation
The polymerase chain reaction (PCR) makes many copies of a chosen piece of DNA in a test tube. Each cycle has three steps, and the cycle is repeated many times.
(1) Denaturation — A step of PCR. The DNA is heated to a high temperature so that its two strands separate.
(2) Transformation — Not a step of PCR. Transformation is the process by which a piece of DNA is put into a host bacterium. It happens after the DNA is ready, in a living cell, not in the PCR tube.
(3) Annealing — A step of PCR. The temperature is lowered so that the two primers (short pieces of DNA) join to their matching places on the single strands.
(4) Extension — A step of PCR. A heat-stable DNA polymerase (such as the one from Thermus aquaticus) adds nucleotides to the primers and makes new strands.
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.”
Cloning Vectors – “… Transformation is a procedure through which a piece of DNA is introduced in a host bacterium (you will study the process in subsequent section).”
Figure 9.6 Polymerase chain reaction (PCR) : Each cycle has three steps: (i) Denaturation; (ii) Primer annealing; and (iii) Extension of primers