Question

During amplification of gene using PCR, Taq polymerase is used between

• A. None of the above
• B. Annealing and amplification
• C. Denaturation and annealing
• D. Annealing and extension

Answer 1

The correct option is D Annealing and extension

Polymerase Chain Reaction
Polymerase chain reaction (PCR) is a technique used for the in vitro amplification of the DNA fragments or gene of interest to create multiple copies.

Steps involved in PCR
The steps involved in PCR are as follows:
a. Denaturation - In this step separation of the two strands of double-stranded DNA is done by application of heat at 94 - 95°C.
b. Annealing - This step involves the hybridization of the primers with specific sequences of each of the strands of the template DNA. Primers are small oligonucleotides up to 18 - 24 nucleotides long synthesised chemically. This process is carried out at 50 - 56°C.
c. Extension or synthesis - In this step, the Taq DNA polymerase synthesizes DNA by adding dNTPs (deoxyribonucleotides) to the free ends of the primers. The extension of DNA is carried out at 72°C. This helps in synthesizing complementary strands.

$Taq$ polymerase
$Taq$ polymerase is a thermostable enzyme obtained from bacterium $Thermusaquaticus$. It is used to extend the length of the primer after annealing the primer at the 3’ ends of the single strands of DNA in the process of PCR. Hence it is used between annealing and extension steps.

Amplification
The formation of multiple copies of the DNA of interest is carried out by multiple cycles of the PCR. This is called amplification.

Final answer
(B) Annealing and extension