CBSE Class 12 Biology: Genetics and Molecular Biology — Important Questions with Answers 2026

Short Answer Questions (2-3 Marks)

• Q: Describe the structure of DNA. What is the significance of base pairing?
  Ans: DNA is a double helix of two antiparallel polynucleotide chains. Each nucleotide has: phosphate group, deoxyribose sugar, nitrogenous base (A, T, G, C). Base pairing (Chargaff's rule): A=T (2 H-bonds), G≡C (3 H-bonds). Sugar-phosphate backbone on outside, bases project inward. Significance: (1) Complementary base pairing ensures accurate replication. (2) A=T means %A = %T and %G = %C in any DNA. (3) G≡C regions are more stable (3 H-bonds). (4) Enables semi-conservative replication.
• Q: Explain the Central Dogma of molecular biology.
  Ans: Central Dogma (Crick, 1958): Genetic information flows in one direction: DNA → RNA → Protein. (1) Transcription: DNA → mRNA. RNA polymerase copies template strand of DNA to form mRNA. Occurs in nucleus. (2) Translation: mRNA → Protein. Ribosomes read mRNA codons (triplets of bases). tRNA brings amino acids. Each codon specifies one amino acid (genetic code). Occurs in cytoplasm. Reverse transcription (RNA → DNA) occurs in retroviruses (HIV) using reverse transcriptase.
• Q: Explain the process of DNA replication.
  Ans: Semi-conservative replication: Each new DNA has one old strand + one new strand (proved by Meselson-Stahl experiment). Steps: (1) Helicase unwinds double helix at origin of replication. (2) SSB proteins stabilise single strands. (3) Primase adds RNA primer. (4) DNA polymerase III adds nucleotides (5'→3' direction). Leading strand: continuous. Lagging strand: Okazaki fragments. (5) DNA polymerase I replaces RNA primers with DNA. (6) DNA ligase joins fragments. Result: Two identical DNA molecules.

Long Answer / Application Questions (4-6 Marks)

• Q: What is a genetic code? State its properties.
  Ans: Genetic code is the relationship between sequence of bases in mRNA and sequence of amino acids in protein. Properties: (1) Triplet — 3 bases (codon) code for 1 amino acid. (2) Universal — same code in almost all organisms. (3) Degenerate — more than one codon for most amino acids (61 codons for 20 amino acids). (4) Non-overlapping — codons read sequentially without overlap. (5) Non-ambiguous — each codon codes for only one amino acid. (6) AUG is start codon (methionine). (7) UAA, UAG, UGA are stop codons.

Exam Tips for This Chapter

• Revise all definitions and laws from Genetics and Molecular Biology — commonly asked as 1-2 mark questions
• Practice diagrams related to Genetics and Molecular Biology — neat labelled diagrams carry 2-3 marks
• For numericals, always show formula → substitution → answer with correct units
• Previous year analysis shows Genetics and Molecular Biology carries significant marks in the board exam