Molecular Basis of Inheritance
MIT‑Level Treatment – First Principles to Edge Cases
Audience – NEET Medical aspirants who want depth beyond the standard curriculum.
Goal – Build a conceptual bridge from chemical fundamentals to the genomic architectures that underlie human disease.
1. Foundations of Molecular Genetics
| Concept | Core Idea | Key Equation / Principle |
|---|---|---|
| Molecular Bonds | Covalent, hydrogen, ionic, Van der Waals | ΔG = ΔH – TΔS (Gibbs free energy governs bond stability) |
| Polymerization | Monomers → Polymers | n = number of monomer units; M = mass of polymer |
| Information Theory | DNA as a binary code | Shannon entropy ( H = -\sum p_i \log_2 p_i ) |
1.1 DNA: The Information Molecule
- Structure: Double helix, antiparallel strands, Watson‑Crick base pairs (A↔T, G↔C).
- Geometry: 10.5 bp/turn, pitch ≈ 34 Å; chirality: B‑DNA (right‑handed).
- Energetics: Base stacking dominates stability (~ 5 kcal/mol per pair).
First‑principle Insight
The free‑energy difference between base pairings arises from hydrogen bonding and π‑stacking, both derivable from quantum‑chemical calculations on nucleobase analogues. For example, the stabilization of G‑C over A‑T can be reproduced via density‑functional theory (DFT) with ~ 1 kcal/mol accuracy, confirming the observed GC bias in genomes.
2. DNA Replication: Kinetics and Fidelity
| Step | Enzyme | Mechanism | Fidelity Factor |
|---|---|---|---|
| Initiation | DNA helicase, DnaB (E. coli) | Unwinds helix via ATP hydrolysis | ~10⁶‑fold increase |
| Elongation | DNA polymerase III | 5’→3’ polymerization, 3’→5’ proofreading | 10⁻⁹ error rate |
| Termination | Tus/Ter system (E. coli) | Crossover inhibition | – |
2.1 Proofreading & Mismatch Repair
- Proofreading: 3’→5’ exonuclease removes mispaired dNTPs.
- MMR: MutS detects mismatches; MutL recruits MutH for nicking; exonuclease II removes 5’ segment.
- Error‑correction probability: ( P_{\text{correct}} = 1 - \epsilon ), where ( \epsilon \approx 10^{-9} ).
- Edge case: Pol α lacks proofreading → higher mutation frequency in lagging strand.
2.2 Thermodynamic Considerations
The Gibbs free energy of dNTP incorporation (ΔG⁰) is negative for correct pairs (e.g., GTP→C) and slightly positive for mismatches, providing a spontaneous selectivity. The kinetic barrier (activation energy) is lowered by polymerase’s conformational changes.
3. Transcription: From DNA to RNA
| Component | Role | Key Kinetic Parameter |
|---|---|---|
| Promoter | Binding site for RNA polymerase | ( k_{\text{init}} ) |
| σ‑factor | Promoter specificity | ( K_d ) |
| RNAP | Catalytic core | ( k_{\text{elong}} ) |
| Transcription factors | Activation/Repression | ( K_{\text{act}}/K_{\text{rep}} ) |
3.1 First‑Principle View of Transcription Initiation
The free‑energy landscape of promoter opening involves a barrier ( \Delta G^\ddagger ) that depends on DNA sequence and nucleosome occupancy. Thermodynamic models predict initiation rates via the Arrhenius equation:
[ k_{\text{init}} = A e^{-\Delta G^\ddagger / RT} ]
where ( A ) is the pre‑exponential factor (~10⁹ s⁻¹).
3.2 Edge Cases
- Promoter leakiness: Some “constitutive” promoters still exhibit basal transcription; explained by high σ‑factor dissociation constants.
- Transcriptional bursting: Stochastic initiation leads to pulse‑like mRNA production; captured by two‑state promoter models (ON/OFF).
4. RNA Processing & Regulation
| Process | Key Players | Functional Outcome |
|---|---|---|
| Splicing | U1, U2, U4, U5, U6 snRNPs | Exon ligation |
| RNA editing | ADAR, APOBEC | A→I, C→U changes |
| Alternative splicing | SR proteins, hnRNPs | Transcript diversity |
| miRNA | Drosha, Dicer, RISC | Post‑transcriptional silencing |
| Epigenetics | DNA methyltransferases, histone acetyltransferases | Chromatin remodeling |
4.1 Thermodynamics of Splice Site Recognition
Splice sites obey the consensus GU–AG rule. The spliceosome’s assembly energy can be modeled as:
[ \Delta G_{\text{splice}} = \