Molecular Basis of Inheritance — NEET Botany MCQs with answers

Molecular Basis of Inheritance is a Class 12 Botany chapter in the NEET (UG) syllabus. NEET720 has 1,314 reviewed practice questions on it, each with a quick answer and a step-by-step explanation. The 8 questions below are free and fixed, so you can bookmark this page; the full chapter, plus mistake tracking and spaced revision, is in the app.

Topics covered

DNA Structure and Packaging · Search for Genetic Material and Replication · Transcription · Genetic Code and Translation · Regulation of Gene Expression · Human Genome Project and DNA Fingerprinting · RNA · Search for genetic material · Human Genome Project and genome organisation · RNA processing · Origin of life / RNA world · Genome organisation · Mutation at the molecular level · Nucleic acid chemistry · DNA vs RNA · Gene Expression · Transcription Unit · Genetics · Molecular Basis of Inheritance · Packaging of DNA · RNA structure · Molecular basis - historical concepts · Molecular basis - biotechnology connection · Molecular basis - genetic material properties · DNA Replication · Transcription and Translation · Gene Regulation · Genetic Code · Human Genome Project · DNA Fingerprinting · Translation · Central Dogma · Discovery of DNA as Genetic Material · Chromatin Organisation · DNA Structure · Mutation · DNA Structure and Replication · Gene Regulation and Translation · Mutation and Translation · DNA Replication and Genetic Fingerprinting

8 free Molecular Basis of Inheritance practice questions with answers

Choose an answer in your head before opening it. Each explanation says why the correct option is right and, where relevant, why the tempting wrong option is wrong.

1. Question 1 · easy · DNA Structure and Packaging

   In the Watson-Crick model of DNA, how many hydrogen bonds hold together a guanine-cytosine (G-C) base pair, and how does this compare with an adenine-thymine (A-T) pair?

   • A.G-C has 2 hydrogen bonds, same as A-T
   • B.G-C has 3 hydrogen bonds, one more than the 2 in A-T, making G-C pairing more thermally stable
   • C.G-C has 2 hydrogen bonds while A-T has 3, making A-T more stable
   • D.Both G-C and A-T are held by 4 hydrogen bonds each
   Show answer and explanation

   Answer: B. G-C has 3 hydrogen bonds, one more than the 2 in A-T, making G-C pairing more thermally stable

   Guanine pairs with cytosine via 3 hydrogen bonds while adenine pairs with thymine via 2 hydrogen bonds; the extra bond makes G-C pairs more thermally stable.

   Watson and Crick proposed that the two antiparallel strands of DNA are held together by specific hydrogen bonding between purines and pyrimidines: adenine (purine) always pairs with thymine (pyrimidine) through 2 hydrogen bonds, and guanine (purine) always pairs with cytosine (pyrimidine) through 3 hydrogen bonds. Because G-C pairs have one additional hydrogen bond, DNA regions rich in G-C content require more energy (higher temperature) to denature/melt than A-T rich regions. Option A and D misstate the bond counts; option C swaps which pair has more bonds.

   Common mistake: Assuming all base pairs share the same number of hydrogen bonds.

   Key point: A-T = 2 H-bonds; G-C = 3 H-bonds — G-C pairing is more thermally stable.

2. Question 2 · easy · DNA Structure and Packaging

   According to the Watson-Crick double helix model, what is the approximate diameter of the DNA helix and the number of base pairs present in one complete turn?

   • A.34 Å diameter; 20 base pairs per turn
   • B.20 Å diameter; 34 base pairs per turn
   • C.20 Å diameter; 10 base pairs per turn (turn length ~34 Å)
   • D.10 Å diameter; 34 base pairs per turn
   Show answer and explanation

   Answer: C. 20 Å diameter; 10 base pairs per turn (turn length ~34 Å)

   The DNA double helix has a diameter of about 20 Å, and one complete turn spans about 34 Å containing approximately 10 base pairs.

   Watson-Crick's model gives precise structural constants: helix diameter ≈ 20 Å (2 nm), and each full turn of the helix is ≈ 34 Å long and contains ≈ 10 base pairs (so successive bases are separated by ≈ 3.4 Å along the axis). Options A and B confuse which number belongs to diameter versus turn length/bp count; option D uses an implausibly small diameter.

   Common mistake: Mixing up the 20 Å diameter value with the 34 Å turn-length value.

   Key point: Diameter ≈ 20 Å; one turn ≈ 34 Å ≈ 10 bp.

3. Question 3 · medium · DNA Structure and Packaging

   The histone octamer that forms the core of a nucleosome consists of two molecules each of which four histone proteins?

   • A.H1, H2A, H2B, H3
   • B.H2A, H2B, H3, H4, H1
   • C.H1, H3, H4, and a non-histone protein
   • D.H2A, H2B, H3, H4
   Show answer and explanation

   Answer: D. H2A, H2B, H3, H4

   The nucleosome core (histone octamer) is made of two copies each of H2A, H2B, H3 and H4; H1 is a separate linker histone, not part of the octamer.

   A nucleosome's core particle consists of eight histone molecules — two copies each of H2A, H2B, H3, and H4 — around which about 146-200 bp of negatively charged DNA is wrapped. Histone H1 is distinct: it is a linker histone that binds DNA between adjacent nucleosomes (at the entry/exit points) rather than forming part of the octamer core. Options A, B and C incorrectly fold H1 into the octamer.

   Common mistake: Including H1 as a fifth histone within the octamer core.

   Key point: Octamer = 2×(H2A+H2B+H3+H4); H1 is the separate linker histone, not part of the octamer.

4. Question 4 · medium · DNA Structure and Packaging

   A phosphodiester bond linking two nucleotides in a DNA strand connects which two carbon positions of the adjacent sugars?

   • A.The 2' carbon of one sugar to the 3' carbon of the next sugar
   • B.The 3' carbon of one sugar to the 5' carbon of the next sugar, via a phosphate group
   • C.The 5' carbon of one sugar to the 5' carbon of the next sugar directly
   • D.The nitrogenous base of one nucleotide to the sugar of the next nucleotide
   Show answer and explanation

   Answer: B. The 3' carbon of one sugar to the 5' carbon of the next sugar, via a phosphate group

   A phosphodiester bond joins the 3' carbon of one deoxyribose to the 5' carbon of the next deoxyribose through an intervening phosphate group, building the sugar-phosphate backbone.

   Each nucleotide is a nitrogenous base + pentose sugar + phosphate group. Polynucleotide chains form when the phosphate group bridges the 3'-OH of one sugar to the 5'-phosphate of the next sugar, creating a phosphodiester bond. Repetition of this linkage builds the continuous, directional sugar-phosphate backbone on the outside of the helix, with bases projecting inward. Option A wrongly invokes the 2' carbon (absent in deoxyribose's relevant chemistry here); C proposes an impossible 5'-5' direct link; D confuses base attachment (glycosidic bond) with the backbone linkage.

   Common mistake: Confusing the glycosidic bond (base-sugar) with the phosphodiester bond (sugar-phosphate-sugar).

   Key point: Phosphodiester bond = 3'-OH of one sugar to 5'-phosphate of next sugar.

5. Question 5 · medium · DNA Structure and Packaging

   What is the role of histone H1 in chromatin organization, and how does it differ from the histones forming the nucleosome core?

   • A.H1 wraps the DNA around itself to form the nucleosome core, just like H2A, H2B, H3, H4
   • B.H1 is a non-histone acidic protein that has no direct DNA-binding role
   • C.H1 degrades excess DNA between nucleosomes to shorten the chromatin fibre
   • D.H1 is a linker histone that binds to DNA between two adjacent nucleosomes, helping organize/seal the linker DNA, whereas core histones form the octamer around which DNA is wrapped
   Show answer and explanation

   Answer: D. H1 is a linker histone that binds to DNA between two adjacent nucleosomes, helping organize/seal the linker DNA, whereas core histones form the octamer around which DNA is wrapped

   Histone H1 is the linker histone: it sits on DNA between adjacent nucleosomes and helps organize/seal the linker region, distinct from the core octamer (H2A, H2B, H3, H4) around which DNA is wrapped.

   The nucleosome's core octamer (two copies each of H2A, H2B, H3, H4) provides the positively charged surface around which the negatively charged DNA wraps. Histone H1 is a separate, linker histone that associates with the DNA connecting successive nucleosomes, helping compact and organize the 'beads on a string' chromatin fibre into higher-order structures. It does not wrap DNA into the bead itself nor does it degrade DNA. Option A misassigns H1 a core-wrapping role; C fabricates an enzymatic function; D wrongly calls H1 non-histone/acidic when it is in fact a basic DNA-binding histone.

   Common mistake: Treating H1 as a fifth core histone rather than the distinct linker histone.

   Key point: H1 is the linker histone sealing DNA between nucleosomes, not part of the wrapping octamer.

6. Question 6 · medium · DNA Structure and Packaging

   A double-stranded DNA molecule with a higher percentage of G-C base pairs shows a higher melting temperature (denatures at a higher temperature) than one rich in A-T pairs. What is the correct biochemical basis for this observation?

   • A.G-C pairs form covalent bonds between strands while A-T pairs form only hydrogen bonds
   • B.Guanine and cytosine are larger molecules, so their increased mass raises the melting point
   • C.G-C pairs are held by 3 hydrogen bonds versus 2 in A-T pairs, so more energy is required to separate G-C rich regions
   • D.A-T rich regions have more phosphate groups per base pair, weakening the backbone
   Show answer and explanation

   Answer: C. G-C pairs are held by 3 hydrogen bonds versus 2 in A-T pairs, so more energy is required to separate G-C rich regions

   G-C pairs have one extra hydrogen bond (3 vs 2 in A-T), so more thermal energy is needed to break them apart, raising the melting temperature of G-C rich DNA.

   DNA denaturation (melting) requires breaking the hydrogen bonds between complementary bases on the two strands. Because G-C pairs are held by 3 hydrogen bonds compared to 2 in A-T pairs, G-C rich duplex regions require more thermal energy to separate, directly raising the melting temperature. This is a purely hydrogen-bond-based phenomenon, not related to molecular mass (B), covalent bonding (C, which never occurs between paired bases), or unequal phosphate distribution (D, which is constant per nucleotide regardless of base).

   Common mistake: Attributing thermal stability to base size/mass rather than hydrogen bond count.

   Key point: Higher G-C content → more H-bonds → higher melting temperature.

7. Question 7 · medium · DNA Structure and Packaging

   The total DNA in a single human cell nucleus, if fully stretched out, would measure approximately 2 metres in length. Why is such extensive DNA packaging into chromatin necessary?

   • A.To protect DNA from UV radiation exclusively, since packaging serves no other structural purpose
   • B.To convert DNA into RNA more efficiently during transcription
   • C.To increase the total amount of genetic information a cell can store
   • D.To compact the very long DNA molecule so that it can fit within a nucleus only a few micrometres in diameter
   Show answer and explanation

   Answer: D. To compact the very long DNA molecule so that it can fit within a nucleus only a few micrometres in diameter

   DNA packaging into nucleosomes and higher-order chromatin structures is essential to fit an approximately 2-metre-long DNA molecule inside a nucleus that is only a few micrometres across.

   There is an enormous mismatch between the length of fully stretched genomic DNA (~2 m in a human cell) and the tiny size of the nucleus (a few micrometres in diameter). Wrapping DNA around histone octamers to form nucleosomes, and further coiling these into chromatin fibres, achieves the compaction needed to accommodate this length within the nucleus while still permitting selective access to genes for replication and transcription. This is a spatial/structural necessity, not related to UV protection alone (A), transcription efficiency (B), or increasing information content (C).

   Common mistake: Believing packaging changes the amount of genetic information rather than just its spatial compaction.

   Key point: Packaging solves a spatial problem: ~2 m of DNA must fit in a nucleus a few µm across.

8. Question 8 · medium · DNA Structure and Packaging

   DNA readily wraps around the histone octamer to form a nucleosome. What property of DNA and of histone proteins makes this tight association possible?

   • A.DNA is positively charged due to its sugar residues, and histones are negatively charged, so they attract electrostatically
   • B.DNA is hydrophobic and histones are hydrophilic, causing them to associate via hydrophobic exclusion
   • C.DNA is negatively charged (due to phosphate groups) and histones are positively charged (rich in basic amino acids like lysine and arginine), so electrostatic attraction draws DNA onto the histone surface
   • D.DNA and histones form covalent bonds at every phosphate group to lock the structure permanently
   Show answer and explanation

   Answer: C. DNA is negatively charged (due to phosphate groups) and histones are positively charged (rich in basic amino acids like lysine and arginine), so electrostatic attraction draws DNA onto the histone surface

   DNA's phosphate backbone carries a net negative charge, while histones are rich in basic amino acids giving them a net positive charge; this electrostatic attraction lets DNA wrap tightly around the histone octamer.

   The phosphate groups along the DNA backbone give it a consistently negative charge. Histone proteins are unusually rich in basic amino acids (lysine, arginine), giving them a net positive charge. This charge complementarity drives a strong, non-covalent electrostatic attraction that allows about 146-200 bp of DNA to wrap around the histone octamer to form a nucleosome, while still permitting the DNA to be unwound when needed for replication/transcription. Option A reverses both charges; B misapplies hydrophobic reasoning; D wrongly invokes covalent (irreversible) bonding.

   Common mistake: Reversing the charges of DNA and histone proteins.

   Key point: DNA (–, phosphate) and histones (+, basic amino acids) associate via electrostatic attraction, not covalent bonds.

Questions about Molecular Basis of Inheritance for NEET

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