CBSE Class 12 Biology: Molecular Basis of Inheritance — Notes 2026

Structure of DNA

• DNA double helix (Watson and Crick, 1953): two antiparallel polynucleotide strands; base pairs held by hydrogen bonds
• A=T (2 H bonds); G≡C (3 H bonds); base stacking (hydrophobic interactions) also contribute to stability
• DNA in eukaryotes: complexed with histone proteins (octamer of H2A, H2B, H3, H4 × 2) around nucleosome

DNA Replication

• Semi-conservative (Meselson-Stahl, 1958): each daughter DNA has one parental and one new strand
• Key enzymes: helicase (unwinds), primase (RNA primer), DNA pol III (synthesises, 5'→3'), DNA pol I (removes primer), ligase (joins Okazaki fragments)
• Leading strand: continuous synthesis; lagging strand: discontinuous (Okazaki fragments); joins later

Transcription

• mRNA synthesised from DNA template strand; RNA pol reads 3'→5', mRNA synthesised 5'→3'
• Promoter: TATA box in eukaryotes; RNA pol binds here; terminator signals RNA pol to stop
• Post-transcriptional modification (eukaryotes): 5' cap, poly-A tail, splicing out introns (pre-mRNA → mature mRNA)

Genetic Code

• 64 codons (4³ = 64); 61 code for amino acids; 3 stop codons (UAA, UAG, UGA)
• Degeneracy: multiple codons for same amino acid; leucine has 6 codons; serine has 6 codons
• Universal: same code in almost all organisms (exceptions: mitochondria); allows genetic engineering

Translation (Protein Synthesis)

• Ribosome: small subunit (decoding) + large subunit (peptide bond formation); A, P, E sites
• tRNA: anticodon loop recognises mRNA codon; acceptor stem carries amino acid; aminoacyl-tRNA synthetase charges tRNA
• Elongation: amino acid added to C-terminus; peptide bond formed by peptidyl transferase (rRNA — ribozyme)

Regulation of Gene Expression

• Lac operon (Jacob-Monod): repressor protein binds operator; lactose (inducer) inactivates repressor → transcription ON
• Negative regulation: repressor blocks transcription; positive regulation: activator enhances transcription
• Eukaryotic gene expression: chromatin structure, DNA methylation, histone acetylation — all affect transcription

CBSE Board Focus

• Molecular Basis: 7–9 marks; DNA replication steps and enzymes, transcription, genetic code features
• Semi-conservative replication: Meselson-Stahl experiment; describe and explain result
• Lac operon: draw diagram; explain repressor-operator-inducer mechanism; 4-mark question