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🧬 Molecular Basis of Inheritance — NEET UG Questions

Topic 1: Introduction & The Search for Genetic Material

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🔷 SUBTOPIC 1.1 — Why DNA is the Genetic Material (Historical Experiments)

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• A) DNA is the transforming principle
• B) Protein is the genetic material
• C) A heat-killed virulent strain could transform non-virulent strain into virulent
• D) RNA acts as genetic material in bacteria

• Live R strain (non-virulent) → mice survived
• Live S strain (virulent) → mice died
• Heat-killed S strain → mice survived
• Heat-killed S + Live R → mice DIED ← KEY RESULT

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• A) Protein
• B) Polysaccharide capsule
• C) RNA
• D) DNA

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• A) ³⁵S was found in the pellet (bacteria), ³²P in supernatant
• B) ³²P was found in the pellet (bacteria), ³⁵S in supernatant
• C) Both ³²P and ³⁵S were in the pellet
• D) Both were in the supernatant

• ³²P (DNA) entered the bacteria → found in pellet after centrifugation
• ³⁵S (Protein) stayed outside → found in supernatant
• This conclusively proved DNA, not protein, is the genetic material

Memory trick: Phosphorus → Pellet → Passed into bacteria

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• A) ³²P and ³³S
• B) ³²P and ³⁵S
• C) ³¹P and ³⁵S
• D) ³³P and ³⁵S

• ³²P labels DNA (phosphate backbone)
• ³⁵S labels protein (sulfur in amino acids like cysteine & methionine)

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🔷 SUBTOPIC 1.2 — Structure of DNA (Watson & Crick Model)

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• A) Griffith and Avery
• B) Watson and Crick (1953)
• C) Meselson and Stahl
• D) Chargaff

• X-ray diffraction data by Rosalind Franklin & Maurice Wilkins
• Chargaff's rules (A=T, G=C)
• Chemical knowledge of nucleotides

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• A) A + T = G + C
• B) A = G and T = C (purines = purines)
• C) A = T and G = C
• D) Amount of purines > pyrimidines always

• A = T (linked by 2 hydrogen bonds)
• G = C (linked by 3 hydrogen bonds)
• Therefore: Purines (A+G) = Pyrimidines (T+C)

G≡C has 3 H-bonds → more stable, higher melting temp A=T has 2 H-bonds

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• A) 20%
• B) 40%
• C) 30%
• D) 60%

• A = 20%, so T = 20% (Chargaff's rule)
• A + T = 40%
• Remaining = G + C = 60%
• G = C = 30%

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• A) Hydrogen bonds between bases
• B) Phosphodiester bonds between sugar-phosphate units
• C) Peptide bonds
• D) Glycosidic bonds

• The sugar-phosphate backbone is the outer framework
• Adjacent nucleotides are joined by 3'→5' phosphodiester bonds
• Nitrogenous bases face inward and pair via H-bonds
• The two strands are antiparallel (one runs 5'→3', other 3'→5')

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• A) Adenine
• B) Guanine
• C) Cytosine
• D) Both A and B

Memory trick: PYrimidines are PYnter (smaller/thinner) — single ring PURines are PURe As Gold — double ring

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• A) They have opposite base sequences
• B) One strand runs 5'→3' and the other 3'→5'
• C) They are made of different types of nucleotides
• D) They differ in their sugar components

5'—A—T—G—C—3'   (Strand 1)
   |   |   |   |
3'—T—A—C—G—5'   (Strand 2)

• This antiparallel orientation is essential for replication and transcription
• DNA polymerase can only synthesize in 5'→3' direction

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• A) 1
• B) 2
• C) 3
• D) 4

G≡C = 3 H-bonds | A=T = 2 H-bonds

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• A) 3'-TACGAATC-5'
• B) 5'-TACGAATC-3'
• C) 3'-UACGAAUC-5'
• D) 5'-AUGCUUAG-3'

• Write complementary bases (A↔T, G↔C)
• The complement runs antiparallel, so 3'→5' under 5'→3'

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🔷 SUBTOPIC 1.3 — Packaging of DNA / Chromatin Structure

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• A) 4 histone proteins (H2A, H2B, H3, H4)
• B) 8 histone proteins — octamer of H2A, H2B, H3, H4
• C) Histone H1 only
• D) Non-histone proteins

• Nucleosome = ~200 bp DNA wound around a histone octamer
  • Core: 2 copies each of H2A, H2B, H3, H4 (= 8 histones)
  • ~146 bp of DNA winds around the core (~1.65 turns)
  • H1 is the linker histone — sits outside, helps compact chromatin further
• String of nucleosomes = "beads-on-a-string" (11 nm fiber)

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• A) H2A
• B) H2B
• C) H1
• D) H3

H1 = linker histone, sits between nucleosomes, NOT part of the core octamer

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• A) Glutamate and Aspartate
• B) Lysine and Arginine
• C) Glycine and Alanine
• D) Proline and Valine

• Histones are basic proteins — rich in Lysine and Arginine (+ve charge)
• DNA phosphate backbone is negatively charged
• Electrostatic interaction holds them together
• H3 & H4 are richest in arginine; H2B & H1 richest in lysine

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• A) Euchromatin is transcriptionally inactive
• B) Heterochromatin is loosely packed
• C) Euchromatin is lightly stained and transcriptionally active ✅
• D) Both are equally condensed

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• A) Ribosomes on mRNA
• B) Nucleosomes on the DNA strand
• C) Amino acids on a polypeptide
• D) Codons on mRNA

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🔷 SUBTOPIC 1.4 — DNA Replication (NEET Favourite!)

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• A) Conservative
• B) Semiconservative ✅
• C) Dispersive
• D) Bidirectional

• Bacteria grown in ¹⁵N (heavy) medium → all DNA is heavy (¹⁵N-¹⁵N)
• Shifted to ¹⁴N (light) medium → after 1 generation: ALL DNA = hybrid (¹⁵N-¹⁴N) → intermediate band
• After 2 generations: 50% hybrid + 50% light (¹⁴N-¹⁴N)
• This pattern = SEMICONSERVATIVE replication

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• A) DNA Polymerase I
• B) DNA Polymerase III
• C) Primase ✅
• D) Helicase

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• A) Exons
• B) Introns
• C) Okazaki fragments ✅
• D) Replicons

• DNA polymerase can ONLY work 5'→3'
• Leading strand: synthesized continuously in 5'→3' direction
• Lagging strand: synthesized discontinuously as short ~100-200 bp Okazaki fragments
• These are later joined by DNA ligase

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• A) Ori C
• B) ARS
• C) Telomere
• D) Centromere

In prokaryotes: oriC (origin of chromosomal replication) In eukaryotes: Multiple origins of replication called ARS (Autonomously Replicating Sequences)

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📝 QUICK REVISION TABLE

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Want me to continue with the next topics?

• Topic 2: Transcription (RNA synthesis, promoters, types of RNA)
• Topic 3: Genetic Code (codons, properties, wobble hypothesis)
• Topic 4: Translation (ribosomes, tRNA, protein synthesis)
• Topic 5: Regulation of Gene Expression (lac operon)
• Topic 6: Human Genome Project & DNA Fingerprinting

Just say "next" or name a specific topic!

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🧬 Molecular Basis of Inheritance

NCERT Pages 79–80 — Introduction & The Genetic Material

Deep NEET UG Level Questions

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🔶 SECTION A: THE NATURE OF GENETIC MATERIAL

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• A) How proteins are synthesized in a cell
• B) What is the chemical nature of the genetic material and how does it work
• C) Why DNA replication is semiconservative
• D) How chromosomes are packaged

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• A) It must be able to replicate faithfully
• B) It must be chemically and structurally stable
• C) It must be able to express itself as "Mendelian characters"
• D) It must always be double-stranded

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• A) HIV
• B) Bacteriophage T4
• C) Tobacco Mosaic Virus (TMV) ✅
• D) Adenovirus

• In TMV (Tobacco Mosaic Virus), the genetic material is single-stranded RNA
• This was one of the first proofs that not all organisms use DNA as genetic material
• Retroviruses (HIV) also have RNA, but TMV is the classic NCERT example
• This supports the idea that RNA was probably the first genetic material (RNA World Hypothesis)

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• A) RNA — because it can both store and express information
• B) DNA — because it is more stable due to deoxyribose sugar and absence of reactive -OH group
• C) RNA — because uracil is more stable than thymine
• D) DNA — because it is always double-stranded

Key NCERT line: "The 2'-OH group present at every nucleotide in RNA is a reactive group and makes RNA labile and easily degradable. DNA is more stable."

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• A) Ribozyme activity
• B) Retroviral activity
• C) Reverse transcription
• D) RNA world hypothesis

• Ribozymes = RNA molecules that act as enzymes (e.g., peptidyl transferase — the ribosomal rRNA that forms peptide bonds)
• RNA acts as:
  • Genetic material (in viruses)
  • Messenger (mRNA)
  • Structural (rRNA)
  • Adapter (tRNA)
  • Catalytic (ribozymes)
• This dual role supports the RNA World Hypothesis — RNA was the original molecule of life before DNA took over as genetic material

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• A) DNA evolved before RNA
• B) Proteins evolved before nucleic acids
• C) RNA was the first self-replicating and catalytic molecule; DNA evolved later
• D) All genetic information is stored in proteins

• The hypothesis states RNA was the primordial genetic molecule because it can:
  • Store information (like DNA)
  • Catalyze reactions (like enzymes)
• Over time, DNA took over as the primary genetic storage molecule due to greater stability
• Proteins became the primary catalysts
• This is supported by the fact that ribosomes (rRNA) catalyze peptide bond formation

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🔶 SECTION B: PROOF THAT DNA IS THE GENETIC MATERIAL

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• A) Neurospora crassa
• B) Streptococcus pneumoniae (Pneumococcus)
• C) Escherichia coli
• D) Tobacco Mosaic Virus

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• A) Mice survived; R strain remained non-virulent
• B) Mice survived; S strain was revived
• C) Mice died; live S strain was recovered from dead mice ✅
• D) Mice died; only R strain was recovered

Live R (non-virulent) + Heat-killed S (virulent) 
         ↓ Injected into mice
    MICE DIED 🐭
         ↓
Live S strain recovered from dead mice!

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• A) DNA from S strain entered R strain
• B) Protein from S strain transformed R strain
• C) Some "transforming principle" from heat-killed S strain transformed R strain into virulent S strain
• D) Heat-killed S strain was revived by R strain

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• A) Watson and Crick
• B) Hershey and Chase
• C) Avery, MacLeod and McCarty (1944) ✅
• D) Meselson and Stahl

• Protease → Transformation still occurred (protein is NOT the principle)
• RNase → Transformation still occurred (RNA is NOT the principle)
• DNase → Transformation ABOLISHED ✅ (DNA IS the principle!)

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• A) DNA only
• B) Proteins and RNA ✅
• C) Carbohydrates and lipids
• D) All biomolecules

• Treatment with Protease → transformation still happened → Proteins are NOT genetic material
• Treatment with RNase → transformation still happened → RNA is NOT genetic material
• Treatment with DNase → transformation stopped → DNA IS the genetic material
• So they eliminated proteins and RNA as candidates

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• A) φX174
• B) Lambda (λ) phage
• C) T2 bacteriophage ✅
• D) M13 phage

• Protein coat (capsid) — contains sulfur (S) → labelled with ³⁵S
• DNA core — contains phosphorus (P) → labelled with ³²P

DNA has phosphorus but NO sulfur. Protein has sulfur (in cysteine, methionine) but NO phosphorus. This is why these isotopes were chosen — they uniquely label each molecule.

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• A) Protein enters the bacterium and directs phage production
• B) DNA enters the bacterium and directs phage production ✅
• C) Both DNA and protein enter the bacterium
• D) Neither enters; bacteria make their own phage

• ³²P-DNA → inside bacteria (pellet) → DNA was injected into bacteria
• ³⁵S-protein → outside (supernatant) → protein coat stayed outside
• New phages produced inside bacterium contained ³²P → DNA was passed to progeny
• Conclusion: DNA is the genetic material that carries all information needed to produce new phages

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• A) To lyse the bacteria
• B) To break open the phage capsids
• C) To shear off the phage protein coats from the surface of bacteria ✅
• D) To denature the DNA

• Centrifuge → Heavy bacteria (with DNA inside) → pellet
• Phage protein ghosts → remain in supernatant

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• A) Watson & Crick → Avery et al. → Griffith → Hershey-Chase
• B) Griffith (1928) → Avery et al. (1944) → Hershey-Chase (1952) → Watson & Crick (1953) ✅
• C) Hershey-Chase → Griffith → Avery et al. → Watson & Crick
• D) Avery et al. → Griffith → Watson & Crick → Hershey-Chase

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🔶 SECTION C: ASSERTION-REASON TYPE (NEET 2024 Pattern)

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• A) Both A and R are true; R is the correct explanation of A ✅
• B) Both A and R are true; R is NOT the correct explanation
• C) A is true, R is false
• D) A is false, R is true

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• A) Both A and R are true; R is the correct explanation
• B) Both A and R are true; R is NOT the correct explanation ✅
• C) A is true, R is false
• D) A is false, R is true

• A is TRUE — heat-killed S alone does NOT kill mice
• R is PARTIALLY wrong — heat killed the cells and denatured proteins, but the DNA was still intact (that's why it could still transform R strain when mixed). The reason heat-killed S alone doesn't kill is that the bacteria are dead and cannot multiply, not because DNA was destroyed.

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• A) Both A and R are true
• B) A is true, R is false
• C) Both A and R are false ✅
• D) A is false, R is true

• ³⁵S labelled protein (NOT DNA) — sulfur is in amino acids cysteine and methionine
• ³²P labelled DNA (NOT protein) — phosphorus is in the DNA backbone
• DNA has NO sulfur; Protein has NO phosphorus (practically)

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🔶 SECTION D: MATCH THE FOLLOWING

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• A) A-1, B-2, C-3, D-4
• B) A-2, B-3, C-4, D-1 ✅
• C) A-3, B-4, C-1, D-2
• D) A-4, B-1, C-2, D-3

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• A) ³²P → Protein; ³⁵S → DNA
• B) ³²P → RNA; ³⁵S → Protein
• C) ³²P → DNA; ³⁵S → Protein ✅
• D) ³²P → DNA; ³⁵S → Carbohydrate

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🔶 SECTION E: TRICKY/HIGH-YIELD ONE-LINERS

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📊 MASTER SUMMARY TABLE — Pages 79–80

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Want to go deeper into the next NCERT pages?

• p. 80–82: DNA Structure (nucleotides, double helix, Watson-Crick model)
• p. 82–84: Packaging of DNA (nucleosome, histone, chromatin)
• p. 84–87: DNA Replication

Just say "next page" or "DNA structure questions"!