Q.Which of the following contributed in popularising the PCR (polymerase chain reactions) technique?

PCR became a practical, widespread tool only after a key technical barrier was removed. The original DNA polymerase used in the reaction got destroyed at the high temperatures needed to separate the DNA strands. This meant fresh enzyme had to be added after every heating cycle, making the process slow, expensive, and impractical.

The breakthrough came with the discovery and use of a thermostable DNA polymerase (like Taq polymerase, isolated from a hot-spring bacterium). This enzyme survives the high-temperature denaturation step and remains active through dozens of cycles. It allowed the entire reaction to be automated in a single closed tube, which is what truly popularised PCR. …

PCR, or polymerase chain reaction, is a technique that amplifies a specific DNA sequence into millions of copies. When Kary Mullis first conceived the idea in the 1980s, the basic ingredients were all known: a DNA template, primers, deoxyribonucleotides (dNTPs), and a DNA polymerase. Yet the technique remained cumbersome and inefficient for routine use. The reason was that the DNA polymerase used initially was derived from E. coli, an organism that lives at moderate temperatures. This enzyme was denatured and destroyed during the high-temperature step required to separate the two DNA strands. Consequently, fresh enzyme had to be added manually after every heating cycle. This made the process slow, labour-intensive, and expensive — hardly a technique that could be popularised for widespread use.

The breakthrough came with the discovery and use of a thermostable DNA polymerase, most famously Taq polymerase, isolated from the thermophilic bacterium Thermus aquaticus. This bacterium thrives in hot springs, and its DNA polymerase remains stable and active even at the high temperatures used in PCR. Because the enzyme could survive the denaturation step, there was no need to add fresh enzyme after each cycle. This single change allowed the entire reaction to be carried out in a closed tube, with the enzyme working through all 30–40 cycles automatically. The process could now be performed by a simple programmable thermal cycler, making PCR rapid, reliable, and accessible to any molecular biology laboratory.

Let us now examine the other options to see why they are not the primary factor that popularised PCR.

Easy availability of DNA template — The DNA template is the starting material, but it has always been relatively easy to obtain from cells or tissues. The difficulty was never in getting the template; it was in amplifying it efficiently. So this did not drive the popularisation. …

Alternative Approach: Identify the Actual Historical Bottleneck, Then Eliminate

Rather than stating which factor mattered, work through what problem PCR had BEFORE the

breakthrough, and test each option against whether it actually solved that specific

problem.

Step 1: State the pre-breakthrough bottleneck.

Early PCR used a standard (non-heat-stable) DNA polymerase. Each cycle's denaturation

step (near-boiling temperature) destroyed the enzyme, so fresh polymerase had to be

manually added after every single cycle -- making the technique slow, labour-intensive,

and impractical for routine, automated use.

Step 2: Test "easy availability of DNA template" (option a).

DNA template was never scarce or hard to obtain -- even a single starting molecule is

enough for PCR to amplify. This was never the bottleneck. Eliminate.

Step 3: Test "availability of synthetic primers" (option b).

Synthetic oligonucleotide primers were already chemically available before PCR became

routine; their existence did not by itself remove the need to keep re-adding enzyme

every cycle. Eliminate as the popularising factor.

Step 4: Test "cheap deoxyribonucleotides" (option c).

dNTP cost fell as demand for PCR grew, making this an effect of PCR's popularity rather

than its cause. Eliminate.

Step 5: Test "thermostable DNA polymerase" (option d). …