Denaturation, annealing, extension. Taq polymerase. Applications.

PCR — Polymerase Chain Reaction Steps

Question

Describe the Polymerase Chain Reaction (PCR) technique. Name the enzyme used and explain each step of the process. State any two applications of PCR.

(CBSE 2024 Board Exam, 5 marks)

Solution — Step by Step

We take the DNA template (the segment we want to amplify), two specific primers (short single-stranded DNA sequences, one complementary to each strand), all four deoxyribonucleoside triphosphates (dNTPs), and Taq DNA polymerase in a buffer solution.

Why primers? Because DNA polymerase cannot start synthesis from scratch — it needs a free 3’-OH end to add nucleotides. Primers provide exactly that.

The reaction mixture is heated to 94°C. At this temperature, the hydrogen bonds between the two strands of the double helix break, separating them into two single-stranded templates.

This is like unzipping a zipper — each strand is now accessible for copying.

The temperature is lowered to 40–60°C (depends on primer design). The primers, being short and complementary, bind specifically to their target sequences on each template strand.

This step determines the specificity of PCR — primers bind only to the exact region we want to amplify.

Temperature is raised to 72°C, which is the optimal working temperature for Taq DNA polymerase. This enzyme extends the primers by adding dNTPs in the 5’→3’ direction, synthesizing new complementary strands.

Why Taq polymerase specifically? It’s isolated from Thermus aquaticus, a bacterium living in hot springs. It remains active at 94°C — ordinary DNA polymerase would denature during the denaturation step.

One cycle (denaturation → annealing → extension) takes about 2–3 minutes. After 30–35 cycles, we get approximately $2^n$ copies of the target sequence, where $n$ is the number of cycles. From a single DNA molecule, we can generate billions of copies in a few hours.

Why This Works

The elegance of PCR is that the product of one cycle becomes the template for the next. After the first few cycles, the dominant product is a precise copy of the region between the two primers — nothing more, nothing less.

The exponential amplification ( $2^n$ ) is what makes PCR so powerful. After 30 cycles, a single molecule theoretically becomes over one billion copies. In practice, efficiency isn’t 100%, but even at 85% efficiency, the amplification is enormous.

Number of copies after $n$ cycles $= 2^n$

After 30 cycles: $2^{30} \approx 10^9$ copies

(Starting from a single template molecule)

Alternative Method — Quick 3-Point Summary for Exam

If a board question asks you to “briefly describe” PCR (2–3 marks), use this structure:

Denaturation (94°C) — double-stranded DNA separates into single strands
Annealing (40–60°C) — specific primers bind to complementary sequences
Extension (72°C) — Taq polymerase synthesizes new strands using dNTPs

Then state: The cycle repeats 30–35 times, exponentially amplifying the target sequence.

For the enzyme, always mention: Taq polymerase from Thermus aquaticus — thermostable, survives repeated heating cycles.

Applications

Two applications frequently asked in CBSE and NEET:

Detection of genetic disorders — PCR amplifies tiny DNA samples to detect mutations responsible for conditions like sickle cell anaemia or thalassaemia.
Forensic science / DNA fingerprinting — even trace amounts of DNA (from a hair follicle, blood spot, or saliva) can be amplified for identification. Crucial in criminal investigations and paternity testing.

NEET 2022 asked the source organism of Taq polymerase directly. Always remember: Thermus aquaticus — a thermophilic bacterium from hot springs. The thermal stability of Taq polymerase is what made automated PCR (using a thermocycler) commercially feasible.

Common Mistake

Students often write that DNA polymerase “starts from the beginning of the DNA strand.” This is wrong. DNA polymerase requires a primer with a free 3’-OH group to begin adding nucleotides. Without primers, extension cannot happen. This is also why two primers are needed — one for each template strand (remember, the two strands run antiparallel).

Another slip: confusing the temperatures. Denaturation is the highest temperature (94°C), not extension. A common exam trap.