MCQs on Molecular Basis of Inheritance – CBSE Class 12 Biology

Course: CBSE Class 12 Biology
Unit II: Genetics and Evolution
Topic: Molecular Basis of Inheritance
NCERT Aligned | CBSE Board Exam Oriented | Concept-Clearing Explanations

Section A: DNA, RNA and Genetic Material (Q1–Q15)

Q1. The genetic material in most organisms is

A. RNA
B. Protein
C. DNA
D. Carbohydrate

Correct Answer: C
Explanation: DNA is the hereditary material in most organisms due to its stability and ability to replicate accurately.

Q2. In which virus is RNA the genetic material?

A. Bacteriophage
B. Tobacco mosaic virus
C. Adenovirus
D. Herpes virus

Correct Answer: B
Explanation: Tobacco mosaic virus (TMV) contains RNA as its genetic material.

Q3. DNA is a polymer of

A. Amino acids
B. Nucleotides
C. Fatty acids
D. Monosaccharides

Correct Answer: B
Explanation: DNA is made of repeating units called deoxyribonucleotides.

Q4. A nucleotide is composed of

A. Sugar + base
B. Sugar + phosphate
C. Base + phosphate
D. Sugar + base + phosphate

Correct Answer: D
Explanation: Each nucleotide consists of a nitrogenous base, a pentose sugar, and a phosphate group.

Q5. Which nitrogenous base is absent in RNA?

A. Adenine
B. Cytosine
C. Thymine
D. Guanine

Correct Answer: C
Explanation: RNA contains uracil instead of thymine.

Q6. Chargaff’s rule states that

A. A = G and T = C
B. A = T and G = C
C. A = C and G = T
D. Purines = Pyrimidines

Correct Answer: B
Explanation: In DNA, adenine pairs with thymine and guanine pairs with cytosine.

Q7. The sugar present in DNA is

A. Ribose
B. Deoxyribose
C. Glucose
D. Fructose

Correct Answer: B
Explanation: DNA contains deoxyribose sugar, lacking one oxygen atom.

Q8. Hydrogen bonds in DNA are formed between

A. Sugar and phosphate
B. Purine bases only
C. Complementary bases
D. Same bases

Correct Answer: C
Explanation: Hydrogen bonds form between complementary nitrogenous bases (A–T and G–C).

Q9. DNA has

A. Single helical structure
B. Triple helix
C. Double helical structure
D. Circular helix only

Correct Answer: C
Explanation: DNA exists as a double helix with two antiparallel strands.

Q10. The backbone of DNA is formed by

A. Base pairing
B. Hydrogen bonds
C. Sugar-phosphate linkage
D. Peptide bonds

Correct Answer: C
Explanation: Alternating sugar and phosphate groups form the backbone of DNA.

Q11. Which bond links nucleotides in DNA?

A. Peptide bond
B. Hydrogen bond
C. Phosphodiester bond
D. Glycosidic bond

Correct Answer: C
Explanation: Nucleotides are linked by phosphodiester bonds.

Q12. Purine bases include

A. Adenine and thymine
B. Cytosine and guanine
C. Adenine and guanine
D. Thymine and uracil

Correct Answer: C
Explanation: Adenine and guanine are purines with double-ring structure.

Q13. Pyrimidine bases include

A. Adenine and guanine
B. Cytosine and thymine
C. Guanine and thymine
D. Adenine and cytosine

Correct Answer: B
Explanation: Cytosine and thymine are pyrimidines with single-ring structure.

Q14. RNA differs from DNA by having

A. Deoxyribose sugar
B. Thymine
C. Single strand
D. Double helix

Correct Answer: C
Explanation: RNA is generally single-stranded, unlike DNA.

Q15. DNA is more stable than RNA because

A. RNA is double stranded
B. DNA has ribose sugar
C. DNA lacks hydroxyl group at 2′ carbon
D. RNA has thymine

Correct Answer: C
Explanation: Absence of 2′-OH group in DNA makes it chemically more stable.

Section B: DNA Replication and Transcription (Q16–Q30)

Q16. DNA replication is

A. Conservative
B. Dispersive
C. Semi-conservative
D. Random

Correct Answer: C
Explanation: Each daughter DNA has one parental and one newly synthesized strand.

Q17. DNA replication occurs during

A. G₁ phase
B. G₂ phase
C. S phase
D. M phase

Correct Answer: C
Explanation: DNA synthesis occurs in the S phase of the cell cycle.

Q18. The enzyme responsible for DNA synthesis is

A. RNA polymerase
B. DNA ligase
C. DNA polymerase
D. Helicase

Correct Answer: C
Explanation: DNA polymerase catalyzes the synthesis of new DNA strands.

Q19. Helicase enzyme helps in

A. Joining DNA fragments
B. Unwinding DNA
C. Proofreading
D. Transcription

Correct Answer: B
Explanation: Helicase separates the two strands of DNA during replication.

Q20. Okazaki fragments are formed on

A. Leading strand
B. Lagging strand
C. Both strands
D. RNA strand

Correct Answer: B
Explanation: Lagging strand is synthesized discontinuously as Okazaki fragments.

Q21. RNA primer is synthesized by

A. DNA polymerase
B. Ligase
C. Primase
D. Helicase

Correct Answer: C
Explanation: Primase synthesizes short RNA primers required for DNA replication.

Q22. DNA ligase helps in

A. Unwinding DNA
B. Joining Okazaki fragments
C. RNA synthesis
D. Base pairing

Correct Answer: B
Explanation: DNA ligase joins the Okazaki fragments on lagging strand.

Q23. Transcription is the synthesis of

A. DNA from RNA
B. RNA from DNA
C. Protein from RNA
D. DNA from protein

Correct Answer: B
Explanation: Transcription produces RNA using DNA as a template.

Q24. The enzyme involved in transcription is

A. DNA polymerase
B. RNA polymerase
C. Ligase
D. Topoisomerase

Correct Answer: B
Explanation: RNA polymerase catalyzes RNA synthesis.

Q25. Transcription occurs in

A. Cytoplasm only
B. Nucleus only
C. Ribosome
D. Golgi apparatus

Correct Answer: B
Explanation: In eukaryotes, transcription takes place in the nucleus.

Q26. Template strand in transcription is also called

A. Coding strand
B. Sense strand
C. Antisense strand
D. Leading strand

Correct Answer: C
Explanation: The antisense strand acts as the template for RNA synthesis.

Q27. mRNA carries information for synthesis of

A. DNA
B. RNA
C. Protein
D. Lipid

Correct Answer: C
Explanation: mRNA carries genetic code from DNA to ribosomes for protein synthesis.

Q28. Promoter region is present

A. At 3′ end of gene
B. At 5′ end of gene
C. In introns
D. On RNA

Correct Answer: B
Explanation: Promoter lies upstream at the 5′ end and initiates transcription.

Q29. Termination of transcription occurs at

A. Operator
B. Promoter
C. Terminator
D. Enhancer

Correct Answer: C
Explanation: Terminator sequence signals the end of transcription.

Q30. RNA polymerase binds to

A. Enhancer
B. Terminator
C. Promoter
D. Operator

Correct Answer: C
Explanation: RNA polymerase recognizes and binds to the promoter region.

Section C: Genetic Code, Translation and Regulation (Q31–Q50)

Q31. Genetic code is

A. Overlapping
B. Continuous
C. Triplet
D. Ambiguous

Correct Answer: C
Explanation: Each amino acid is coded by a triplet of nucleotides called codon.

Q32. Start codon for protein synthesis is

A. UAA
B. UAG
C. UGA
D. AUG

Correct Answer: D
Explanation: AUG codes for methionine and initiates translation.

Q33. Stop codons do not code for

A. Amino acids
B. Proteins
C. RNA
D. Enzymes

Correct Answer: A
Explanation: Stop codons signal termination and do not code for amino acids.

Q34. tRNA helps in

A. DNA replication
B. Transcription
C. Amino acid transport
D. Protein folding

Correct Answer: C
Explanation: tRNA carries specific amino acids to ribosomes during translation.

Q35. Ribosomes are the site of

A. Replication
B. Transcription
C. Translation
D. Mutation

Correct Answer: C
Explanation: Protein synthesis occurs on ribosomes.

Q36. Anticodon is present on

A. mRNA
B. tRNA
C. rRNA
D. DNA

Correct Answer: B
Explanation: Anticodon on tRNA pairs with codon on mRNA.

Q37. rRNA forms

A. Genetic code
B. Ribosomal structure
C. Amino acids
D. Enzymes

Correct Answer: B
Explanation: rRNA combines with proteins to form ribosomes.

Q38. Genetic code is degenerate because

A. One codon codes multiple amino acids
B. One amino acid is coded by multiple codons
C. Codons overlap
D. It is universal

Correct Answer: B
Explanation: Many amino acids are coded by more than one codon.

Q39. Lac operon is an example of

A. Structural gene
B. Repressor gene
C. Inducible operon
D. Constitutive operon

Correct Answer: C
Explanation: Lac operon is activated in presence of lactose.

Q40. The repressor in lac operon binds to

A. Promoter
B. Operator
C. Structural gene
D. Enhancer

Correct Answer: B
Explanation: Repressor protein binds to operator to block transcription.

Q41. Inducer of lac operon is

A. Glucose
B. Lactose
C. Galactose
D. Sucrose

Correct Answer: B
Explanation: Lactose inactivates repressor, allowing transcription.

Q42. Operon concept was proposed by

A. Watson and Crick
B. Jacob and Monod
C. Mendel
D. Morgan

Correct Answer: B
Explanation: Jacob and Monod proposed operon model in prokaryotes.

Q43. Structural genes of lac operon are

A. i, p, o
B. z, y, a
C. lac, rep, op
D. cap, lac, z

Correct Answer: B
Explanation: lacZ, lacY, and lacA code for enzymes involved in lactose metabolism.

Q44. Mutation is a change in

A. Protein only
B. DNA sequence
C. RNA sequence
D. Cell membrane

Correct Answer: B
Explanation: Mutation involves alteration in DNA nucleotide sequence.

Q45. Frame shift mutation is caused by

A. Base substitution
B. Deletion or insertion
C. Duplication
D. Translocation

Correct Answer: B
Explanation: Insertion or deletion of nucleotides alters reading frame.

Q46. Silent mutation does not change

A. DNA sequence
B. Amino acid sequence
C. Codon
D. Gene

Correct Answer: B
Explanation: Due to degeneracy, amino acid remains same.

Q47. The flow of genetic information is called

A. Central dogma
B. Operon
C. Replicon
D. Codon

Correct Answer: A
Explanation: Central dogma describes DNA → RNA → Protein flow.

Q48. Reverse transcription occurs in

A. Bacteria
B. Plants
C. Retroviruses
D. Fungi

Correct Answer: C
Explanation: Retroviruses synthesize DNA from RNA.

Q49. Translation begins at

A. 3′ end of mRNA
B. 5′ end of mRNA
C. Middle of mRNA
D. Anticodon

Correct Answer: B
Explanation: Ribosome binds near 5′ end of mRNA.

Q50. The ultimate product of gene expression is

A. DNA
B. RNA
C. Protein
D. Lipid

Correct Answer: C
Explanation: Gene expression results in functional proteins.