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Zoology · Biotechnology — Principles and Processes

PCR — Amplifying the Gene of Interest

Section 9.3.3 of NCERT Class 12 Biology introduces the Polymerase Chain Reaction as the in-vitro method of choice for amplifying a specific DNA sequence before cloning or diagnosis. NEET consistently tests PCR step order, temperatures, the source and rationale for Taq polymerase, and the exponential copy formula — often through assertion-reason and sequence-arrangement formats. Mastery of this single topic has contributed marks in every paper from 2018 through 2025.

NCERT grounding

The authoritative syllabus text is NCERT Class 12 Biology, Chapter 9 — Biotechnology: Principles and Processes, Section 9.3.3, titled "Amplification of Gene of Interest using PCR." The section is brief — roughly half a page of prose plus Figure 9.6 — but every word is examinable. The key sentence reads: "Such repeated amplification is achieved by the use of a thermostable DNA polymerase (isolated from a bacterium, Thermus aquaticus), which remain active during the high temperature induced denaturation of double stranded DNA." This one sentence anchors two of the most frequently tested NEET facts: the organism of origin and the reason for thermostability.

"The segment of DNA can be amplified to approximately billion times, i.e., 1 billion copies are made."

NCERT Class 12 Biology, Chapter 9, §9.3.3

What is PCR?

Polymerase Chain Reaction (PCR) is an in-vitro enzymatic technique that produces millions to billions of copies of a specific DNA segment from a minute starting quantity of template. Conceived by Kary Mullis in 1983 and first published in 1985, the method earned Mullis the 1993 Nobel Prize in Chemistry. Before PCR, amplifying a gene required time-consuming cloning into a vector followed by propagation in a host organism. PCR accomplishes the same goal in a matter of hours, using only a test tube, a thermocycler, and a handful of reagents.

The reaction exploits the same logic as cellular DNA replication: a polymerase reads a single-stranded template in the 3′→5′ direction and synthesises a new complementary strand in the 5′→3′ direction. The innovation of PCR is the use of short synthetic oligonucleotides called primers that bracket the target sequence, so that amplification is selective rather than genome-wide, and the use of a heat-stable polymerase that survives the repeated high-temperature steps needed to melt (denature) double-stranded DNA between cycles.

PCR components

Component Role Notes
DNA template Provides the target sequence to be copied Can be genomic DNA, plasmid, or cDNA; only nanogram quantities needed
Forward primer Anneals to the antisense strand; defines the 5′ boundary of the amplicon Short oligonucleotide (~18–22 nt), complementary to one strand
Reverse primer Anneals to the sense strand; defines the 3′ boundary Two primers together bracket the region of interest
dNTPs (dATP, dGTP, dCTP, dTTP) Building blocks for new DNA strand synthesis Deoxynucleoside triphosphates; all four must be present
Taq DNA polymerase Synthesises new DNA strand from primer 3′-OH end Thermostable; from Thermus aquaticus; optimal at ~72°C
Buffer + Mg²⁺ Maintains optimal pH; Mg²⁺ is essential cofactor for Taq MgCl₂ concentration affects primer annealing specificity
Thermocycler Automates precise temperature cycling across all three steps Programmable heating/cooling block; replaces manual water baths

The three-step cycle

Each round of PCR amplification consists of three precisely temperature-controlled steps. The cycle is repeated 25–35 times in a thermocycler.

One PCR cycle (repeated ~30 times)

NCERT Figure 9.6 sequence
  1. Step 1

    Denaturation

    Temperature raised to ~94°C. Hydrogen bonds holding the two DNA strands together break. The double helix unwinds into two single-stranded templates.

    ~94°C · 30–60 s
  2. Step 2

    Annealing

    Temperature lowered to ~50–60°C. Forward and reverse primers hybridise to their complementary sequences on opposite strands, flanking the target region.

    50–60°C · 30–60 s
  3. Step 3

    Extension

    Temperature raised to ~72°C (optimum for Taq polymerase). Taq reads the template 3′→5′ and extends each primer by adding dNTPs, synthesising a new complementary strand.

    ~72°C · 1 min/kb

After the extension step, each original double-stranded DNA molecule has become two. The products of one cycle serve as templates for the next. By cycle 3, molecules of the exact target length — bounded at both ends by primer sequences — begin to accumulate. These "short" products then double with every subsequent cycle, making them the dominant species after ~20 cycles.

Figure 1 PCR Three-Step Cycle Diagram One PCR Cycle — Temperature Profile 94°C 72°C 55°C DENATURE ANNEAL EXTEND dsDNA → ssDNA Primers bind Taq extends Time →

Figure 1. Idealised temperature profile for one PCR cycle. The coral zone (94°C) is denaturation; amber zone (50–60°C) is primer annealing; teal zone (72°C) is Taq-mediated extension. The cycle repeats 25–35 times in a thermocycler.

Taq polymerase — the key to automation

The pivotal problem in early PCR attempts was that normal DNA polymerases (such as E. coli Pol I) are heat-labile: they denature and lose catalytic activity above ~45°C. Each denaturation step at 94°C would destroy the enzyme, requiring fresh polymerase to be added manually before every extension step — a prohibitively tedious process.

The solution emerged from the biology of extreme environments. Thermus aquaticus is a thermophilic bacterium first isolated from hot springs in Yellowstone National Park in 1969 by Thomas Brock and Hudson Freeze. It thrives at temperatures of 70–80°C. Its DNA polymerase — commercially named Taq polymerase — is intrinsically thermostable, retaining activity even after prolonged exposure to 95°C. Because Taq survives the denaturation step, a single addition at the start of the reaction suffices for the entire run. This property made PCR a practical, automatable procedure and enabled the invention of the programmable thermocycler.

Taq polymerase vs. E. coli DNA polymerase I — key differences

Taq polymerase

~95°C

Upper stability limit

  • Source: Thermus aquaticus (thermophile)
  • Optimal activity at ~72°C
  • Survives repeated denaturation cycles
  • Lacks 3′→5′ proofreading exonuclease (higher error rate)
  • Used in all standard PCR protocols
VS

E. coli DNA Pol I

~37°C

Optimal temperature

  • Source: mesophilic bacterium
  • Denatures above ~45°C
  • Cannot survive PCR denaturation steps
  • Has 3′→5′ proofreading activity
  • Used for nick translation, not PCR

In high-fidelity applications (e.g., cloning where sequence accuracy matters), Pfu polymerase from Pyrococcus furiosus is preferred because it possesses 3′→5′ proofreading activity. However, for NEET purposes, the examinable organism is exclusively Thermus aquaticus and the enzyme is Taq polymerase.

Exponential amplification — the 2ⁿ formula

The defining feature of PCR is exponential, not linear, amplification. After the first cycle, one dsDNA molecule becomes two. After the second cycle, two become four. The relationship is:

2ⁿ

Copies after n cycles

Starting from a single template molecule. After 30 cycles, this equals 2³⁰ ≈ 1.07 × 10⁹ copies (approximately one billion). NCERT states the segment can be amplified "approximately billion times." The NEET 2025 paper asked directly: PCR amplifies DNA following the equation — answer is 2ⁿ.

It is important to understand that not all 2ⁿ products are of the exact target length. In the first two cycles, the new strands extend beyond the primer-defined region (their 3′ ends are undefined). Starting from cycle 3, strands bounded precisely at both ends by primer sequences accumulate. By cycle 20–25, these "short" products dominate overwhelmingly, essentially making the final product a purified copy of the target sequence.

Figure 2 PCR Exponential Amplification Chart PCR — Exponential Copy Number Growth 0 1 2 3 4 5 6 Cycle number (n) 1 2 4 8 16 32 64 1 2 4 8 16 32 64 copies = 2ⁿ

Figure 2. Exponential amplification in PCR. Each cycle doubles the copy number. After n cycles starting from one template molecule, 2ⁿ copies exist. Thirty cycles yield approximately 10⁹ copies.

RT-PCR — reverse transcription PCR

RT-PCR (Reverse Transcription PCR) is a variant that begins not with DNA but with an mRNA template. The enzyme reverse transcriptase first converts the mRNA into a single-stranded complementary DNA (cDNA). This cDNA then serves as the template for standard PCR amplification using Taq polymerase and primers.

RT-PCR has become indispensable in two contexts. First, it is used to detect and quantify the expression of specific genes: since only expressed genes produce mRNA, RT-PCR on a tissue-derived RNA sample reveals which genes are active in that tissue. Second, RT-PCR is the standard diagnostic tool for RNA viruses — including HIV, influenza, and SARS-CoV-2 — because their genetic material is RNA, not DNA, and would be invisible to standard PCR without the reverse transcription step. In molecular diagnosis, quantitative RT-PCR (qRT-PCR or real-time RT-PCR) provides both detection and copy-number quantification in a single closed-tube reaction.

Applications of PCR

Application domain Specific use Variant used
Gene cloning Amplify target gene before ligation into a vector Standard PCR
Molecular diagnosis Detect bacterial/viral DNA in patient samples (HIV, TB, hepatitis) Standard or RT-PCR
Forensic science DNA fingerprinting from trace biological evidence (blood, hair) Standard PCR + STR analysis
Prenatal diagnosis Detect chromosomal or single-gene disorders from foetal cells Standard PCR
Gene expression analysis Measure mRNA levels; identify which genes are active in a tissue RT-PCR / qRT-PCR
Mutation detection Identify SNPs or point mutations in cancer or inherited disease genes Allele-specific PCR
Ancient DNA studies Amplify degraded DNA from museum specimens or archaeological remains Standard PCR

Worked examples

Worked example 1

A scientist performs 30 cycles of PCR starting with a single double-stranded DNA molecule. How many copies of the target DNA will be present at the end?

Answer: Using the formula copies = 2ⁿ, where n = 30: 2³⁰ = 1,073,741,824 ≈ 1.07 × 10⁹ (approximately one billion copies). NCERT rounds this to "approximately billion times" amplification. Note: the question specifies "copies" not "new molecules" — the original molecule counts, so technically there are 2³⁰ double-stranded molecules.

Worked example 2

Which of the following is not an application of PCR?
(1) Detection of gene mutation   (2) Molecular diagnosis
(3) Gene amplification   (4) Purification of isolated protein

Answer: (4) Purification of isolated protein. PCR amplifies nucleic acids (DNA and RNA), not proteins. Protein purification involves biochemical separation techniques such as chromatography. Options 1, 2, and 3 are all legitimate PCR applications documented in NCERT and NIOS sources. This is a NEET 2021 question.

Worked example 3

Taq polymerase used in PCR is obtained from which organism, and why is it specifically chosen over E. coli DNA polymerase?

Answer: Taq polymerase is obtained from Thermus aquaticus, a thermophilic bacterium found in hot springs. It is chosen because it is thermostable — it remains active at the high temperatures (~94°C) used during the denaturation step of each PCR cycle. E. coli DNA polymerase would denature and lose activity at these temperatures, requiring fresh enzyme to be added before every extension step, making automation impossible. The thermostability of Taq polymerase allows a single addition at the start of the reaction to suffice for all 30+ cycles.

Worked example 4

In PCR, if very high temperature is not maintained at the beginning, which step will be affected first?
(1) Ligation   (2) Annealing   (3) Extension   (4) Denaturation

Answer: (4) Denaturation. Denaturation is the first step of the PCR cycle and requires ~94°C to separate the two DNA strands. If the temperature is insufficient to denature the double-stranded DNA, the strands will not separate, preventing primers from accessing the template in the subsequent annealing step, and blocking the entire reaction. This is NEET 2021 Q.157.

Common confusion and NEET traps

PCR vs. RT-PCR — key distinctions tested in NEET

Standard PCR

DNA

Starting material

  • Template is genomic DNA or plasmid DNA
  • Enzyme used: Taq DNA polymerase
  • Does not detect RNA viruses directly
  • Used for gene amplification before cloning
  • Detects presence of DNA sequences
VS

RT-PCR

mRNA

Starting material

  • Template is mRNA; converted to cDNA first
  • Enzymes: reverse transcriptase → then Taq polymerase
  • Detects RNA viruses (HIV, influenza, SARS-CoV-2)
  • Used for gene expression studies
  • Detects actively transcribed genes only

NEET PYQ Snapshot — PCR Amplification

Verified NEET previous-year questions on PCR from 2016–2025 with solutions.

NEET 2025

Polymerase chain reaction (PCR) amplifies DNA following the equation:

  1. 2N²
  2. N²
  3. 2ⁿ
  4. 2ⁿ⁺¹
Answer: (3) 2ⁿ

Why: PCR doubles the number of copies with each cycle. After n cycles starting from one molecule: 2¹ = 2, 2² = 4, 2³ = 8, etc. The relationship is exponential with base 2. Option 1 (2N²) and Option 2 (N²) are algebraically distinct and incorrect — they would give linear or polynomial growth. Option 4 (2ⁿ⁺¹) would be correct only if counting both original strands as distinct products at cycle 0.

NEET 2021

Which of the following is a correct sequence of steps in a PCR (Polymerase Chain Reaction)?

  1. Annealing, Denaturation, Extension
  2. Denaturation, Annealing, Extension
  3. Denaturation, Extension, Annealing
  4. Extension, Denaturation, Annealing
Answer: (2) Denaturation, Annealing, Extension

Why: Denaturation must come first (~94°C) to separate the dsDNA into single strands. Primers can only bind (anneal) to single-stranded DNA, so annealing follows (~50–60°C). Extension by Taq polymerase occurs last at ~72°C. Any sequence that places Annealing before Denaturation is incorrect because primers cannot displace a complementary strand at low temperature.

NEET 2021

During the process of gene amplification using PCR, if very high temperature is not maintained in the beginning, then which of the following steps of PCR will be affected first?

  1. Ligation
  2. Annealing
  3. Extension
  4. Denaturation
Answer: (4) Denaturation

Why: Denaturation is the first step of PCR and requires ~94°C to separate the two strands of dsDNA. If this temperature is not reached, the strands remain hydrogen-bonded together, preventing primer access in the next step. Ligation is not part of the PCR cycle at all — it is a separate recombinant DNA step.

NEET 2021

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

  1. Detection of gene mutation
  2. Molecular diagnosis
  3. Gene amplification
  4. Purification of isolated protein
Answer: (4) Purification of isolated protein

Why: PCR is a technique for amplifying nucleic acids (DNA/RNA). It has no role in protein purification, which requires chromatographic or precipitation-based biochemical techniques. Detecting mutations (1), molecular diagnosis (2), and gene amplification (3) are all documented applications of PCR.

NEET 2020

Match the organism with its use in biotechnology:
(b) Thermus aquaticus — (iii) DNA polymerase
(Which of the following correctly matches Thermus aquaticus?)

  1. Cry proteins
  2. Cloning vector
  3. DNA polymerase
  4. Construction of first DNA molecule
Answer: (3) DNA polymerase

Why: Thermus aquaticus is the source of the thermostable Taq DNA polymerase used in PCR. It does not produce Cry proteins (Bacillus thuringiensis), is not used as a cloning vector (Agrobacterium / plasmids), and was not involved in the first recombinant DNA (Salmonella typhimurium).

NEET 2016

The Taq polymerase enzyme is obtained from:

  1. Thiobacillus ferroxidans
  2. Bacillus subtilis
  3. Pseudomonas putida
  4. Thermus aquaticus
Answer: (4) Thermus aquaticus

Why: Taq polymerase was first isolated from the thermophile Thermus aquaticus, which thrives in hot springs at ~70–80°C. Its polymerase is intrinsically heat-stable, unlike those from mesophiles such as B. subtilis or Pseudomonas putida. This is one of the most directly testable facts in the PCR section.

FAQs — PCR Amplification

High-frequency doubts from NEET aspirants on PCR concepts and applications.

Who invented PCR and in which year?

PCR (Polymerase Chain Reaction) was invented by Kary Mullis in 1983. He was awarded the Nobel Prize in Chemistry in 1993 for this discovery. The technique was first published in 1985.

Why is Taq polymerase used in PCR instead of E. coli DNA polymerase?

Taq polymerase, isolated from the thermophilic bacterium Thermus aquaticus, is heat-stable and remains active at temperatures up to ~95°C. E. coli DNA polymerase denatures and loses activity at the high temperatures used during the denaturation step (~94°C) of each PCR cycle. Taq polymerase survives repeated denaturation cycles without needing to be replenished.

What are the three steps in one PCR cycle, and what temperature is used for each?

One PCR cycle has three steps: (1) Denaturation at ~94°C — the double-stranded DNA template is unwound into two single strands by heat; (2) Annealing at ~50–60°C — short oligonucleotide primers bind to complementary sequences on each template strand; (3) Extension at ~72°C — Taq polymerase synthesises new DNA by adding dNTPs from the 3′ end of each primer.

How many copies of DNA are produced after n cycles of PCR?

After n cycles, PCR produces 2ⁿ copies of the target DNA segment. Starting from a single double-stranded template, 30 cycles theoretically generate over 1 billion (2³⁰ ≈ 10⁹) copies. This is the exponential amplification described in NCERT section 9.3.3.

What is RT-PCR and how does it differ from standard PCR?

RT-PCR (Reverse Transcription PCR) begins with an mRNA template rather than genomic DNA. The enzyme reverse transcriptase first converts mRNA into complementary DNA (cDNA), which then serves as the template for standard PCR amplification. RT-PCR is used to detect expressed genes, diagnose RNA viruses (e.g., HIV, SARS-CoV-2), and quantify gene expression.

What are the components required for a PCR reaction?

A PCR reaction requires: (1) DNA template (the target sequence to be amplified); (2) two oligonucleotide primers — one for each strand, flanking the target region; (3) four deoxyribonucleoside triphosphates (dATP, dGTP, dCTP, dTTP) as building blocks; (4) thermostable Taq DNA polymerase; and (5) buffer with Mg²⁺ ions (cofactor for Taq polymerase).

What are the main applications of PCR?

PCR applications include: (1) amplifying the gene of interest before cloning into a vector; (2) molecular diagnosis of genetic diseases and infections (e.g., HIV, tuberculosis); (3) DNA fingerprinting in forensic science; (4) detection of gene mutations; (5) prenatal diagnosis; (6) RT-PCR for quantifying gene expression and detecting RNA viruses.

Related Topics
Biotechnology — Principles and Processes Back to the full chapter notes. Restriction Enzymes Molecular scissors that cut DNA at palindromic recognition sequences. Cloning Vectors Plasmids and phages used to carry foreign DNA into host cells. Cutting DNA at Specific Locations Restriction digestion and gel electrophoresis for isolating gene fragments. Insertion into Host Cells Methods for introducing recombinant DNA into competent host cells. Molecular Diagnosis — ELISA and PCR Diagnostic applications of PCR for detecting infections and genetic disease.