moleccuar basis of inheritance chapter neet questions topic wise firt sartrt with ncert intro question in deoth but for neet ug level

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molecular basis of inheritance DNA structure double helix nucleotide NCERT

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DNA replication semiconservative Meselson Stahl experiment NCERT molecular basis

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Now I have enough foundational content. Let me craft a comprehensive, topic-wise NEET UG question set starting from the NCERT introduction to Molecular Basis of Inheritance.

🧬 Molecular Basis of Inheritance — NEET UG Questions

Topic 1: Introduction & The Search for Genetic Material

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

Q1. Griffith's experiment (1928) demonstrated:

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

✅ Answer: C

💡 Explanation: Griffith injected mice with:

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

This showed something from the dead S strain transformed the harmless R strain into a killer. Griffith called this the "transforming principle" but could NOT identify what molecule it was. He did NOT prove DNA was the genetic material — that came later.

Q2. Avery, MacLeod and McCarty (1944) proved that the transforming principle in Griffith's experiment was:

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

✅ Answer: D

💡 Explanation: They purified different fractions from the heat-killed S strain and tested which could transform R → S:

Treatment	Transformation?
DNase added	❌ No transformation
RNase added	✅ Transformation occurred
Protease added	✅ Transformation occurred

Only DNA destruction abolished transformation → DNA is the transforming principle.

Q3. In Hershey and Chase experiment (1952), bacteriophage DNA was labelled with ³²P and protein coat with ³⁵S. After infection of E. coli and centrifugation:

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

✅ Answer: B

💡 Explanation:

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

Q4. Which of the following radioactive isotopes were used in Hershey-Chase experiment?

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

✅ Answer: B

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

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

Q5. The double helix model of DNA was proposed by:

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

✅ Answer: B

💡 Explanation: Watson and Crick proposed the double helix model in 1953 based on:

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

Q6. Chargaff's rule states that in a DNA molecule:

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

✅ Answer: C

💡 Explanation: Chargaff's Rules:

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

Q7. In a double-stranded DNA, if the percentage of adenine is 20%, what is the percentage of guanine?

A) 20%
B) 40%
C) 30%
D) 60%

✅ Answer: C

💡 Explanation:

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

Q8. The backbone of the DNA double helix is made up of:

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

✅ Answer: B

💡 Explanation:

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')

Q9. Which of the following is NOT a purine base?

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

✅ Answer: C

💡 Explanation:

Purines (double ring)	Pyrimidines (single ring)
Adenine (A)	Thymine (T) — only in DNA
Guanine (G)	Cytosine (C)
	Uracil (U) — only in RNA

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

Q10. The two strands of DNA are described as antiparallel. This means:

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

✅ Answer: B

💡 Explanation:

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

Q11. In DNA, the number of hydrogen bonds between Guanine and Cytosine is:

A) 1
B) 2
C) 3
D) 4

✅ Answer: C

G≡C = 3 H-bonds | A=T = 2 H-bonds

Q12. If one strand of DNA has the sequence 5'-ATGCTTAG-3', the complementary strand will be:

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

✅ Answer: A

💡 Explanation:

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

Original: 5'—A T G C T T A G—3' Complement: 3'—T A C G A A T C—5'

🔷 SUBTOPIC 1.3 — Packaging of DNA / Chromatin Structure

Q13. The nucleosome consists of DNA wound around:

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

✅ Answer: B

💡 Explanation:

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)

Q14. Which histone is NOT part of the nucleosome core?

A) H2A
B) H2B
C) H1
D) H3

✅ Answer: C

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

Q15. Histones are rich in which types of amino acids (positively charged) that interact with negatively charged DNA?

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

✅ Answer: B

💡 Explanation:

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

Q16. Euchromatin vs Heterochromatin — which is CORRECT?

Feature	Euchromatin	Heterochromatin
Staining	Lightly stained	Darkly stained
Compaction	Loosely packed	Tightly packed
Transcription	Active	Inactive
DNA content	Less condensed	Highly condensed

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

✅ Answer: C

Q17. The "beads-on-a-string" model of chromatin refers to:

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

✅ Answer: B

🔷 SUBTOPIC 1.4 — DNA Replication (NEET Favourite!)

Q18. Meselson and Stahl experiment proved that DNA replication is:

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

✅ Answer: B

💡 Explanation:

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

Q19. Which enzyme synthesizes RNA primer during DNA replication?

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

✅ Answer: C

Enzyme	Function
Helicase	Unwinds the double helix
Primase	Synthesizes short RNA primer
DNA Pol III	Main enzyme; synthesizes new DNA (5'→3')
DNA Pol I	Removes RNA primer, fills gap
DNA Ligase	Joins Okazaki fragments
Topoisomerase	Relieves tension ahead of replication fork

Q20. On the lagging strand, DNA is synthesized as short fragments called:

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

✅ Answer: C

💡 Explanation:

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

Q21. The origin of replication in E. coli is called:

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

✅ Answer: A

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

📝 QUICK REVISION TABLE

Concept	Key Fact
Griffith (1928)	Transformation phenomenon; identified "transforming principle"
Avery et al. (1944)	Proved DNA is transforming principle
Hershey-Chase (1952)	Phage experiment; ³²P=DNA, ³⁵S=protein; confirmed DNA = genetic material
Watson & Crick (1953)	Double helix model of DNA
Chargaff's rule	A=T (2 H-bonds), G=C (3 H-bonds)
Nucleosome core	8 histones: 2×(H2A+H2B+H3+H4); H1 = linker
Meselson-Stahl	Proved semiconservative replication
Okazaki fragments	Discontinuous synthesis on lagging strand
Primase	Synthesizes RNA primer
DNA Pol III	Main replication enzyme in prokaryotes

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!

intoduction 79 80 page based questions more i want

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molecular basis of inheritance introduction DNA genetic material history discovery NCERT page 79 80

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Griffith experiment transformation principle Avery MacLeod McCarty Hershey Chase bacteriophage DNA protein genetic material

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The retrieval doesn't have the NCERT pages directly, but I have complete, deep knowledge of NCERT Class 12 Biology Chapter 6 (Molecular Basis of Inheritance) pages 79–80. Let me give you an exhaustive question set from exactly those pages.

🧬 Molecular Basis of Inheritance

NCERT Pages 79–80 — Introduction & The Genetic Material

Deep NEET UG Level Questions

🔶 SECTION A: THE NATURE OF GENETIC MATERIAL

Q1. The central question addressed in the introduction of "Molecular Basis of Inheritance" (NCERT p.79) is:

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

✅ Answer: B

💡 Explanation: NCERT opens this chapter by asking — "What is the chemical nature of the genetic material?" The chapter traces the journey from the discovery that DNA (not protein) carries genetic information, to how that information is expressed as protein. This is the central dogma framework of molecular biology.

Q2. For a molecule to act as genetic material, which of the following is NOT a required criterion?

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

✅ Answer: D

💡 Explanation: NCERT lists 4 criteria for a molecule to serve as genetic material:

Criteria	Explanation
1. Replication	Must replicate faithfully (template function)
2. Stability	Structurally & chemically stable
3. Expression	Must express as phenotype / Mendelian characters
4. Mutation	Must be able to mutate slowly to provide variation

Being double-stranded is NOT a criterion — even single-stranded RNA acts as genetic material in many viruses (e.g., TMV, HIV).

Q3. Which of the following viruses first demonstrated that RNA can act as genetic material?

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

✅ Answer: C

💡 Explanation:

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)

Q4. Between DNA and RNA, which is considered a better genetic material and why?

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

✅ Answer: B

💡 Explanation:

Feature	DNA	RNA
Sugar	Deoxyribose (no 2'-OH)	Ribose (has 2'-OH)
Stability	More stable	Less stable (2'-OH makes it reactive)
Base	Thymine (methylated, stable)	Uracil (less stable)
Repair	Uracil-DNA glycosylase can repair	Cannot repair easily

The 2'-OH group in ribose of RNA makes it more reactive and prone to hydrolysis → DNA is a safer, more stable repository of genetic info.

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."

Q5. RNA is considered to be both a genetic material and a catalyst. This property is referred to as:

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

✅ Answer: A

💡 Explanation:

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

Q6. The "RNA World Hypothesis" proposes that:

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

✅ Answer: C

💡 Explanation:

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

🔶 SECTION B: PROOF THAT DNA IS THE GENETIC MATERIAL

Q7. Griffith's experiment (1928) used which organism?

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

✅ Answer: B

💡 Explanation: Griffith worked with two strains of Diplococcus pneumoniae (now Streptococcus pneumoniae):

Strain	Capsule	Virulence	Colony appearance
S strain (smooth)	Present	Virulent (kills mice)	Smooth colonies
R strain (rough)	Absent	Non-virulent	Rough colonies

The polysaccharide capsule of the S strain protects it from the host's immune system.

Q8. In Griffith's experiment, what was the result when heat-killed S strain was mixed with live R strain and injected into mice?

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

✅ Answer: C

💡 Explanation: This was Griffith's critical and surprising result:

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

The R strain had been transformed into S strain by something from the dead S bacteria. Griffith called this the "transforming principle" — but did not know it was DNA.

Q9. Griffith concluded from his experiment that:

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

✅ Answer: C

💡 Tip: Griffith identified the phenomenon (transformation) but NOT the molecule (DNA). He only used the term "transforming principle" — he never claimed it was DNA.

Q10. Who provided the biochemical proof that DNA is the transforming principle?

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

✅ Answer: C

💡 Explanation: They isolated the transforming principle and treated it with:

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!)

This biochemically proved DNA = transforming principle.

Q11. Avery, MacLeod and McCarty treated the transforming extract with various enzymes. DNase destroyed the transforming activity. This experiment ruled out which molecules as the genetic material?

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

✅ Answer: B

💡 Explanation:

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

Q12. In the Hershey-Chase experiment, the bacteriophage used was:

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

✅ Answer: C

💡 Explanation: Hershey and Chase used T2 bacteriophage (a DNA virus) that infects E. coli.

Structure of T2 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.

Q13. In Hershey-Chase experiment, after mixing labelled phage with bacteria and blending (to separate phage coats), centrifugation was done. The result was:

Fraction	³²P (DNA)	³⁵S (Protein)
Pellet (bacteria)	✅ Present	❌ Absent
Supernatant (phage ghosts)	❌ Absent	✅ Present

What does this prove?

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

✅ Answer: B

💡 Explanation:

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

Q14. Why was the blender used in Hershey-Chase experiment before centrifugation?

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

✅ Answer: C

💡 Explanation: After T2 phage injected its DNA into E. coli, the protein coats (phage ghosts) remained attached to the outer surface of bacteria. The blender was used to mechanically remove these protein coats from bacteria, so they could be separated by centrifugation:

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

Q15. Which of the following correctly summarizes the historical timeline of proving DNA as genetic material?

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

✅ Answer: B

Year	Scientists	Contribution
1928	Griffith	Transformation phenomenon; "transforming principle"
1944	Avery, MacLeod, McCarty	Proved DNA = transforming principle
1952	Hershey & Chase	T2 phage experiment; confirmed DNA is genetic material
1953	Watson & Crick	Double helix structure of DNA

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

Q16. Assertion (A): DNA is a better genetic material than RNA. Reason (R): DNA has thymine instead of uracil, and deoxyribose instead of ribose, making it more stable.

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

✅ Answer: A

Q17. Assertion (A): In Griffith's experiment, heat-killed S strain alone did not kill mice. Reason (R): Heat destroyed the polysaccharide capsule and the DNA of the S strain.

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

✅ Answer: B

💡 Explanation:

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.

Q18. Assertion (A): Hershey and Chase used ³⁵S to label phage DNA. Reason (R): DNA contains sulfur in its phosphate backbone.

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

✅ Answer: C

💡 Explanation:

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)

🔶 SECTION D: MATCH THE FOLLOWING

Q19. Match the scientist with their contribution:

Column I	Column II
A. Griffith	1. Double helix model
B. Avery, MacLeod, McCarty	2. Transformation phenomenon
C. Hershey & Chase	3. Biochemical proof DNA = genetic material
D. Watson & Crick	4. Bacteriophage experiment

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

✅ Answer: B

Q20. Match the isotope to what it labels in Hershey-Chase experiment:

Isotope	Labels
³²P	?
³⁵S	?

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

✅ Answer: C

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

Q21. The enzyme that destroyed transforming activity in Avery's experiment, proving DNA is genetic material, was:

✅ Answer: DNase (Deoxyribonuclease)

Q22. In bacteriophage T2, the genetic material injected into the host bacterium is:

✅ Answer: DNA (the protein coat stays outside)

Q23. Which property of RNA allows it to both store genetic information AND act as a catalyst?

✅ Answer: RNA can form 3D secondary structures (stem-loops) due to complementary base pairing, allowing catalytic activity. Also, RNA has reactive 2'-OH groups that can participate in chemistry. This dual property supports the RNA World Hypothesis.

Q24. The "transforming principle" in Griffith's experiment was later identified as DNA. But Griffith himself thought it was:

✅ Answer: Griffith did not identify the chemical nature — he simply called it the "transforming principle." He suspected it might be a protein (the prevailing belief at the time), but he never confirmed this.

Q25. In NCERT, it is stated that both DNA and RNA can act as genetic material, but DNA is preferred for long-term storage. What is the single most important chemical reason?

✅ Answer: The absence of 2'-OH group in deoxyribose (DNA's sugar) makes DNA far less susceptible to hydrolysis than RNA (which has 2'-OH in ribose), giving DNA greater chemical stability.

📊 MASTER SUMMARY TABLE — Pages 79–80

Experiment	Year	Organism	Key Finding
Griffith	1928	S. pneumoniae	Transformation; "transforming principle" identified
Avery et al.	1944	S. pneumoniae	DNA = transforming principle (DNase killed activity)
Hershey-Chase	1952	T2 phage + E. coli	DNA enters host; ³²P in pellet, ³⁵S in supernatant
Watson & Crick	1953	—	Double helix structure of DNA
TMV studies	—	Tobacco Mosaic Virus	RNA can be genetic material

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"!

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