Chapter 5 — Molecular Basis of Inheritance

Chapter 5 of NCERT Class 12 Biology covers the Molecular Basis of Inheritance, explaining the structure of DNA, its replication, transcription, genetic code, translation, gene regulation, the Human Genome Project, and DNA fingerprinting.

NCERT Class 12 Biology Chapter 5, Molecular Basis of Inheritance, explains how DNA — a double-stranded helical polymer of deoxyribonucleotides — stores and transmits genetic information. It covers DNA's double helix structure (Watson and Crick, 1953), semiconservative replication (proved by Meselson and Stahl), transcription of DNA into RNA, the triplet genetic code deciphered by Nirenberg and Khorana, and protein synthesis (translation) at ribosomes. The chapter also details gene regulation through the lac operon, the Human Genome Project (launched 1990, completed 2003), and DNA fingerprinting using VNTR polymorphisms.

1. 01DNA is a double helix with anti-parallel strands held by hydrogen bonds: A-T (2 H-bonds) and G-C (3 H-bonds), with a pitch of 3.4 nm and ~10 base pairs per turn.
2. 02DNA replicates semiconservatively — each daughter molecule retains one parental strand — proved by Meselson and Stahl (1958) using heavy nitrogen (15N) in E. coli.
3. 03The genetic code is a triplet codon system: 61 codons specify amino acids and 3 are stop codons (UAA, UAG, UGA); AUG serves as the start codon and codes for methionine.
4. 04In eukaryotes, primary RNA transcripts (hnRNA) undergo splicing (introns removed, exons joined), capping, and tailing before becoming functional mRNA for translation.
5. 05The lac operon in E. coli is the classic model of gene regulation: in the absence of lactose the repressor blocks transcription; lactose (inducer) inactivates the repressor, allowing gene expression.
6. 06The Human Genome Project (1990–2003) sequenced the ~3164.7 million base pairs of human DNA, identified ~20,000–25,000 genes, and found that less than 2% of the genome codes for proteins.