Molecular Basis of Inheritance

What you'll learn

A from-scratch guide to DNA's double-helix structure, the classic experiments proving DNA is the genetic material, semiconservative replication, and the central dogma of transcription, the genetic code, and translation — built for CBSE Class 12 boards, NEET and CUET.

This chapter has

Molecular Basis of Inheritance — solved practice questions

8 Class 12 Biology questions with step-by-step solutions. Attempt each, then reveal the answer.

1. Q1easy

   In a DNA double helix, adenine pairs with thymine through how many hydrogen bonds?

   • AOne
   • BTwo
   • CThree
   • DFour
   Show answer & solution

   Correct answer: (B) Two

   Adenine (A) and thymine (T) form a complementary base pair held by two hydrogen bonds.

2. Q2easy

   The number of hydrogen bonds formed between guanine and cytosine in DNA is:

   • ATwo
   • BOne
   • CThree
   • DFour
   Show answer & solution

   Correct answer: (C) Three

   Guanine (G) pairs with cytosine (C) through three hydrogen bonds, making G-C pairs more stable than A-T pairs.

3. Q3medium

   According to Chargaff's rule, in a double-stranded DNA molecule:

   • AA = C and G = T
   • BA + T = G + C
   • CA = G and T = C
   • DA = T and G = C
   Show answer & solution

   Correct answer: (D) A = T and G = C

   Because of complementary base pairing, the amount of adenine equals thymine and guanine equals cytosine, i.e. A = T and G = C.

4. Q4easy

   The semiconservative mode of DNA replication was experimentally proved in Escherichia coli by:

   • AMeselson and Stahl
   • BGriffith
   • CHershey and Chase
   • DWatson and Crick
   Show answer & solution

   Correct answer: (A) Meselson and Stahl

   Meselson and Stahl (1958) used density-gradient centrifugation with 15N/14N labelling of E. coli DNA to demonstrate semiconservative replication.

5. Q5medium

   In the Watson–Crick model of B-DNA, the distance between two consecutive base pairs along the axis is about:

   • A0.34 nm
   • B3.4 nm
   • C34 nm
   • D0.034 nm
   Show answer & solution

   Correct answer: (A) 0.34 nm

   The rise per base pair is 0.34 nm; one full turn of the helix is 3.4 nm and contains about 10 base pairs (3.4 nm / 0.34 nm).

6. Q6medium

   The enzyme that catalyzes the polymerization of deoxyribonucleotides during DNA replication is:

   • ARNA polymerase
   • BDNA polymerase
   • CDNA ligase
   • DHelicase
   Show answer & solution

   Correct answer: (B) DNA polymerase

   DNA-dependent DNA polymerase adds deoxyribonucleotides using the template strand; ligase joins fragments and helicase unwinds the helix.

7. Q7hard

   During DNA replication, the strand synthesized discontinuously as short Okazaki fragments is the:

   • ALeading strand
   • BTemplate strand
   • CLagging strand
   • DCoding strand
   Show answer & solution

   Correct answer: (C) Lagging strand

   Because DNA polymerase works only 5'→3', the lagging strand (with template of 5'→3' polarity in the direction of the fork) is made discontinuously as Okazaki fragments later joined by ligase.

8. Q8medium

   In eukaryotes the primary transcript (hnRNA) contains exons and introns; the removal of introns and joining of exons is called:

   • ACapping
   • BTailing
   • CTranscription
   • DSplicing
   Show answer & solution

   Correct answer: (D) Splicing

   Splicing removes the non-coding introns and joins the coding exons to form mature mRNA. Capping and tailing are other processing steps.

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