The trap first: Students confuse the role of PCR with the role of blotting. PCR amplifies — it makes millions of copies of a specific DNA segment. Blotting detects — it separates molecules, transfers them to a membrane, and visualises a specific target using a probe. They are different stages of molecular analysis, and NEET questions routinely test whether you can distinguish "amplify" from "detect."
PCR (Polymerase Chain Reaction): Invented by Kary Mullis. Uses a thermostable DNA polymerase (Taq polymerase, from Thermus aquaticus) to amplify a target DNA sequence in vitro. Each cycle has three steps — denaturation (~95°C), annealing (~55–65°C), and extension (~72°C). Two primers are essential: a forward primer and a reverse primer, each flanking the target region on opposite strands, oriented inward. After n cycles, the target is amplified approximately 2ⁿ times (NCERT Class 12 Biology Chapter 10, page 225).
Gel electrophoresis: Separates DNA fragments by size through an agarose matrix under an electric field. Smaller fragments migrate faster toward the anode. Visualised using ethidium bromide staining under UV.
Blotting techniques:
Watch-out: "Which technique amplifies DNA?" — the answer is always PCR, never a blotting method. Blotting identifies what is already present; PCR generates copies.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
The enzyme used in PCR is isolated from a thermophilic bacterium because it must withstand:
PCR requires two primers because:
After 10 cycles of PCR starting from a single double-stranded DNA molecule, the approximate number of copies of the target region is:
In gel electrophoresis of DNA fragments, smaller fragments migrate:
Southern blotting is used to detect:
A researcher wants to determine whether a patient carries a specific mutant allele. She amplifies the region using PCR, then separates the products by gel electrophoresis. The technique she uses to confirm the identity of the band is:
During PCR, the step in which primers bind to their complementary sequences on the template DNA is called:
A student claims that PCR uses only one primer per reaction because "polymerase extends in both directions from a single primer." The error in this reasoning is:
Given
A forensic scientist has a blood sample containing trace DNA from a suspect. She needs to determine whether the suspect's DNA contains a specific 500 bp sequence linked to a genetic marker. Available tools: Taq polymerase, primers flanking the 500 bp region, agarose gel apparatus, Southern blot kit with a probe complementary to the 500 bp marker.
Required
Identify the correct sequence of techniques and their roles in confirming the presence of the specific 500 bp marker.
Concept
PCR amplifies a specific DNA region (defined by primers) to detectable quantities. Gel electrophoresis separates the amplified product by size. Southern blotting confirms sequence identity by probe hybridisation — because a 500 bp band on a gel could be any 500 bp fragment; only probe binding proves it is the target.
Formula/Rule
- PCR: 2ⁿ amplification (exponential, *n* = number of cycles) - Gel: migration ∝ 1/log(size) — smaller fragments travel farther - Southern blot: sequence-specific hybridisation (complementary probe binds target)
Substitution/Application
1. **PCR** the trace DNA using the two primers → amplify the 500 bp target region. 2. **Gel electrophoresis** → run the PCR product; look for a band at ~500 bp. 3. **Southern blot** → transfer gel to membrane → hybridise with the labelled probe → confirm the 500 bp band IS the marker (not a non-specific product of similar size).
Calculation/Reasoning
If PCR runs for 30 cycles: 2³⁰ ≈ 1.07 × 10⁹ copies of the target — sufficient for gel visualisation and blotting. The number 30 is an exact counting integer (does not limit significant figures).
Final answer
Correct technique sequence: **PCR → Gel electrophoresis → Southern blotting.** PCR amplifies, gel separates by size, Southern confirms identity. Each technique has a distinct, non-interchangeable role.
Common trap
Confusing amplification (PCR) with detection (blotting). A common wrong answer would say "Southern blotting amplifies DNA" or "PCR confirms identity." Remember: **Amplify = PCR. Detect specific sequence = blot.**
Similar NEET-style question
"Which of the following correctly pairs the technique with its primary function? (a) PCR — sequence-specific detection (b) Southern blot — DNA amplification (c) Northern blot — RNA detection (d) Western blot — DNA transfer to membrane" Answer: (c). Northern blot detects specific RNA by hybridisation. ---
PCR: denature 94°C → anneal 50-65°C → extend 72°C with Taq pol. Gel electrophoresis: agarose (DNA) or PAGE (proteins) — separation by size in electric field. Southern (DNA), Northern (RNA), Western (protein) blots.
-- NCERT Class 12 Biology, Ch. 10, p. 225These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Similar Terms
Bt: insect resistance (Cry toxin). RNAi: nematode resistance via gene silencing. Herbicide-tolerant: bar/EPSPS gene.
Question on which gene/strategy gives which trait.
Cry → insects. RNAi → nematodes. EPSPS → herbicide tolerance.
Category: Similar Terms
PCR amplifies (makes copies); blotting transfers + visualises specific bands. Different stages of analysis.
Question asks which technique amplifies vs detects.
Amplify = PCR. Detect specific = blot (Southern DNA, Northern RNA, Western protein).
Category: Similar Terms
Restriction enzymes CUT at recognition sites; ligase JOINS sticky ends with phosphodiester bonds; polymerase synthesises strands.
Question asks which tool performs which step.
Cut = restriction. Paste = ligase. Copy = polymerase. Cohen-Boyer used EcoRI + ligase for first rDNA.
Root cause: concept gap
Eli Lilly's Humulin: A and B chains expressed SEPARATELY in two E. coli strains; chains then combined in vitro and joined by disulfide bonds.
Root cause: concept gap
PCR requires TWO primers — one for each end of target DNA, anneal to opposite strands, point inward. Together they bracket the amplicon.
Root cause: concept gap
EcoRI: GAATTC palindrome, cuts to leave 5' AATT overhangs (sticky ends). Blunt-ending enzymes: HaeIII, AluI.
Root cause: term confusion
Bt cotton expresses Cry protein INSIDE its tissues — endogenous; insects ingesting plant tissue are killed. Not externally sprayed.
26 questions from NEET 2020, 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
Why can’t insulin be given orally to diabetic patients?
Silencing of specific mRNA is possible via RNAi because of
Identify the part of a bio-reactor which is used as a foam braker from the given figure.
Polymerase chain reaction (PCR) amplifies DNA following the equation.
The “Ti plasmid” of Agrobacterium tumefaciens stands for
Upon exposure to UV radiation, DNA stained with ethidium bromide will show
Which of the following is not a cloning vector?
Which one of the following statement is not true regarding gel electrophoresis technique?
Transposons can be used during which one of the following ?
¬˝ÁÃ’ œŸ ∞ ¡Êß◊Ê ∑ § Áfl ÿ ◊ ª‹Ã ∑§ÕŸ ∑§Ê ¬„øÊÁŸ∞– (4) After zygote formation
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
similar term confusion
Biology relies on precise terminology; close terms tempt selection.
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