Genetic Material DNA Packaging

8 MCQs9-step worked example
Source: NCERT Genetics and EvolutionPYQ coverage: NEET 2020, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast updated: 25 Sep 2026

Genetic Material DNA Packaging, explained for NEET

A high-frequency trap in this chapter is mixing up which experiment actually proved DNA is the genetic material. Griffith (1928) only showed that something — a "transforming principle" — passed from heat-killed virulent S strain Streptococcus pneumoniae into live R strain, making the R strain virulent. He never identified the chemical. Avery, MacLeod and McCarty (1933–44) went further: they treated the transforming extract with proteases, RNases and DNase in turn. Only DNase stopped transformation, pointing to DNA — but this still left doubters. The unequivocal proof came from Hershey and Chase (1952): bacteriophages grown with radioactive phosphorus (labelling DNA) transferred radioactivity into infected bacteria, while phages grown with radioactive sulfur (labelling protein) did not. Since DNA carries phosphorus and protein does not, this showed DNA — not protein — enters the host and is inherited.

Separately, DNA packaging: a positively charged histone octamer (eight histone molecules, rich in lysine and arginine) is wrapped by negatively charged DNA to form one nucleosome, holding 200 bp of DNA. Nucleosomes give chromatin its "beads-on-string" look. Loosely packed, transcriptionally active chromatin is euchromatin; densely packed, inactive chromatin is heterochromatin.

Why DNA and not RNA for long-term storage? Both can replicate via complementary base pairing, but RNA's 2′-OH group makes it reactive and RNA is also catalytic — both make it unstable. DNA's thymine (over RNA's uracil) adds further stability. DNA is therefore chemically less reactive and structurally more stable, so it stores genetic information; RNA, being labile, is better suited to transmission and messenger roles. This asymmetry is also why RNA — not DNA — is thought to be the first genetic material (RNA World): RNA alone could once both store information and catalyse reactions, before DNA evolved from it for stability.

Watch out: "who proved it" (Griffith vs Avery-MacLeod-McCarty vs Hershey-Chase) and "200 bp vs 8 molecules" are two separate numbers from two separate sentences — do not swap them.

Can you answer these Genetic Material DNA Packaging MCQs?

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

A histone octamer, around which DNA is wrapped to form a nucleosome, is made up of how many histone molecules?

Show answer and why every option is right or wrong

Answer: A. Histones are organised into a unit of eight molecules called the histone octamer (NCERT Class 12 Biology, Chapter 5, page 83).

Why B is wrong: 200 is the number of DNA base pairs wrapped in one nucleosome, not the number of histone molecules in the octamer — the two figures come from the same sentence but describe different things.

Why C is wrong: Four is neither the histone-octamer count nor the bp count; it does not appear in the chapter's packaging description.

Why D is wrong: Ten is the approximate number of base pairs per turn of the DNA double helix (a different fact from section 5.1), not the histone count.

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MCQ 2Easy RecallPractice

How many base pairs of DNA helix does a typical nucleosome contain?

Show answer and why every option is right or wrong

Answer: B. A typical nucleosome contains 200 bp of DNA helix wrapped around the histone octamer (NCERT Class 12 Biology, Chapter 5, page 83).

Why A is wrong: Eight is the number of histone molecules in the octamer, not the bp count — swapping these two numbers is a common negative-marking trap.

Why C is wrong: 34 bp is not a chapter figure; 0.34 nm is the rise per base pair and 3.4 nm is the helix pitch, both length measures, not a bp count.

Why D is wrong: 2.2 metres is the total length of DNA in a typical mammalian cell, an unrelated packaging figure, not a per-nucleosome bp count.

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MCQ 3Easy RecallPractice

Which region of chromatin is loosely packed, stains light, and is transcriptionally active?

Show answer and why every option is right or wrong

Answer: C. Euchromatin is the loosely packed, light-staining, transcriptionally active region of chromatin (NCERT Class 12 Biology, Chapter 5, page 84).

Why A is wrong: Heterochromatin is the densely packed, dark-staining, transcriptionally inactive region — the opposite of what the stem describes.

Why B is wrong: The nucleoid is the region in prokaryotes like E. coli where DNA is held by proteins; prokaryotes lack a defined nucleus, so euchromatin/heterochromatin does not apply there.

Why D is wrong: Satellite DNA refers to repetitive DNA sequences separated during density-gradient centrifugation (used in DNA fingerprinting), not a chromatin-packing state.

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MCQ 4Direct ApplicationPractice

Griffith's 1928 experiment with heat-killed S strain and live R strain Streptococcus pneumoniae established which conclusion?

Show answer and why every option is right or wrong

Answer: D. Griffith showed that a transforming principle from the heat-killed S strain converted R strain to virulent S strain, but he did not determine its biochemical nature (NCERT Class 12 Biology, Chapter 5, page 84-85).

Why A is wrong: Identifying DNA specifically as the transforming principle is Avery-MacLeod-McCarty's 1933-44 conclusion, not Griffith's — a frequent scientist mix-up in this chapter.

Why B is wrong: This describes the logic of the Hershey-Chase phage experiment (1952), a different experiment on a different organism (bacteriophages, not pneumococcus).

Why C is wrong: This enzyme-digestion method (proteases, RNases, DNase) is Avery-MacLeod-McCarty's technique, not Griffith's mouse-injection experiment.

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MCQ 5Direct ApplicationPractice

Avery, MacLeod and McCarty treated the transforming extract from heat-killed S strain with protein-digesting enzymes, RNA-digesting enzymes, and DNA-digesting enzymes in separate trials. Which treatment blocked transformation of R strain into S strain?

Show answer and why every option is right or wrong

Answer: A. Digestion with DNase, but not proteases or RNases, inhibited transformation, showing the transforming substance was DNA (NCERT Class 12 Biology, Chapter 5, page 85).

Why B is wrong: Protease (protein-digesting enzyme) treatment did NOT affect transformation — this was the evidence used to rule protein out as the transforming principle, not confirm it.

Why C is wrong: RNase treatment also did NOT affect transformation, ruling out RNA as the transforming principle.

Why D is wrong: Only DNase blocked transformation; protease and RNase left transformation unaffected, so the three treatments did not behave equally.

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MCQ 6Direct ApplicationPractice

In the Hershey-Chase experiment, bacteriophages were grown separately in media containing radioactive phosphorus or radioactive sulfur. What did the results show?

Show answer and why every option is right or wrong

Answer: B. DNA contains phosphorus but not sulfur, and protein contains sulfur but not phosphorus; only the phosphorus-labelled (DNA) radioactivity entered the bacteria, proving DNA passes from virus to bacterium (NCERT Class 12 Biology, Chapter 5, page 85-86).

Why A is wrong: This reverses the actual result: sulfur labels protein, and protein-associated radioactivity stayed with the empty viral coats outside the bacteria, not inside them.

Why C is wrong: The two labels gave opposite results, not equal ones — that asymmetry is exactly what let Hershey and Chase distinguish DNA from protein.

Why D is wrong: Phosphorus-labelled (DNA) radioactivity DID enter the bacteria; only the sulfur-labelled (protein) radioactivity stayed outside.

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MCQ 7Concept TrapPractice

A genetic material must be stable enough to survive without changing across an organism's life cycle. Comparing DNA and RNA on this criterion, which statement is correct?

Show answer and why every option is right or wrong

Answer: C. RNA's 2′-OH group is reactive and RNA is also catalytic, making it labile; DNA is chemically less reactive and structurally more stable, and thymine in place of uracil adds further stability — so DNA is the preferred molecule for storing genetic information (NCERT Class 12 Biology, Chapter 5, page 87).

Why A is wrong: This reverses the actual comparison: chemical 'activity' (reactivity via the 2′-OH group) is exactly why RNA is LESS stable, not more.

Why B is wrong: Replication ability is one separate criterion (both DNA and RNA can duplicate via complementarity); stability is a different criterion on which DNA and RNA are NOT equal.

Why D is wrong: This also reverses the comparison: thymine adds stability compared to uracil, it does not reduce it — so this does not make RNA more stable.

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MCQ 8Concept TrapPractice

The RNA World hypothesis proposes that RNA, not DNA, was the first genetic material. Which fact from the chapter best supports this?

Show answer and why every option is right or wrong

Answer: D. RNA used to act as a genetic material as well as a catalyst; because this catalytic activity made it reactive and unstable, DNA evolved from RNA with chemical modifications for greater stability (NCERT Class 12 Biology, Chapter 5, page 88).

Why A is wrong: This base difference is a structural fact about DNA vs RNA composition, not the reason RNA is believed to be the earlier, more primitive genetic material.

Why B is wrong: This describes present-day transcription (DNA to RNA), the reverse chronological direction from what the RNA World hypothesis proposes for life's origin.

Why C is wrong: The 2′-OH group is the chemical feature that makes RNA reactive and unstable; on its own it explains RNA's instability, not why RNA came first.

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How do you solve a Genetic Material DNA Packaging question? A worked example

  1. 1

    Given

    Bacteriophages were grown in two separate batches: one in a medium containing radioactive phosphorus (³²P), the other in a medium containing radioactive sulfur (³⁵S). Both radioactive batches were then allowed to infect separate cultures of E. coli, agitated in a blender to strip off the empty viral coats, and centrifuged to separate virus particles from bacteria.

  2. 2

    Required

    Determine whether DNA or protein is the genetic material that physically enters the bacterial cell during phage infection.

  3. 3

    Concept

    DNA contains phosphorus but no sulfur; protein contains sulfur but no phosphorus. Labelling phages with one radioactive element at a time marks either their DNA or their protein exclusively, so tracking which label ends up inside the bacteria reveals which molecule entered.

  4. 4

    Formula

    Not applicable — this is a tracer-logic experiment, not a calculation.

  5. 5

    Substitution

    ³²P labels phage DNA only (phosphorus is in DNA, not protein). ³⁵S labels phage protein only (sulfur is in protein, not DNA). After infection and blending, bacteria infected with ³²P-phages were tested for radioactivity, and bacteria infected with ³⁵S-phages were tested separately.

  6. 6

    Calculation

    Bacteria infected with ³²P-labelled phages were found to be radioactive (DNA had entered). Bacteria infected with ³⁵S-labelled phages were not radioactive (protein had stayed in the emptied viral coats, removed by blending).

  7. 7

    Final answer

    DNA — not protein — is the genetic material that passes from bacteriophage into the bacterium, giving the unequivocal proof that eluded Griffith's and Avery-MacLeod-McCarty's earlier experiments.

  8. 8

    Common trap

    Do not credit this "unequivocal proof" result to Griffith (who only showed a transforming principle existed, without identifying it) or to Avery-MacLeod-McCarty (who showed DNA alone causes transformation biochemically, but left some biologists unconvinced) — only Hershey-Chase (1952) is described as unequivocal in the chapter.

  9. 9

    Similar NEET-style question

    If phages were instead labelled with radioactive carbon (¹⁴C), which is present in both DNA and protein, would this isotope be able to distinguish DNA from protein the way ³²P and ³⁵S did? (Answer: No. ¹⁴C is present in both DNA and protein, so radioactivity from a ¹⁴C label would appear in bacteria regardless of whether DNA or protein entered — the experiment's power comes specifically from ³²P marking DNA only and ³⁵S marking protein only, an exclusivity ¹⁴C does not have.)

What to remember before solving Genetic Material DNA Packaging questions

Key Fact

Packaging of DNA Helix

The distance between two consecutive base pairs in DNA is 0.34 nm. Multiplying the total base pairs in a typical mammalian cell (6.6 x 10^9 bp) by 0.34 x 10^-9 m/bp gives a DNA length of approximately 2.2 metres, far greater than the nucleus dimension of about 10^-6 m. In prokaryotes such as E. coli, which lack a defined nucleus, negatively charged DNA is held with positively charged proteins in a region called the 'nucleoid', organised in large loops held by proteins.

-- NCERT Class 12 Biology, Ch. 5, p. 83
Key Fact

Packaging of DNA Helix

In eukaryotes, positively charged basic proteins called histones, rich in the basic amino acid residues lysine and arginine, organise into a unit of eight molecules called a histone octamer. Negatively charged DNA wraps around the positively charged histone octamer to form a nucleosome; a typical nucleosome contains 200 bp of DNA helix. Nucleosomes are the repeating unit of chromatin and appear as a 'beads-on-string' structure under the electron microscope (EM).

-- NCERT Class 12 Biology, Ch. 5, p. 83
Key Fact

Packaging of DNA Helix

The beads-on-string chromatin is packaged into chromatin fibers, further coiled and condensed at the metaphase stage of cell division to form chromosomes. Higher-level packaging requires additional proteins collectively called Non-histone Chromosomal (NHC) proteins. In a typical nucleus, loosely packed, lightly staining chromatin is called euchromatin, and densely packed, dark-staining chromatin is called heterochromatin. Euchromatin is transcriptionally active, whereas heterochromatin is inactive.

-- NCERT Class 12 Biology, Ch. 5, p. 84
Key Fact

Transforming Principle (Griffith)

In 1928, Frederick Griffith worked with Streptococcus pneumoniae, which forms smooth (S, mucous/polysaccharide coat, virulent) or rough (R, no coat, non-virulent) colonies. Mice infected with live S strain die; mice infected with live R strain or heat-killed S strain alone do not. When Griffith injected a mixture of heat-killed S and live R bacteria, the mice died, and he recovered living S bacteria from them. He concluded the R strain had been transformed by a 'transforming principle' transferred from the heat-killed S strain, enabling R strain to synthesise a smooth polysaccharide coat and become virulent.

-- NCERT Class 12 Biology, Ch. 5, p. 84
Key Fact

Biochemical Characterisation of Transforming Principle

Oswald Avery, Colin MacLeod and Maclyn McCarty (1933-44) purified biochemicals (proteins, DNA, RNA) from heat-killed S cells to find which could transform live R cells into S cells. They discovered that DNA alone from S bacteria caused transformation; protein-digesting enzymes (proteases) and RNA-digesting enzymes (RNases) did not affect transformation, but digestion with DNase did inhibit it. They concluded that DNA is the hereditary material, though not all biologists were convinced at the time.

-- NCERT Class 12 Biology, Ch. 5, p. 85
Key Fact

The Genetic Material is DNA (Hershey-Chase)

Alfred Hershey and Martha Chase (1952) worked with bacteriophages. They grew viruses on media with radioactive phosphorus (labels DNA, which contains phosphorus) or radioactive sulfur (labels protein, which contains sulfur but not DNA). Radioactive phages were attached to E. coli; viral coats were then removed by agitation in a blender, and virus particles separated by centrifugation. Bacteria infected with radioactive-DNA phages were radioactive, while bacteria infected with radioactive-protein phages were not, showing DNA is the genetic material passed from virus to bacteria.

-- NCERT Class 12 Biology, Ch. 5, p. 85
Key Fact

Properties of Genetic Material (DNA versus RNA)

A genetic material must be able to replicate, be chemically and structurally stable, provide scope for mutation needed for evolution, and be able to express itself as 'Mendelian Characters'. Both DNA and RNA can direct duplication via base pairing and complementarity. The 2'-OH group present at every nucleotide in RNA is a reactive group that makes RNA labile and easily degradable, and RNA is also catalytic, hence reactive; DNA is chemically less reactive and structurally more stable than RNA, making DNA the better genetic material. The presence of thymine in place of uracil also confers additional stability to DNA.

-- NCERT Class 12 Biology, Ch. 5, p. 87
Key Fact

RNA World

RNA was the first genetic material. Essential life processes such as metabolism, translation and splicing evolved around RNA. RNA acted both as genetic material and as a catalyst (some important biochemical reactions in living systems are catalysed by RNA catalysts, not protein enzymes), but RNA as a catalyst was reactive and hence unstable. Therefore DNA evolved from RNA with chemical modifications that make it more stable; being double stranded with a complementary strand, DNA further resists change by evolving a process of repair.

-- NCERT Class 12 Biology, Ch. 5, p. 88

Where do students lose marks on Genetic Material DNA Packaging?

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

Griffith vs Avery-MacLeod-McCarty vs Hershey-Chase mixup

Category: Similar Terms

NEET questions swap which experiment proved DNA is the genetic material versus which only showed a 'transforming principle' existed.

When it triggers

A question describes the heat-killed S + live R mouse experiment and asks who proved DNA is the genetic material, expecting Griffith when it was Avery-MacLeod-McCarty (biochemical proof) or Hershey-Chase (unequivocal proof with bacteriophages).

How to avoid

Griffith (1928) only showed a 'transforming principle' existed without identifying its biochemical nature; Avery-MacLeod-McCarty (1933-44) showed DNA alone causes transformation; Hershey-Chase (1952) gave the unequivocal proof using radiolabeled phosphorus/sulfur in bacteriophages.

Trap

Nucleosome bp count vs histone octamer count confusion

Category: Negative Marking

Students confuse the number of DNA base pairs per nucleosome (200 bp) with the number of histone molecules per octamer (8).

When it triggers

A question asks 'how many base pairs are wrapped in a typical nucleosome' and offers 8 as a distractor, or asks the number of histone molecules and offers 200.

How to avoid

A histone octamer is made of eight histone molecules; a typical nucleosome (DNA wrapped around that octamer) contains 200 bp of DNA helix — these are two distinct numbers from the same sentence.

Mistake

Students attribute the unequivocal proof that DNA is the genetic material to Griffith or to Avery-MacLeod-McCarty instead of Hershey and Chase.

Root cause: term confusion

Correction

Griffith (1928) demonstrated a 'transforming principle' without identifying it biochemically; Avery-MacLeod-McCarty (1933-44) showed DNA alone causes transformation biochemically; Hershey and Chase (1952) gave the unequivocal proof using radiolabeled bacteriophages.

Wrong option pattern

selects Griffith or Avery-MacLeod-McCarty as the 'unequivocal proof' source instead of Hershey-Chase

Mistake

Students believe RNA is more chemically stable than DNA because RNA is more 'active' in cells, reversing the actual stability comparison.

Root cause: concept gap

Correction

DNA is chemically less reactive and structurally more stable than RNA because RNA's 2'-OH group is reactive and RNA is also catalytic; the presence of thymine instead of uracil confers additional stability to DNA, making DNA the preferred molecule for storage of genetic information.

Wrong option pattern

claims RNA is more stable than DNA or that thymine is less stabilising than uracil

More in Genetics and Evolution: 17 exam traps and mistakes · 4 formulas · 1 question pattern from its other lessons.

Genetic Material DNA Packaging questions from past NEET papers

9 questions from NEET 2020, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.

NEET 2025

Given below are two statements : Statement I : In the RNA world, RNA is considered the first genetic material evolved to carry out essential life processes. RNA acts as a genetic material and also as a catalyst for some important biochemical reactions in living systems. Being reactive, RNA is unstable. Statement II : DNA evolved from RNA and is a more stable genetic material. Its double helical strands being complementary, resist changes by evolving repairing mechanism. In the light of the above statements, choose the most appropriate answer from the options given below :

1Statement I is incorrect but statement II is correct
2Both statement I and statement II are correct
3Both statement I and statement II are incorrect
4Statement I is correct but statement II is incorrect
NTA Answer: Option 2(final)

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NEET 2025

Match List I with List II: List-I List-II A. Alfred Hershey and Martha Chase I. Streptococcus pneumoniae B. Euchromatin II. Densely packed and dark-stained C. Frederick Griffith III. Loosely packed and light-stained D. Heterochromatin IV. DNA as genetic material confirmation Choose the correct answer from the options given below:

1A-III, B-II, C-IV, D-I
2A-II, B-IV, C-I, D-III
3A-IV, B-II, C-I, D-III
4A-IV, B-III, C-I, D-II
NTA Answer: Option 4(final)

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NEET 2023

Given below are two statements: Statement I: In prokaryotes, the positively charged DNA is held with some negatively charged proteins in a region called nucleoid. Statement II: In eukaryotes, the negatively charged DNA is wrapped around the positively charged histone octamer to form nucleosome. In the light of the above statements, choose the correct answer from the options given below:

1Both Statement I and Statement II are false.
2Statement I is correct but Statement II is false.
3Statement I is incorrect but Statement II is true.
4Both Statement I and Statement II are true.
NTA Answer: Option 3(final)

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NEET 2022

Read the following statements and choose the set of correct statements : (a) Euchromatin is loosely packed chromatin (b) Heterochromatin is transcriptionally active (c) Histone octomer is wrapped by negatively charged DNA in nucleosome (d) Histones are rich in lysine and arginine (e) A typical nucleosome contains 400 bp of DNA helix Choose the correct answer from the options given below :

1(a), (c), (e) Only
2(b), (d), (e) Only
3(a), (c), (d) Only
4(b), (e) Only
NTA Answer: Option 3(final)

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Sources

Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.

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