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Schooling · Class 12 · Biology · Molecular Basis of Inheritance · Chapter on a page

CBSE Class 12 · Biology · NCERT Biology

Molecular Basis of Inheritance: chapter on a page

A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).

About the notes: Written by Shishya's AI on 27 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.

Fact-checked on 21 Oct 2026: an AI-assisted check of this note against cited sources; the errors it found were corrected that day. Some errors may remain.

Mind map

Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.

Mind map of Molecular Basis of Inheritance: Key ideas, Formulas and facts, Worked examples, Watch out forKey ideasDNA is the genetic materialDouble-helix structure of DNASemi-conservative replicationTranscriptionGenetic codeTranslationRegulation of gene expressionFormulas and factsChargaff's rulesBase-pairing ruleDNA replication is semi-conservativeDirection of synthesisNumber of codonsStart codonWobble positionWorked examplesCalculating base compositionDetermining mRNA and amino-acid…Predicting replication productsWatch out forConfusing template strand with…Writing DNA base pairs as A–G or T–CForgetting that DNA polymerase needs…Assuming introns stay in mature mRNATreating the genetic code as…Molecular Basis ofInheritance
The mind map as a list
  • Key ideas
    • DNA is the genetic material
    • Double-helix structure of DNA
    • Semi-conservative replication
    • Transcription
    • Genetic code
    • Translation
    • Regulation of gene expression
  • Formulas and facts
    • Chargaff's rules
    • Base-pairing rule
    • DNA replication is semi-conservative
    • Direction of synthesis
    • Number of codons
    • Start codon
    • Wobble position
  • Worked examples
    • Calculating base composition
    • Determining mRNA and amino-acid…
    • Predicting replication products
  • Watch out for
    • Confusing template strand with…
    • Writing DNA base pairs as A–G or T–C
    • Forgetting that DNA polymerase needs…
    • Assuming introns stay in mature mRNA
    • Treating the genetic code as…

What this chapter is about

This chapter explains how genetic information is stored, copied and expressed at the molecular level. You will learn that DNA (deoxyribonucleic acid) is the genetic material in most organisms, understand how its double-helix structure allows faithful replication, and see how the information in DNA flows to RNA and then to proteins. This sequence—DNA → RNA → Protein—is often called the central dogma of molecular biology.

Key ideas

  • DNA is the genetic material: Experiments by Griffith (transformation in bacteria), by Avery, MacLeod and McCarty (who showed that the transforming principle is DNA) and by Hershey–Chase (using radioactive isotopes to label protein and DNA of bacteriophages) established that DNA, not protein, carries hereditary information.
  • Double-helix structure of DNA: Two antiparallel polynucleotide strands wind around each other; adenine pairs with thymine (A–T, two hydrogen bonds) and guanine pairs with cytosine (G–C, three hydrogen bonds). The sugar-phosphate backbone lies outside; nitrogenous bases stack inside.
  • Semi-conservative replication: Each original strand serves as a template; two daughter molecules each contain one parental strand and one newly synthesised strand. Key enzymes include helicase (unwinding), DNA polymerase (synthesising the new strand 5′→3′), primase (making RNA primers) and ligase (joining Okazaki fragments on the lagging strand).
  • Transcription: RNA polymerase synthesises a single-stranded mRNA complementary to the template (antisense) strand of a gene. In eukaryotes, the primary transcript undergoes capping, polyadenylation and splicing (removal of introns) before leaving the nucleus.
  • Genetic code: A sequence of three bases (codon) in mRNA specifies one amino acid. The code is nearly universal, degenerate (most amino acids have more than one codon), non-overlapping and has start (AUG) and stop (UAA, UAG, UGA) codons.
  • Translation: Ribosomes read mRNA codons; tRNA molecules carry specific amino acids and have anticodons complementary to mRNA codons. Peptide bonds form between successive amino acids, producing a polypeptide.
  • Regulation of gene expression: The lac operon model in E. coli shows how genes can be switched on or off depending on environmental signals (presence or absence of lactose).
  • Human Genome Project: This international effort sequenced approximately 3 × 10⁹ base pairs of human DNA, identified about 20 000–25 000 protein-coding genes, and revealed that coding sequences make up only about 2 % of the genome.

Formulas and facts to remember

  • Chargaff's rules: In double-stranded DNA, percentage of A equals percentage of T; percentage of G equals percentage of C; hence (A + G) = (T + C).
  • Base-pairing rule: A pairs with T (or U in RNA) by two hydrogen bonds; G pairs with C by three hydrogen bonds.
  • DNA replication is semi-conservative: Each daughter DNA molecule has one old strand and one new strand.
  • Direction of synthesis: DNA and RNA polymerases add nucleotides only in the 5′ → 3′ direction.
  • Number of codons: 4³ = 64 possible triplet codons; 61 code for amino acids, 3 are stop codons.
  • Start codon: AUG codes for methionine and signals the beginning of translation.
  • Wobble position: The third base of a codon often shows flexibility in pairing, explaining degeneracy.
  • Human genome facts: About 3.2 × 10⁹ bp; roughly 20 000–25 000 genes; most DNA is non-coding (introns, repetitive sequences).

Common mistakes

  • Confusing template strand with coding strand → Remember: RNA polymerase reads the template (antisense) strand 3′→5′ and synthesises mRNA 5′→3′.
  • Writing DNA base pairs as A–G or T–C → Only A–T and G–C pairs are allowed; mismatched pairs destabilise the helix.
  • Forgetting that DNA polymerase needs a primer → Replication cannot begin de novo; primase must first lay down a short RNA primer.
  • Assuming introns stay in mature mRNA → Introns are removed by splicing; only exons appear in the final mRNA that is translated.
  • Treating the genetic code as organism-specific → The code is nearly universal; the same codons specify the same amino acids in bacteria, plants and humans (with minor exceptions).

Quick revision

  • DNA is made of two antiparallel strands; A pairs with T, G pairs with C.
  • Replication is semi-conservative and proceeds 5′ → 3′.
  • Transcription produces mRNA from the template strand; eukaryotic mRNA is processed (cap, poly-A tail, splicing).
  • Translation occurs on ribosomes; tRNA anticodons match mRNA codons to assemble amino acids into polypeptides.
  • The lac operon illustrates inducible gene regulation in prokaryotes.
  • The Human Genome Project revealed roughly 20 000–25 000 genes in about 3 × 10⁹ base pairs.