Molecular Basis of Inheritance – important questions for Boards 2027

CBSE Class 12 Biology

1. VSA · 0.5 marks

   Name the two scientists who demonstrated semiconservative DNA replication in E. coli.

   Answer

   Matthew Meselson and Franklin Stahl demonstrated semiconservative DNA replication in E. coli in 1958.

2. MCQ · 1 mark

   Which set lists the three components of a nucleotide? (A) Nitrogenous base, pentose sugar and phosphate group (B) Nitrogenous base, amino acid and phosphate group (C) Pentose sugar, amino acid and fatty acid (D) Nitrogenous base, glycerol and pentose sugar

   Answer

   (A) Nitrogenous base, pentose sugar and phosphate group — these are the three components of a nucleotide.

3. SA · 2 marks

   State how the sugar and the presence of thymine or uracil differ between DNA and RNA. Name the purine bases and the pyrimidine bases.

   Answer

   DNA contains deoxyribose, while RNA contains ribose. Thymine is present in DNA, whereas uracil is present in RNA in place of thymine. Purines are Adenine and Guanine; pyrimidines are Cytosine, Uracil and Thymine.

4. MA · 3 marks

   Describe how DNA is organised in the nucleoid of E. coli. Then explain the role of histones and the histone octamer in eukaryotic DNA packaging, including how a nucleosome forms.

   Answer

   1. In E. coli, DNA is not scattered throughout the cell despite the absence of a defined nucleus. Negatively charged DNA is held with positively charged proteins in a region termed the nucleoid, where it is organised in large loops held by proteins. 2. Eukaryotic histones are positively charged, basic proteins rich in lysine and arginine residues. Eight histone molecules form a histone octamer. 3. Negatively charged DNA wraps around the positively charged histone octamer to form a nucleosome, which typically contains 200 bp of DNA helix.

5. MCQ · 4 marks

   Which option correctly describes the Hershey–Chase experiment and its inference? (This question is worth 4 marks.) (A) Blending removed viral coats and centrifugation separated phages from bacteria; bacteria became radioactive when infected with phages labelled in DNA, but not when infected with phages labelled in protein. (B) Blending separated DNA from protein; bacteria became radioactive only when infected with protein-labelled phages. (C) Centrifugation removed the bacterial cells; both DNA-labelled and protein-labelled phages entered the bacteria. (D) Blending caused the phages to inject their coats; the experiment showed that protein, not DNA, entered bacteria.

   Answer

   (A) Blending removed viral coats and centrifugation separated phages from bacteria; bacteria became radioactive when infected with phages labelled in DNA, but not when infected with phages labelled in protein. — DNA entered the bacteria, whereas protein did not.

6. LA · 5 marks

   Explain how a nitrogenous base, a pentose sugar and a phosphate group are linked to form a nucleotide. Then describe how nucleotides join to form a polynucleotide chain, naming the linkages involved.

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7. VSA · 0.5 marks

   After it was demonstrated in E. coli, in what broader group of organisms was semiconservative DNA replication subsequently shown?

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8. MCQ · 1 mark

   Which description best fits the arrangement of the two polynucleotide chains in DNA? (A) They have sugar-phosphate backbones with bases projecting inward. (B) They have bases on the outside and phosphate groups paired inward. (C) They are parallel, with both 5′ ends at the same end. (D) They are identical in sequence and run in the same direction.

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9. SA · 2 marks

   Describe the antiparallel orientation of the two DNA strands and state the base-pairing rules, including the number of hydrogen bonds in each pair.

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10. MA · 3 marks

    Explain why DNA synthesis is continuous on one template strand and discontinuous on the other at a replication fork. State the direction in which DNA polymerase synthesises DNA and the role of DNA ligase. (3 marks)

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