Mutation and DNA repair

In short

A mutation is any heritable change in the DNA sequence — from a single base substitution to loss of a whole chromosome segment — arising spontaneously from replication errors and chemical decay of DNA, or induced by physical and chemical mutagens. DNA repair is the set of enzymatic pathways (direct reversal, base excision, nucleotide excision, mismatch, recombinational and translesion systems) that restore sequence before the damage is fixed as a mutation. The balance between damage and repair explains genetic variation, ageing, inherited disorders such as xeroderma pigmentosum and Lynch syndrome, and almost all cancer. The topic is examined in B.Sc/M.Sc molecular genetics papers, CSIR-UGC NET Life Sciences, GATE BT/XL, ICMR-JRF and DBT-BET, and is used in Indian pharma genotoxicity testing (Ames and micronucleus assays under the New Drugs and Clinical Trials Rules, 2019, which replaced the former Schedule Y), in agri-biotech mutation breeding at BARC and IARI, and in clinical genomics roles at labs such as MedGenome, Strand and Tata Memorial.

Explained

What a mutation is, and how mutations are classified

• Mutation: a permanent, heritable alteration in nucleotide sequence; mutant = organism carrying it; wild type = the prevalent reference allele. Polymorphism is conventionally a variant at >1% frequency in a population.
• By cell type: germline mutations are transmitted to progeny and cause inherited disease; somatic mutations are confined to a cell lineage and cause cancer and mosaicism.
• By size/level: gene (point) mutations — one or few bases; chromosomal mutations — deletion, duplication, inversion, translocation; genomic mutations — aneuploidy, polyploidy.
• Base substitutions: transition = purine↔purine or pyrimidine↔pyrimidine (A↔G, C↔T); transversion = purine↔pyrimidine. Transitions are commoner, largely from 5-methylcytosine deamination at CpG sites.
• Coding consequences: silent (synonymous) — same amino acid, usually third-base; missense — amino acid change (conservative if chemically similar, e.g. HbS β6 Glu→Val in sickle-cell anaemia); nonsense — premature stop (β-thalassaemia alleles common in India); frameshift — insertion/deletion not a multiple of three, altering every downstream codon.
• Other functional classes: neutral, loss-of-function (amorph/hypomorph), gain-of-function (hypermorph/neomorph), conditional (e.g. temperature-sensitive), lethal, suppressor (second mutation restoring phenotype — intragenic or intergenic, e.g. a tRNA nonsense suppressor), back mutation (reversion) versus forward mutation.
• Splice-site, promoter and regulatory mutations change expression without altering coding sequence; triplet (trinucleotide) repeat expansion causes dynamic mutations — CAG in Huntington disease and SCA, CGG in fragile X, GAA in Friedreich ataxia — showing anticipation.
• Mutation hotspots: CpG dinucleotides, short direct repeats (slipped-strand mispairing), and runs of the same base (polymerase slippage).

Spontaneous mutation: sources

• Replication errors: DNA polymerase misincorporation (~10⁻⁵ per base), cut to ~10⁻⁷ by 3′→5′ proofreading exonuclease and to ~10⁻¹⁰ by mismatch repair.
• Tautomeric shift: bases flip between keto/enol and amino/imino forms; the rare tautomer mispairs (enol-T with G) giving transitions. Wobble/ionised base pairing has a similar effect.
• Depurination: spontaneous hydrolysis of the N-glycosidic bond — thousands of apurinic (AP) sites per mammalian cell per day.
• Deamination: cytosine → uracil (read as T, giving C→T transitions); 5-methylcytosine → thymine (not removed by uracil-DNA glycosylase, hence a hotspot); adenine → hypoxanthine; guanine → xanthine.
• Oxidative damage: reactive oxygen species give 8-oxoguanine (pairs with A → G:C→T:A transversions) and thymine glycol; single- and double-strand breaks.
• Strand slippage in repeats → microsatellite instability; transposable elements inserting into genes (e.g. Alu insertions).
• Luria–Delbrück fluctuation test (1943) proved spontaneous mutations arise randomly before selection, not as an adaptive response; replica plating (Lederbergs, 1952) confirmed it without exposing cells to the selective agent.

Induced mutation: mutagens and their mechanisms

• Base analogues: 5-bromouracil (thymine analogue, enolises and pairs with G → transitions); 2-aminopurine (pairs with C).
• Deaminating agents: nitrous acid (HNO₂) — C→U, A→hypoxanthine → transitions.
• Alkylating agents: EMS, MMS, dimethyl sulphate, nitrogen mustard, nitrosoguanidine — add alkyl groups (O⁶-methylguanine pairs with T → G:C→A:T transitions); bifunctional agents cross-link strands.
• Intercalating agents: ethidium bromide, acridine orange, proflavine — slot between base pairs and cause single-base insertions/deletions → frameshifts (used by Crick and Brenner to establish the triplet code).
• Hydroxylamine: hydroxylates cytosine, which then pairs with A — a one-direction (C→T) mutagen.
• Ultraviolet radiation (254 nm, absorption peak of DNA ~260 nm): adjacent pyrimidines form cyclobutane pyrimidine dimers and 6-4 photoproducts, blocking replication and transcription.
• Ionising radiation (X-rays, γ-rays): directly and through water radiolysis gives base damage, single- and double-strand breaks, chromosome aberrations; dose measured in gray (Gy = J/kg), biological dose in sievert (Sv). Radiation protection in India is regulated by the Atomic Energy Regulatory Board (AERB); the SHANTI Act, 2025 repeals the Atomic Energy Act 1962 and the Civil Liability for Nuclear Damage Act 2010.
• Chemical carcinogens needing activation: benzo[a]pyrene (cigarette smoke) → diol epoxide forming bulky guanine adducts; aflatoxin B₁ (Aspergillus on groundnut and maize, a monitored contaminant under FSSAI limits) → AFB₁-N7-guanine → G→T transversion at TP53 codon 249.
• Site-directed and transposon mutagenesis are laboratory tools to make defined mutations; chemical/physical mutagenesis (gamma rays, EMS) underlies mutation breeding — India has released a large number of mutant crop varieties through BARC's Trombay groundnut, mungbean and other lines.

Direct reversal, excision and mismatch repair

• Photoreactivation: photolyase uses 300–500 nm light energy (FADH⁻ and MTHF/8-HDF chromophores) to split pyrimidine dimers directly. Present in bacteria, plants, many animals — absent in placental mammals.
• Alkyltransferase: MGMT (Ada/Ogt in *E. coli*) transfers the methyl group of O⁶-methylguanine onto its own cysteine — suicide enzyme, no excision. MGMT promoter methylation predicts temozolomide response in glioma.
• Base excision repair (BER) — small, non-helix-distorting lesions (uracil, 8-oxoG, AP sites):
  1. A lesion-specific DNA glycosylase (UNG, OGG1, MUTYH) cleaves the N-glycosidic bond, releasing the base and leaving an AP site.
  2. AP endonuclease (APE1 / Exo III, Endo IV) nicks the backbone 5′ to the AP site; dRPase/polymerase removes the sugar-phosphate.
  3. DNA polymerase β (Pol I in *E. coli*) fills the single nucleotide (short-patch) or Pol δ/ε fills 2–10 nt (long-patch).
  4. DNA ligase III–XRCC1 (LigA in bacteria) seals the nick.
• Nucleotide excision repair (NER) — bulky, helix-distorting lesions (pyrimidine dimers, adducts):
  • *E. coli*: UvrA₂B scans, UvrB verifies, UvrC incises 8 nt 5′ and 4–5 nt 3′ of the lesion, UvrD (helicase II) removes the 12–13 nt oligomer, Pol I fills, ligase seals.
  • Human: XPC-RAD23B (global genome) or stalled RNA Pol II with CSA/CSB (transcription-coupled repair) recognises the lesion; TFIIH (XPB, XPD helicases) opens the bubble; XPA and RPA verify; XPF-ERCC1 and XPG incise, excising ~24–32 nt; Pol δ/ε/κ fill; ligase I or III seals.
  • Defects: xeroderma pigmentosum (XPA–XPG; extreme UV sensitivity, >1,000-fold skin cancer risk), Cockayne syndrome and trichothiodystrophy (transcription-coupled defects).
• Mismatch repair (MMR) — corrects replication mismatches and small indels; must identify the new strand:
  • *E. coli*: MutS binds the mismatch, MutL couples, MutH (methyl-directed) nicks the unmethylated new strand at a hemimethylated GATC (Dam site); helicase II and exonucleases excise; Pol III and ligase restore.
  • Human: MSH2–MSH6/MSH3 and MLH1–PMS2; strand discrimination uses nicks and PCNA orientation, not Dam methylation (humans lack the MutH equivalent).
  • Defects cause microsatellite instability and Lynch syndrome (HNPCC) — hereditary colorectal and endometrial cancer; MSI/IHC testing is routine in Indian tertiary oncology centres and predicts immune-checkpoint-inhibitor response.
• Mismatch repair of 8-oxoG: the GO system — MutT hydrolyses 8-oxo-dGTP, MutM removes 8-oxoG opposite C, MutY removes the mispaired A.

Double-strand break repair, tolerance and damage response

• Homologous recombination (HR) — error-free, needs a sister chromatid, so works in S/G2: MRN (Mre11-Rad50-Nbs1) resects, RPA then RAD51 (with BRCA2, RAD52 in yeast; RecA in bacteria) forms the invading nucleoprotein filament, D-loop formation, extension, resolution through Holliday junctions (resolvase/GEN1) or synthesis-dependent strand annealing.
• Non-homologous end joining (NHEJ) — fast, template-free, works throughout the cycle: Ku70/Ku80 binds the ends, DNA-PKcs, Artemis trims, and DNA ligase IV–XRCC4–XLF joins; may add or lose a few bases, so it is error-prone. It is also the physiological mechanism of V(D)J recombination.
• Microhomology-mediated end joining (alt-EJ) and single-strand annealing are backup, deletion-prone routes.
• Interstrand cross-link repair: the Fanconi anaemia pathway (FANCD2/FANCI) with NER and HR components; defects give chromosome breakage and sensitivity to mitomycin C/diepoxybutane (the DEB test).
• Damage signalling: ATM (double-strand breaks) and ATR (replication stress) phosphorylate H2AX (γH2AX, the standard DSB marker), CHK1/CHK2 and p53, producing cell-cycle arrest at G1/S and G2/M, repair, senescence or apoptosis.
• SOS response in *E. coli*: RecA coated on single-stranded DNA activates cleavage of the repressor LexA, derepressing ~40 genes (uvrA/B/C, recA, sulA, umuDC); translesion polymerases Pol V (UmuD′₂C) and Pol IV (DinB) copy past lesions without proofreading — error-prone, so SOS repair is itself mutagenic.
• Translesion synthesis in humans: Pol η (XPV; defective in XP variant), Pol ι, Pol κ, REV1/Pol ζ — damage tolerance rather than repair.
• Therapeutic exploitation: PARP inhibitors (olaparib, talazoparib) are synthetically lethal in BRCA1/BRCA2-mutant breast and ovarian cancer — BRCA testing and these drugs are in Indian clinical practice; platinum agents and radiotherapy work by overwhelming repair.

Detecting mutations and measuring mutagenicity

• Phenotypic/genetic screens: replica plating for auxotrophs, CIB and attached-X tests in *Drosophila* for sex-linked lethals, specific-locus (mouse) tests.
• Ames test (Salmonella/microsome assay): histidine-auxotroph *S. typhimurium* strains (TA1535 base-substitution, TA98 frameshift, TA100, TA1537) with *rfa* (permeable wall) and *uvrB* (repair-deficient) markers; revertant colonies on minimal medium with trace histidine; S9 liver microsomal fraction supplies metabolic activation. A doubling (or ≥2× dose-related increase) of revertants is positive.
• Other genotoxicity assays: in-vitro chromosomal aberration, micronucleus test, comet assay (single-cell gel electrophoresis), mouse lymphoma TK assay, in-vivo rodent studies. In India the battery required for new drugs follows the New Drugs and Clinical Trials Rules, 2019 (which replaced Schedule Y of the Drugs and Cosmetics Rules) and ICH S2(R1); GLP compliance is certified by the National GLP Compliance Monitoring Authority (DST), and institutional animal work needs CPCSEA clearance under the Prevention of Cruelty to Animals Act, 1960.
• Molecular detection: Sanger sequencing and NGS panels, allele-specific PCR/ARMS, RFLP for known point mutations (e.g. MstII for HbS), MLPA for deletions, repeat-primed PCR for expansions, ddPCR for low-frequency somatic variants.
• Variant interpretation: ACMG/AMP five-tier classification (pathogenic, likely pathogenic, VUS, likely benign, benign) with databases ClinVar, gnomAD, COSMIC and India-specific resources (IndiGen, GenomeIndia); HGVS nomenclature — c. for coding (c.76A>T), p. for protein (p.Glu6Val), g. for genomic.
• Regulatory/ethical frame in India: GM and recombinant work under the Rules, 1989 of the Environment (Protection) Act, 1986 with RCGM/GEAC and IBSC oversight; genetic testing and counselling practice guided by the ICMR National Ethical Guidelines (2017) and the DNA-based testing provisions debated under the Assisted Reproductive Technology Act, 2021 and the PCPNDT Act, 1994 (which bars sex selection).

Key points

• Mutation - heritable change in DNA sequence; germline mutations are inherited, somatic ones drive cancer.
• Transition vs transversion - purine↔purine/pyrimidine↔pyrimidine vs purine↔pyrimidine; transitions dominate, especially C→T at CpG.
• Frameshift - indel not a multiple of 3; shifts the reading frame and usually truncates the protein.
• Silent / missense / nonsense - no change, amino-acid change, premature stop respectively.
• Tautomeric shift - rare enol/imino base forms mispair during replication, the classic spontaneous mechanism.
• Deamination - C→U corrected by uracil-DNA glycosylase; 5-mC→T escapes it, making CpG a hotspot.
• Depurination and 8-oxoG - the commonest spontaneous lesions, repaired by base excision repair.
• Base analogue vs intercalator - 5-bromouracil gives substitutions, proflavine/acridine gives frameshifts.
• UV damage - cyclobutane pyrimidine dimers and 6-4 photoproducts, removed by photolyase (not in placental mammals) or NER.
• BER vs NER - single damaged base cut out by a glycosylase vs a 24–32 nt oligonucleotide excised around a bulky lesion.
• Mismatch repair - corrects polymerase errors; strand discrimination by Dam methylation (MutH) in *E. coli*, by nicks/PCNA in humans.
• Three-tier accuracy - polymerase selectivity, 3′→5′ proofreading, mismatch repair: together ~10⁻¹⁰ errors per base.
• HR vs NHEJ - template-dependent and error-free (S/G2) vs template-free, fast and error-prone (all phases).
• SOS response - RecA–LexA derepression; Pol V translesion synthesis makes survival possible but mutagenic.
• Disease links - XP (NER), Lynch syndrome (MMR), Fanconi anaemia (cross-link repair), ataxia telangiectasia (ATM), BRCA1/2 (HR).
• Ames test - his⁻ *Salmonella* reversion with S9 activation; the standard bacterial mutagenicity screen.
• Synthetic lethality - PARP inhibitors kill HR-deficient (BRCA-mutant) tumour cells selectively.
• Mutation breeding - gamma-ray/EMS mutagenesis has produced many released Indian crop varieties (BARC, IARI programmes).

Formulas and examples

Example 1 — mutation frequency and rate. A culture of 2 × 10⁸ *E. coli* cells is plated; 40 rifampicin-resistant colonies appear.

• Mutation frequency = 40 / (2 × 10⁸) = 2 × 10⁻⁷ per cell.
• If the culture grew from 2 × 10⁵ cells, the number of generations ≈ log₂(10³) ≈ 9.97, and cell divisions = 2 × 10⁸ − 2 × 10⁵ ≈ 1.998 × 10⁸.
• Rate per cell division ≈ 40 / 1.998 × 10⁸ ≈ 2.0 × 10⁻⁷ per division (close to the frequency here only because growth was large; the two are not the same quantity).

Example 2 — genome-wide spontaneous mutations. Human per-base mutation rate ≈ 1.2 × 10⁻⁸ per generation; diploid genome ≈ 6.4 × 10⁹ bp.

• New mutations per zygote = 1.2 × 10⁻⁸ × 6.4 × 10⁹ = ≈ 77.
• Coding sequence is ~1.5% of the genome: 0.015 × 77 ≈ 1.2 coding mutations per individual per generation — a figure that explains why most new variants are non-coding.

Example 3 — effect of proofreading and mismatch repair. Polymerase misinsertion 1 in 10⁵; proofreading removes 99% of these; mismatch repair removes 99.9% of what remains.

• After proofreading: 10⁻⁵ × 0.01 = 10⁻⁷.
• After MMR: 10⁻⁷ × 0.001 = 10⁻¹⁰ per base, i.e. ~4.6 × 10⁻⁴ error per copy of a 4.6 × 10⁶ bp *E. coli* genome, about one error every ~2,000 replications.

Example 4 — frameshift versus in-frame deletion. Normal mRNA: 5′-AUG GCU UCA GGU AAC UGA-3′ (Met-Ala-Ser-Gly-Asn-Stop).

• Delete the underlined single C (codon 2 GCU → GU…): 5′-AUG GUU CAG GUA ACU GA-3′ → Met-Val-Gln-Val-Thr… — every codon after the deletion changes; the stop is lost and read-through continues.
• Delete three bases (GCU): 5′-AUG UCA GGU AAC UGA-3′ → Met-Ser-Gly-Asn-Stop — one residue short, frame intact. This is why Δ3 (in-frame, e.g. ΔF508 in cystic fibrosis) is far milder than Δ1 or Δ2.

Example 5 — Ames test interpretation. Solvent control gives 25 revertants/plate; test compound gives 30, 58, 112 and 230 revertants at 1, 10, 100 and 500 µg/plate with S9.

• Ratios: 30/25 = 1.2; 58/25 = 2.3; 112/25 = 4.5; 230/25 = 9.2.
• Positive from 10 µg/plate with a clear dose-response → mutagenic, requiring metabolic activation (compare the −S9 plates; if negative there, the parent compound is a promutagen).
• Specific activity between 10 and 100 µg: (112 − 58)/(100 − 10) = 0.6 revertants per µg.

Where it is used

• Pharma genotoxicity and safety testing: Ames, micronucleus, comet and chromosomal aberration assays in CRO and in-house tox labs (Syngene, Jubilant Biosys, Sai Life, Advinus, Intox) for IND/NDA filings under the New Drugs and Clinical Trials Rules, 2019 and ICH S2(R1).
• Clinical genetics and oncology: NGS variant calling and reporting at MedGenome, Strand Life Sciences, Neuberg, Metropolis, AIIMS, Tata Memorial and SGPGI — BRCA1/2, MMR/MSI, EGFR and thalassaemia/sickle-cell mutation panels; the national sickle-cell anaemia elimination programme screens tribal districts.
• Agricultural mutation breeding and plant genomics: gamma-ray and EMS mutagenesis at BARC, IARI, ICAR institutes and TNAU to create and select mutant lines; mutation-based markers in crop improvement.
• Industrial strain improvement: random mutagenesis plus selection for higher-yielding antibiotic, enzyme, amino-acid and biofuel strains in fermentation companies.
• Bioinformatics roles: variant annotation pipelines (GATK, VEP, ANNOVAR), mutational signature analysis (COSMIC SBS signatures), population databases IndiGen and GenomeIndia — the commonest entry job for a B.Sc/M.Sc genetics graduate.
• Radiation protection and environmental monitoring: AERB-regulated dosimetry and cytogenetic biodosimetry (dicentric assay) for radiation workers; genotoxicity screening of effluents and pesticides for CPCB/FSSAI compliance.

Interview questions

• Q: Why are C→T transitions the most common point mutation in human genomes? A: 5-methylcytosine at CpG sites deaminates to thymine, which uracil-DNA glycosylase cannot recognise as foreign, so the lesion often escapes repair and is fixed.
• Q: Distinguish a mutation rate from a mutation frequency. A: Rate is new mutations per gene or base per cell division or generation; frequency is the proportion of mutants in a population at a given time, which also reflects selection and past history.
• Q: Why is non-homologous end joining error-prone while homologous recombination is not? A: NHEJ ligates broken ends directly, often after trimming, with no template; HR copies an intact homologous sister chromatid, so the original sequence is restored.
• Q: How does *E. coli* mismatch repair know which strand is new? A: The new strand is transiently unmethylated at GATC sites; MutH nicks that strand of the hemimethylated duplex, directing excision there.
• Q: Why do placental mammals get skin cancer from UV when bacteria can simply photoreactivate the damage? A: Placental mammals lack photolyase and rely on nucleotide excision repair; when NER is defective, as in xeroderma pigmentosum, dimers persist and cause C→T mutations in TP53 and other genes.
• Q: What does a positive Ames result only with S9 mean? A: The compound is a promutagen — it is not mutagenic itself but is converted to a reactive metabolite by liver microsomal enzymes.
• Q: Explain the principle behind PARP inhibitors in BRCA-mutant cancer. A: PARP inhibition blocks single-strand break repair; cells lacking BRCA-dependent homologous recombination cannot handle the resulting double-strand breaks, so they die while normal cells survive — synthetic lethality.
• Q: Why is the SOS response called error-prone repair? A: It induces translesion polymerases Pol IV and Pol V that replicate past damage without proofreading, allowing survival at the cost of introducing mutations.
• Q: How did the Luria–Delbrück experiment prove mutations are spontaneous? A: The number of phage-resistant colonies varied enormously between parallel cultures — far more than Poisson expectation — showing mutations arose randomly at different times during growth, not in response to phage exposure.
• Q: A patient's tumour shows microsatellite instability. What does that imply and what is the clinical use? A: Mismatch repair is deficient (sporadic MLH1 methylation or Lynch syndrome), so repeat tracts expand or contract; MSI-high tumours carry a high mutation burden and respond well to immune checkpoint inhibitors, and the finding triggers germline testing and family counselling.
• Q: Why does a nonsense mutation near the 3′ end of a gene often cause milder disease than one near the 5′ end? A: A late stop still produces most of the polypeptide and may escape nonsense-mediated decay, so residual function can remain; an early stop gives a severely truncated or absent protein.
• Q: Can a silent mutation ever be pathogenic? A: Yes — it can disrupt a splice site or exonic splicing enhancer, change mRNA secondary structure, or alter codon usage and translation rate, all of which can reduce functional protein.

Common mistakes

• Treating transition and transversion as "same base changed" versus "different base" — the distinction is purine/pyrimidine class, not direction.
• Saying a frameshift occurs with any insertion or deletion; only indels that are not multiples of three shift the frame.
• Writing that human cells repair UV dimers by photoreactivation — placental mammals have no photolyase; they use NER.
• Confusing BER with NER, or crediting *E. coli* NER to Uvr proteins and then giving the human pathway bacterial names in the same answer.
• Giving MutH-style Dam methylation strand discrimination for human mismatch repair, which uses nicks and PCNA instead.
• Calling the SOS response a repair pathway that restores sequence; it is a damage-tolerance, mutagenic response.
• Reporting an Ames test as positive from a single raised count without a dose-response and without comparing +S9 and −S9 plates; and in exam calculations, dividing mutants by surviving colonies rather than total cells plated.

Related topics

Molecular biology: replication, transcription and translation · Gene regulation and epigenetics · Recombinant DNA technology · PCR and DNA sequencing · Cancer genetics and oncogenes · Human cytogenetics and chromosomal disorders