Molecular Basis of Inheritance

• DNA structure: double helix, antiparallel strands, A-T (2 H-bonds), G-C (3 H-bonds), major/minor groove.
• Central dogma: DNA → (transcription) → RNA → (translation) → protein; exceptions (reverse transcription).
• Replication: semiconservative, bidirectional, leading/lagging strands, Okazaki fragments, DNA polymerase.
• Transcription in prokaryotes vs eukaryotes; mRNA processing (capping, tail, splicing).
• Genetic code — triplet, degenerate, universal (mostly); translation at ribosome.

Level 1 — Foundations

Verbal: The molecular basis of inheritance is DNA storing genetic information in sequence of nucleotides, replicated faithfully and expressed as proteins.

DNA monomer: Nucleotide = phosphate + deoxyribose + nitrogenous base (A, T, G, C).

Chargaff's rules: A = T, G = C in double-stranded DNA.

Watson-Crick model: Right-handed double helix; complementary base pairing stabilises structure.

RNA differences: Uracil replaces thymine; ribose sugar; often single-stranded.

Level 2 — JEE / NEET depth

Replication (Meselson-Stahl): Semiconservative — each new duplex has one old and one new strand.

Enzymes: Helicase unwinds; primase lays RNA primer; DNA pol III elongates; ligase joins Okazaki fragments.

Transcription: DNA template strand → mRNA (5′→3′). Promoter, RNA polymerase. Eukaryotes: exons spliced, introns removed.

Translation: mRNA codons read at ribosome; tRNA brings amino acids; starts AUG (Met); stop codons UAA, UAG, UGA.

Code features: 64 codons, 61 sense; degenerate (multiple codons per amino acid); universal with minor exceptions (mitochondria).

Operon concept (lac): Prokaryotic gene regulation — inducible system (brief NCERT).

NEET/JEE bio: Codon table usage; replication enzyme matching; central dogma diagram.

Complementary DNA strand

Template strand 3′-TACGGCAA-5′.

Step 1 — Pair A-T, G-C antiparallel.
Step 2 — New strand 5′-ATGCCGTT-3′ (read 5′→3′ convention).
Step 3 — Verify Chargaff pairing.
Step 4 — Directionality critical for replication fork models.

Translate mRNA codon sequence

mRNA: 5′-AUG UUC GCA UAA-3′.

Step 1 — AUG → Methionine (start).
Step 2 — UUC → Phe; GCA → Ala.
Step 3 — UAA → stop — peptide Met-Phe-Ala released.
Step 4 — tRNA anticodons 3′-UAC-5′ etc pair with codons.

• Draw DNA base pairing rules.
• Define semiconservative replication.
• Function of RNA polymerase?
• How many nucleotides for one codon?
• Stretch: List three mRNA processing steps in eukaryotes.

NCERT Chapter 6 link: DNA structure, replication, transcription, translation form central dogma. Semiconservative replication proven by Meselson-Stahl experiment. Eukaryotic mRNA processing: 5′ cap, poly-A tail, splicing.

Exam connections: Complementary strand writing with antiparallel 5′→3′ convention. Codon table usage — AUG start, stop codons UAA/UAG/UGA. Replication enzymes: helicase, primase, DNA polymerase, ligase — match function to name. lac operon induction concept for prokaryotic regulation.

Study strategy: Central dogma diagram with reverse transcription exception (retroviruses). Okazaki fragments on lagging strand only. Chargaff A=T, G=C for double-stranded DNA.

When studying Molecular Basis of Inheritance within Genetics, start by listing every formula and definition on one page without looking at the textbook. Compare your list to NCERT — missing items indicate gaps to fix immediately. Work through at least two NCERT Examples for this section with steps written in full; examiners award method marks even when arithmetic slips.

For board exams (CBSE), long answers benefit from a clear structure: definition → explanation → diagram or formula → example → brief conclusion. Underline key terms. For JEE Main and NEET, prioritise conceptual traps and quick calculation paths; timed mixed quizzes of 10 questions after revision simulate exam pressure.

Cross-topic link: Diagrams and terminology precision matter; link molecular genetics to biotechnology applications chapters.

Spaced revision: Review this note at 1 day, 3 days, and 7 days after first study. Attempt the Quick check questions closed-book, then open the Practice tab for graded reinforcement. Maintain an error log — repeated mistake patterns reveal whether the issue is concept, formula recall, or careless reading.

Diagram and terminology drill: For Biology, redraw key figures from memory and define every labelled part in one sentence. Vocabulary precision prevents mark loss in descriptive answers — use NCERT terms exactly as printed in the textbook.

Revision tip: Link this topic to adjacent Class 12 chapters before attempting mixed practice.

Open the Practice tab for graded questions on Molecular Basis of Inheritance.