Q.Which of the following steps are catalysed by Taq DNA polymerase in a PCR reaction?

Taq DNA polymerase is a thermostable enzyme isolated from the bacterium Thermus aquaticus, and its role in PCR is highly specific. Understanding what it does—and what it doesn't do—is crucial for grasping how PCR amplifies DNA.

PCR involves three main steps repeated in cycles:

• Denaturation (a high-temperature step): The double-stranded DNA template separates into single strands. This is purely a physical process driven by high temperature; no enzyme catalyzes it.

• Annealing (a lower-temperature step): Primers bind to their complementary sequences on the single-stranded template. This is also a spontaneous process based on base-pairing; again, no enzyme is required.

• Extension (Taq DNA polymerase's optimal working temperature): Taq DNA polymerase adds nucleotides to the 3' end of each primer, synthesizing new DNA strands complementary to the template. This is the only step that requires enzymatic activity. …

The polymerase chain reaction is a molecular technique that amplifies specific DNA sequences through repeated cycles of three temperature-dependent steps. Understanding what Taq polymerase actually does—and what it doesn't—requires clarity about the nature of each step.

Denaturation is the first step, occurring at a high temperature (the NCERT text does not give an exact figure). At this high temperature, the hydrogen bonds between complementary base pairs in the double-stranded DNA template break, separating the two strands. This is purely a physical process driven by thermal energy. No enzyme catalyses this separation; heat alone disrupts the bonds. Taq polymerase, though thermostable, plays no active role here—it simply survives the high temperature without denaturing itself, which is why it was chosen from the thermophilic bacterium Thermus aquaticus.

Annealing follows at a lower temperature, depending on the primers used. The short, single-stranded primers bind to their complementary sequences on the now-separated template strands. This is again a spontaneous process governed by base-pairing rules and temperature. The primers find their target sequences through molecular collisions and hydrogen bonding. Taq polymerase does not catalyse this binding; it waits, inactive, until the primers are in place.

Alternative Approach: Assign Each PCR Step to Its ACTUAL Cause

Instead of recalling "Taq only does extension" as an isolated fact, work out what

physically CAUSES each of the three PCR steps, and check whether that cause is

enzyme-catalysed or not.

Step 1: What causes denaturation?

Simply raising the temperature high enough supplies enough thermal energy to

break the hydrogen bonds holding the two DNA strands together. This is a purely physical

process -- no enzyme is involved, and none is needed.

Step 2: What causes annealing?

Lowering the temperature allows primers to find and hydrogen-bond with their

complementary sequences on the now-separated template strands, through ordinary

base-pairing chemistry. Again, this happens on its own, driven by temperature and

sequence complementarity -- not by any enzyme.

Step 3: What causes extension?

Here, and only here, an enzyme is actually doing catalytic work: Taq DNA polymerase

binds the primer-template junction and catalyses formation of new phosphodiester bonds

as it adds nucleotides one by one, extending the new strand.

Step 4: Match this back to the options. …