Menu

Molecular Basis of Inheritance

NCERT Class 12 Biology Chapter 5

Molecular Basis of Inheritance for Class 12 Biology and NEET: DNA, replication, transcription, genetic code, translation, lac operon, DNA fingerprinting.

10 topics Updated 8 Nov 2026

For: Class 12 and NEET

In the syllabus: NEET Class 12 syllabus

With your textbook

NCERT Biology (Class 12), Chapter 5 “Molecular Basis of Inheritance”, reprint 2026-27.

Open the official NCERT chapter (PDF, 4.1 MB, ncert.nic.in)

Checked on 19 Oct 2026.

Learn in this order

  1. DNA: structure and packaging

    Structure of DNA for Class 12 Biology and NEET: nucleotides and the double helix with diagrams, Chargaff’s rule, central dogma, nucleosomes, DNA packaging.

    NCERT Section 5.1 15 min read 4 solved examples 8 practice questions

  2. The search for genetic material

    Griffith experiment and the search for genetic material for Class 12 Biology and NEET: Avery’s work, Hershey–Chase and DNA vs RNA, with diagrams.

    NCERT Section 5.2 13 min read 3 solved examples 8 practice questions

  3. RNA world

    RNA world hypothesis for Class 12 Biology and NEET: why RNA is thought to be the first genetic material, RNA as a catalyst (ribozymes) and DNA from RNA.

    NCERT Section 5.3 7 min read 3 solved examples 6 practice questions

  4. DNA replication

    DNA replication for Class 12 Biology and NEET: semiconservative copying, the Meselson–Stahl and Taylor experiments, DNA polymerase, the fork and ligase.

    NCERT Section 5.4 16 min read 4 solved examples 9 practice questions

  5. Transcription

    Transcription for Class 12 Biology and NEET: transcription unit, template and coding strands, types of RNA, RNA polymerases, splicing, capping, tailing.

    NCERT Section 5.5 15 min read 5 solved examples 9 practice questions

  6. Genetic code

    The genetic code for Class 12 Biology and NEET: the triplet codon table, start and stop codons, degeneracy, frameshift mutations and tRNA as adapter.

    NCERT Section 5.6 14 min read 5 solved examples 8 practice questions

  7. Translation

    Translation in biology for Class 12 Biology and NEET: tRNA charging, the 23S rRNA ribozyme, UTRs and the steps of protein synthesis, with a diagram.

    NCERT Section 5.7 10 min read 4 solved examples 8 practice questions

  8. Regulation of gene expression

    Lac operon and regulation of gene expression for Class 12 Biology and NEET: levels of control, operator, repressor, induction by lactose and a diagram.

    NCERT Section 5.8 11 min read 4 solved examples 8 practice questions

  9. Human Genome Project

    The Human Genome Project for Class 12 Biology and NEET: why a mega project, its goals, ESTs and BAC/YAC cloning, Sanger sequencing and genome features.

    NCERT Section 5.9 12 min read 4 solved examples 8 practice questions

  10. DNA fingerprinting

    DNA fingerprinting for Class 12 Biology and NEET: satellite DNA, polymorphism, VNTR probes, the Southern blot steps, applications and a gel diagram.

    NCERT Section 5.10 13 min read 4 solved examples 8 practice questions

Key formulas in this chapter

DNA: structure and packaging

  • nucleoside: base + sugar
  • nucleotide: nucleoside + phosphate
  • Chargaff: A = T, G = C; A + G = T + C
  • A=T: 2 H-bonds; G≡C: 3 H-bonds
  • pitch 3.4 nm; 10 bp per turn
  • 0.34 nm per bp
  • length = bp × 0.34 nm
  • 6.6 × 10 bp → about 2.2 m
  • nucleosome: octamer + 200 bp

Search for genetic material

  • Griffith 1928: transforming principle
  • Avery, MacLeod, McCarty 1933–44: DNA
  • Hershey, Chase 1952: unequivocal proof, DNA
  • radioactive P → DNA; radioactive S → protein
  • DNA: storage; RNA: transmission

RNA world

  • RNA-world hypothesis: early RNA-based inheritance/catalysis
  • RNA: information + catalysis (ribozyme)
  • DNA-based storage: more stable backbone and repair advantages
  • ribozyme example: 23S rRNA (bacteria)

DNA replication

  • semiconservative: one old + one new strand
  • Meselson and Stahl 1958: N, then N
  • assuming 20-min generations: 20 min all hybrid; 40 min half hybrid/half light
  • at 20 min/generation, 80 min: 1/8 hybrid, 7/8 light
  • polymerase builds 5′ → 3′ only
  • template 3′ → 5′ along fork movement: continuous
  • template 5′ → 3′ along fork movement: discontinuous
  • DNA ligase joins the fragments

Transcription

  • template strand: 3′ → 5′
  • coding strand: 5′ → 3′, sequence as RNA
  • promoter upstream; terminator downstream
  • σ: initiation factor; ρ: termination factor
  • polymerase I: 28S, 18S, 5.8S rRNA
  • polymerase II: hnRNA (precursor of mRNA)
  • polymerase III: tRNA, 5S rRNA, snRNAs
  • cap at 5′; tail of 200–300 A at 3′

Genetic code

  • codon: triplet; 4 = 64 codons
  • 61 for amino acids; 3 stop: UAA, UAG, UGA
  • AUG: methionine and start
  • degenerate: several codons for one amino acid
  • coding indel not divisible by 3: frameshift
  • coding indel divisible by 3: downstream frame preserved
  • tRNA: anticodon loop, amino acid acceptor end

Translation

  • charging: amino acid + tRNA, using ATP
  • ribosome: structural RNAs + about 80 proteins; two sites in the large subunit
  • peptide bond catalyst: 23S rRNA (ribozyme)
  • translational unit: AUG to stop codon
  • UTRs at the 5′ and 3′ ends: not translated
  • stop codon: release factor ends translation

Regulation of gene expression

  • lac operon order: p, i, p, o, z, y, a
  • i: repressor (i from inhibitor)
  • z: β-galactosidase; y: permease
  • a: transacetylase
  • lactose → allolactose → reduced repressor binding
  • repressor on operator: operon off

Human Genome Project

  • HGP: 1990–2003 (13 years)
  • NCERT historical genome figure: 3164.7 million bp
  • NCERT illustrative cost: 3 dollars/bp × 3 billion bp = 9 billion dollars
  • NCERT historical gene-length figures: average 3000; dystrophin 2.4 million
  • under 2 per cent codes for proteins
  • NCERT historical counts: chromosome 1 = 2968 genes; Y = 231
  • NCERT historical SNP tally: 1.4 million sites

DNA fingerprinting

  • 0.1 per cent of 3 × 10 = 3 × 10
  • polymorphism: allele frequency above 0.01
  • VNTR: mini-satellite, 0.1 to 20 kb
  • steps: isolate, digest, electrophoresis, blot
  • then hybridise with VNTR probe, autoradiograph
  • unique pattern, except identical twins

More in Biology