Chapter 5 — Molecular Basis of Inheritance

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

• DNA as the genetic material — The chapter establishes DNA — a double-stranded helical polymer — as the molecule that stores and transmits heredity, using the Watson-Crick structure to explain how base pairing enables faithful information storage.
• Copying and reading the code — It links replication, transcription and translation into one flow of information, showing how semiconservative copying preserves DNA and how the triplet genetic code directs protein synthesis at ribosomes.
• Regulating gene expression — Using the lac operon as a model, the chapter explains that genes are switched on and off in response to conditions, introducing regulation as a key layer over the raw genetic sequence.
• Genomics and identity — It closes by connecting molecular genetics to large-scale applications — the Human Genome Project's sequencing of human DNA and DNA fingerprinting using variable repeat polymorphisms for identification.

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.