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What is PCR (Polymerase Chain Reaction)?

Biology · Biotechnology: Principles and Processes · NEET

PCR (Polymerase Chain Reaction) is a lab technique that makes many copies of one chosen piece of DNA in a test tube (in vitro). It needs two primers, the enzyme DNA polymerase, free nucleotides, and the DNA to be copied. Memory hook: PCR is a "photocopy machine for DNA" — each cycle doubles the copies (2ⁿ), so 30 cycles give about a billion copies.
PCR: one DNA piece doubles each cycle (2ⁿ)1StartCycle 1 = 2Cycle 2 = 48Cycle 3 = 8→≈ 30 cycles2³⁰ ≈ 1 billion copiesIn a test tube (in vitro) · needs 2 primers + Taq polymerase (from Thermus aquaticus)
Each PCR cycle doubles the DNA, so copies grow as 2ⁿ — from one segment to about a billion copies after roughly 30 cycles, all done in a test tube using two primers and heat-stable Taq polymerase.

Your doubts, answered

Is PCR done inside a living cell or in a test tube?

PCR happens in a test tube (in vitro), not inside a cell. This is a key NEET point. Cloning inside bacteria also makes copies, but that needs living host cells and takes time. PCR skips the cell — you mix DNA, primers, DNA polymerase and nucleotides in a tube and a machine heats and cools it. So PCR = amplification without a host cell.

Why do we need PCR if restriction enzymes already give us the gene?

Restriction enzymes only cut out the gene from a big DNA sample — you may end up with just one or a few copies. To do experiments (cloning, diagnosis, fingerprinting) you need many copies. PCR amplifies that single gene of interest to about a billion copies, so there is enough DNA to work with. Cutting gives you the piece; PCR multiplies it.

Does PCR copy the whole DNA or only the gene of interest?

Only the gene (or DNA segment) of interest. The two primers decide the exact start and end of the copied region. Primers are short DNA pieces (oligonucleotides) that match the two ends of your target. DNA polymerase can only extend from a primer, so only the region between the two primers gets copied — not the whole genome.

How does PCR make about a billion copies from one DNA molecule?

Each cycle doubles the number of DNA copies, so the growth is 2ⁿ, where n is the number of cycles. After 1 cycle you have 2 copies, after 2 cycles 4, after 3 cycles 8, and so on. After about 30 cycles you get 2³⁰, which is roughly one billion copies. This doubling is why NEET 2025 asked for the amplification equation (answer: 2ⁿ).

Is PCR the same thing as DNA fingerprinting?

No. DNA fingerprinting is a whole method to identify a person from their DNA (using VNTR repeats). PCR is just one tool used inside it. NCERT says the sensitivity of fingerprinting improved because PCR can amplify DNA from even a single cell, so a tiny sample is enough. PCR = the amplifier; fingerprinting = the full identification technique.

Which enzyme is used in PCR and why must it survive heat?

A thermostable (heat-stable) DNA polymerase called Taq polymerase, isolated from the bacterium Thermus aquaticus. In each cycle the DNA is heated to a high temperature to separate the strands (denaturation). A normal enzyme would be destroyed by that heat. Taq stays active through it, so you do not have to add fresh enzyme every cycle. This is a frequently asked NEET fact.

⚠️ The NEET trap
✗ PCR takes place inside bacterial host cells, like gene cloning.
✓ PCR is done in vitro (in a test tube), with no living host cell — the machine just heats and cools the reaction mix.
🧠 'Amplification' does NOT mean inside a cell. Cloning uses a host cell; PCR uses a test tube. If the option says 'in vivo' or 'inside host', reject it.

Real NEET questions

2025

Polymerase chain reaction (PCR) amplifies DNA following the equation

A · 2n + 1
B · 2N²
C · N²
D · 2ⁿ ✓
Solution: In PCR, each cycle doubles the number of DNA copies, so after n cycles the copies follow 2ⁿ. NCERT notes the segment can be amplified to about a billion copies (≈ 2³⁰), which matches the doubling rule. NCERT Ch 9, p.173.
2023

Thermostable DNA polymerase used in PCR was isolated from :

A · Agrobacterium tumefaciens
B · Bacillus thuringiensis
C · Thermus aquaticus ✓
D · Escherichia coli
Solution: PCR repeatedly heats DNA to a high temperature to separate strands, so the polymerase must be heat-stable. This thermostable enzyme (Taq polymerase) comes from the bacterium Thermus aquaticus and stays active during the high-temperature denaturation. NCERT Ch 9, p.173.
2021

Which of the following is not an application of PCR (Polymerase Chain Reaction)

A · Purification of isolated protein ✓
B · Detection of gene mutation
C · Molecular diagnosis
D · Gene amplification
Solution: PCR amplifies DNA in vitro, so its uses are DNA-based: gene amplification, detecting gene mutations, and molecular diagnosis. Purifying an already-isolated protein is a separation (downstream) step, not nucleic-acid amplification, so it is NOT a PCR application. NCERT Ch 9, p.172.

Solved Biotechnology: Principles and Processes NEET PYQs

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Frequently asked

What does PCR stand for?

PCR stands for Polymerase Chain Reaction. It is named after DNA polymerase, the enzyme that builds the new DNA copies in a repeated (chain) reaction of heating and cooling cycles.

What are the four things needed for PCR?

Four essentials: (1) the DNA template (gene of interest), (2) two primers that mark the start and end, (3) DNA polymerase (Taq) to build new strands, and (4) free nucleotides as building blocks. A thermal cycler machine provides the heating and cooling.

Why is PCR important for NEET?

PCR appears almost every year — usually about its 2ⁿ amplification rule, the Taq/Thermus aquaticus source, or its applications (HIV detection, mutation detection, diagnosis, fingerprinting). It is a high-yield one-mark topic from Biotechnology: Principles and Processes.

What is the role of primers in PCR?

Primers are short DNA pieces that base-pair with the two ends of the target region. DNA polymerase can only add nucleotides next to a primer, so primers decide exactly which segment gets copied. Without primers, PCR cannot start.

How many copies does PCR make?

Copies double every cycle (2ⁿ). After about 30 cycles you get roughly one billion copies of the original DNA segment, which is why PCR can start from even a single cell's DNA.

Related concepts:
The Three Steps of PCR: Denaturation, Annealing, ExtensionWhat is Taq Polymerase (Thermus aquaticus)?Role of Primers in PCRApplications of PCRProcesses of Recombinant DNA Technology: Overview
Practise Biology for NEET:
Biology MCQs (free, chapter-wise)Biotechnology: Principles and Processes PYQs