Molecular Basis of Inheritance: The 10 Most Repeated NTA Questions

If there is one chapter in Class 12 Biology that can single-handedly swing your NEET score by 20 to 24 marks, it is Molecular Basis of Inheritance.

NTA loves this chapter. Every year, without fail, you will see 5 to 6 questions picked directly from NCERT lines, diagrams, and processes. But here’s the problem: most students panic because of heavy terms like Transcription, Translation, Lac Operon, and Semi-Conservative Replication.

The truth? NTA repeats the exact same 10 conceptual patterns every single year.

At www.biology300.in, Manpreet Sir has analyzed the last 10 years of PYQs (Previous Year Questions) to bring you these top 10 concepts—along with the exact tools you need to master them under exam pressure.

1. The Lac Operon Regulation Mechanism

• The Question Pattern: "In the Lac Operon, what happens when lactose is added to the medium?" or "Match the structural genes (z, y, a) with their respective enzymes."

• The Concept: The Lac Operon is an inducible, negative regulation system.

  • i-gene: Codes for the Repressor protein.

  • z-gene: Codes for beta-galactosidase (breaks lactose into glucose + galactose).

  • y-gene: Codes for Permease (increases membrane permeability to beta-galactosides).

  • a-gene: Codes for Transacetylase.

• The Trap: NTA tricks students by asking what acts as the inducer. Remember: Lactose (or allolactose) binds to the repressor and inactivates it, allowing transcription to start!

2. Direction of DNA Replication & Enzymes

• The Question Pattern: "In which direction does DNA Polymerase catalyze polymerization?" or "Which enzyme joins Okazaki fragments?"

• The Concept:

  • DNA Polymerase always catalyzes polymerization in the 5' → 3' direction.

  • The Leading strand (3' → 5' template) is synthesized continuously.

  • The Lagging strand (5' → 3' template) is synthesized discontinuously into Okazaki fragments.

  • DNA Ligase joins these Okazaki fragments together.

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3. The Hershey-Chase Experiment (Radioactive Isotopes)

• The Question Pattern: "Which radioisotopes were used by Hershey and Chase to prove that DNA is the genetic material?"

• The Concept:

  • 35S (Radioactive Sulfur): Used to label Proteins (present in cysteine/methionine, but not in DNA).

  • 32P (Radioactive Phosphorus): Used to label DNA (present in the nucleic acid backbone, not in proteins).

• The Result: Radioactivity (32P) was found inside the bacterial cell, confirming DNA as the genetic material.

4. Transcription Unit & Strand Identification

• The Question Pattern: "Given a 5' → 3' sequence of a coding strand, write the sequence of the transcribed mRNA."

• The Concept:

  • Template Strand (3' → 5'): Acts as the template for RNA synthesis.

  • Coding Strand (5' → 3'): Does NOT code for anything, but its sequence is identical to the mRNA, except Thymine (T) is replaced by Uracil (U).

• Pro-Tip by Manpreet Sir: If NTA gives you the Coding strand sequence, just swap 'T' with 'U'—don't waste time reversing the base pairs!

5. Properties of Genetic Code

• The Question Pattern: "Which of the following statements about the genetic code is INCORRECT?"

• The Concept: NTA loves testing these 5 features:

  1. Triplet: 61 codons code for amino acids; 3 are stop codons (UAA, UAG, UGA).

  2. Unambiguous & Specific: One codon codes for only ONE specific amino acid.

  3. Degenerate: Some amino acids are coded by more than one codon.

  4. Universal: AUG codes for Methionine in bacteria as well as humans.

  5. Initiator Codon: AUG acts as the start codon.

6. Chargaff's Rule Calculations

• The Question Pattern: "If a double-stranded DNA has 20% Cytosine, calculate the percentage of Adenine."

• The Concept: According to Erwin Chargaff:

• % A =% T and % G =% C

• Calculation:

  • If %C = 20%, then %G = 20% (Total C + G = 40%).

  • Remaining A + T = 100% - 40% = 60%.

  • Therefore, %A = 30% and %T = 30%.

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7. Types of RNA Polymerases in Eukaryotes

• The Question Pattern: "Which RNA Polymerase transcribes tRNA in eukaryotic cells?"

• The Concept: Unlike prokaryotes (which have a single RNA polymerase), eukaryotes have 3 distinct types:

  • RNA Polymerase I: Transcribes rRNA (28S, 18S, 5.8S).

  • RNA Polymerase II: Transcribes hnRNA (precursor of mRNA).

  • RNA Polymerase III: Transcribes tRNA, 5tS rRNA, and snRNA.

8. Post-Transcriptional Modifications (Splicing, Capping, Tailing)

• The Question Pattern: "What happens during Capping and Tailing of hnRNA?"

• The Concept:

  • Splicing: Removal of Introns (non-coding) and joining of Exons (coding).

  • Capping: Addition of an unusual nucleotide (Methyl guanosine triphosphate) to the 5'-end.

  • Tailing: Addition of Adenylate residues (200–300) to the 3'-end in a template-independent manner.

9. Structure of tRNA (Clover-Leaf Model)

• The Question Pattern: "Which loop of tRNA binds to the codon on mRNA?"

• The Concept:

  • Anticodon Loop: Has bases complementary to the codon on mtRNA.

  • Amino Acid Acceptor End: The 3'-end where the specific amino acid binds.

  • tRNA is called an adapter molecule.

10. Human Genome Project (HGP) & DNA Fingerprinting Metrics

• The Question Pattern: "Select the correct statements regarding the Human Genome Project."

• The Concept: High-yield stats NTA frequently picks:

  • Human genome contains 3.1647times 10^9 bp.

  • Average gene consists of 3000 bases (Largest human gene: Dystrophin at 2.4 million bases).

  • Total estimated genes: 30,000.

  • Chromosome 1 has the most genes (2,968), and Y chromosome has the fewest (231).

  • Less than 2% of the genome codes for proteins.

Final Verdict: Turn Conceptual Knowledge into a 360 Score

Reading through these 10 points gives you the strategy, but consistent execution is what secures your rank. Don't leave your Biology score to chance.

• Step 1: Revise these concepts using Biology300+ Study Material.

• Step 2: Test your speed and accuracy with our NTA-Pattern Test Series.

• Step 3: Watch Manpreet Sir's daily breakdown sessions on the Biology300+ YouTube Channel.

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