⚡ Mutations
Introduction
- Mutation = A sudden, heritable change in the DNA sequence of an organism. Mutations are the ultimate source of all genetic variation — the raw material for evolution.
- Term coined by Hugo de Vries (1901) based on his work on Oenothera lamarckiana (evening primrose).
- Mutation Theory of Evolution (de Vries): evolution occurs by large, sudden changes rather than gradual variation. We now know both mutations and gradual change contribute to evolution.
- Mutagen = Agent that causes mutations (physical, chemical, or biological).
- Mutant = Organism carrying a mutation.
Types of Mutations
I. Based on Cell Type
| Type | Occurrence | Inheritance |
|---|---|---|
| Somatic mutations | In body (somatic) cells | Not inherited; affect only the individual |
| Germinal (Germ-line) mutations | In reproductive (germ) cells | Inherited by offspring |
IMPORTANT
Only germline mutations are passed to the next generation and are therefore relevant to evolution and hereditary diseases. Somatic mutations affect only the individual (e.g., cancer) but are not passed to offspring.
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Introduction
- Mutation = A sudden, heritable change in the DNA sequence of an organism. Mutations are the ultimate source of all genetic variation — the raw material for evolution.
- Term coined by Hugo de Vries (1901) based on his work on Oenothera lamarckiana (evening primrose).
- Mutation Theory of Evolution (de Vries): evolution occurs by large, sudden changes rather than gradual variation. We now know both mutations and gradual change contribute to evolution.
- Mutagen = Agent that causes mutations (physical, chemical, or biological).
- Mutant = Organism carrying a mutation.
Types of Mutations
I. Based on Cell Type
| Type | Occurrence | Inheritance |
|---|---|---|
| Somatic mutations | In body (somatic) cells | Not inherited; affect only the individual |
| Germinal (Germ-line) mutations | In reproductive (germ) cells | Inherited by offspring |
IMPORTANT
Only germline mutations are passed to the next generation and are therefore relevant to evolution and hereditary diseases. Somatic mutations affect only the individual (e.g., cancer) but are not passed to offspring.
II. Chromosomal Mutations
Changes in the number or structure of chromosomes. Large-scale mutations visible under a microscope.
A. Changes in Chromosome Number
1. Euploidy — Changes involving complete sets of chromosomes.
| Type | Chromosome Number | Description | Example |
|---|---|---|---|
| Monoploidy (Haploid) | n | One set | Male honey bee (drone) |
| Diploidy | 2n | Normal two sets | Most organisms |
| Triploidy | 3n | Three sets | Banana (seedless), watermelon |
| Tetraploidy | 4n | Four sets | Potato, coffee |
| Polyploidy | >2n | More than two sets | Common in plants |
- Autopolyploidy — multiple sets from the same species (e.g., AAAA in autotetraploid).
- Allopolyploidy — multiple sets from different species (e.g., AABB). Example: Triticum aestivum (bread wheat, 6n = AABBDD) is an allohexaploid — genomes from three different ancestral species.
- Colchicine — alkaloid from Colchicum autumnale; inhibits spindle formation (prevents microtubule assembly) → induces polyploidy; used in plant breeding.
2. Aneuploidy — Changes involving individual chromosomes (gain or loss of one or a few), NOT in complete sets.
| Type | Formula | Description | Human Example |
|---|---|---|---|
| Monosomy | 2n - 1 | Loss of one chromosome | Turner syndrome (45, X0) |
| Trisomy | 2n + 1 | Gain of one extra chromosome | Down syndrome (47, +21) |
| Tetrasomy | 2n + 2 | Gain of two extra homologous chromosomes | Rare |
| Nullisomy | 2n - 2 | Loss of both homologs | Lethal in most organisms |
Common Human Aneuploidies:
| Syndrome | Karyotype | Chromosomal Basis | Features |
|---|---|---|---|
| Down syndrome | 47, +21 (Trisomy 21) | Extra chromosome 21 | Mental retardation, flat face, short stature, simian crease |
| Turner syndrome | 45, X0 (Monosomy X) | Missing one X chromosome | Female, short stature, webbed neck, infertile |
| Klinefelter syndrome | 47, XXY | Extra X in males | Male, tall, gynecomastia, infertile |
| Patau syndrome | 47, +13 (Trisomy 13) | Extra chromosome 13 | Cleft lip/palate, polydactyly, severe defects |
| Edwards syndrome | 47, +18 (Trisomy 18) | Extra chromosome 18 | Clenched fists, rocker-bottom feet |
| Super female | 47, XXX | Extra X in females | Tall, usually normal fertility |
| Super male (XYY) | 47, XYY | Extra Y in males | Tall |
B. Changes in Chromosome Structure (Chromosomal Aberrations)
| Type | Description | Effect / Example |
|---|---|---|
| Deletion (Deficiency) | Loss of a segment | Loss of genes; often lethal. Example: Cri-du-chat (5p deletion) |
| Duplication | Segment repeated on same chromosome | Gene dosage effect. Example: Bar eye in Drosophila (16A duplication on X) |
| Inversion | Segment reversed 180° | Genes in reverse order. Paracentric (excludes centromere); Pericentric (includes centromere) |
| Translocation | Segment moved to non-homologous chromosome | Reciprocal (exchange); Robertsonian (fusion). Example: Philadelphia chromosome (t(9;22)) → CML |
Image Generation Prompt
Four panel diagram showing chromosomal structural aberrations: (1) Deletion — chromosome ABCDEF losing segment CD → ABEF; (2) Duplication — segment BC repeated → ABBCDEF; (3) Inversion — segment BCD reversed → ADCBEF; (4) Translocation — segment exchange between two non-homologous chromosomes. Label each type clearly. Scientific illustration.
III. Gene Mutations (Point Mutations)
Changes at the level of a single gene — one or a few nucleotide base pairs. Most common type.
A. Substitution Mutations
One base pair replaced by another.
| Type | Change | Example |
|---|---|---|
| Transition | Purine↔Purine (A↔G) or Pyrimidine↔Pyrimidine (C↔T) | Most common; swaps bases of similar size |
| Transversion | Purine↔Pyrimidine (A↔C, A↔T, G↔C, G↔T) | Less common; swaps large for small |
Effects of substitution on the protein:
| Type | Effect |
|---|---|
| Silent (Synonymous) mutation | New codon codes for the same amino acid (due to degeneracy). Protein unaffected. |
| Missense mutation | New codon codes for a different amino acid. Example: Sickle cell anemia — GAG→GUG, Glu→Val at 6th position of β-globin |
| Nonsense mutation | New codon is a stop codon → premature termination → truncated, non-functional protein |
Why is sickle cell anemia the classic missense mutation example?
A single nucleotide change (A→T) in the β-globin gene changes codon 6 from GAG (glutamic acid) to GUG (valine). This single amino acid substitution causes hemoglobin to polymerize under low oxygen, distorting RBCs into a sickle shape — demonstrating how one base change can have profound phenotypic effects.B. Frame-Shift Mutations
Insertion or deletion of nucleotides not in multiples of 3 → shifts the reading frame, corrupting all downstream codons.
| Type | Effect |
|---|---|
| Insertion | Addition of 1+ base pairs → shifts reading frame from that point |
| Deletion | Loss of 1+ base pairs → shifts reading frame from that point |
- Frame-shifts usually produce non-functional proteins.
- Insertions/deletions of multiples of 3 are in-frame (only add/remove amino acids, don't shift the frame).
Mutagens
Physical Mutagens
| Mutagen | Effect |
|---|---|
| UV radiation | Causes thymine dimers (covalent bonds between adjacent thymines on the same strand) |
| X-rays, γ-rays (ionizing radiation) | Cause chromosomal breaks, deletions, translocations |
| Heat | Causes depurination (loss of purine bases) and deamination |
Chemical Mutagens
| Mutagen | Effect |
|---|---|
| Nitrous acid (HNO₂) | Deamination: C→U (reads as T), A→Hypoxanthine (reads as G) → transitions |
| Base analogs (5-bromouracil, 2-aminopurine) | Incorporated instead of normal bases → cause transitions during replication |
| Alkylating agents (EMS, MMS, nitrogen mustard) | Add alkyl groups → mispairing → transitions. EMS is the most commonly used chemical mutagen in plant breeding |
| Acridine dyes (acridine orange, proflavine, ethidium bromide) | Intercalate between base pairs → cause insertions or deletions (frame-shifts) |
| Colchicine | Inhibits spindle formation → polyploidy |
NOTE
EMS (Ethyl Methane Sulfonate) is preferred in mutation breeding because it induces high-frequency point mutations without chromosomal damage and is easy to handle.
DNA Repair Mechanisms
Cells have evolved repair systems to correct DNA damage and prevent mutation accumulation.
| Mechanism | Description |
|---|---|
| Photoreactivation (Light repair) | Photolyase enzyme uses visible light energy to break thymine dimers. Simplest repair mechanism. |
| Excision repair (Dark repair) | Endonuclease cuts damaged strand → exonuclease removes damaged bases → DNA Pol I fills gap → ligase seals. Most common. Does not require light. |
| Mismatch repair | Corrects mismatched bases after replication; distinguishes new strand (unmethylated) from old strand (methylated) to know which to correct. |
| SOS repair | Error-prone repair activated under severe DNA damage; allows replication past lesions but introduces mutations. Last resort. |
Key Points to Remember
- Mutation coined by Hugo de Vries (1901), Oenothera lamarckiana.
- Germline mutations → inherited; somatic mutations → not inherited (affect individual only).
- Euploidy = change in complete chromosome sets; Aneuploidy = change in individual chromosomes.
- Allopolyploidy example: bread wheat (6n = AABBDD); Colchicine (Colchicum autumnale) induces polyploidy.
- Down syndrome = Trisomy 21 (47, +21); Turner = Monosomy X (45, X0); Klinefelter = 47, XXY.
- Cri-du-chat = 5p deletion; Philadelphia chromosome = t(9;22) → CML.
- Transitions (purine↔purine, pyrimidine↔pyrimidine) more common than transversions.
- Sickle cell anemia = missense point mutation: GAG→GUG (Glu→Val, 6th position β-globin).
- UV → thymine dimers; EMS → alkylation → transitions; acridine dyes → frame-shifts.
- Photolyase (light-dependent) fixes thymine dimers; excision repair (dark) is most common.
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Mutation | Sudden, heritable change in DNA; ultimate source of all genetic variation |
| Term coined by | Hugo de Vries (1901); worked on Oenothera lamarckiana (evening primrose) |
| Mutagen | Agent causing mutations (physical, chemical, biological) |
| Somatic mutations | In body cells; not inherited; affect only individual (e.g., cancer) |
| Germinal mutations | In reproductive cells; inherited by offspring; relevant to evolution |
| Euploidy | Changes in complete chromosome sets (monoploidy, triploidy, tetraploidy, etc.) |
| Autopolyploidy | Multiple sets from same species (AAAA) |
| Allopolyploidy | Multiple sets from different species (AABB); bread wheat = 6n = AABBDD (allohexaploid) |
| Colchicine | From Colchicum autumnale; inhibits spindle formation → induces polyploidy; used in plant breeding |
| Aneuploidy | Changes in individual chromosomes (not complete sets) |
| Monosomy (2n−1) | Loss of one chromosome; e.g., Turner syndrome (45, X0) |
| Trisomy (2n+1) | Extra chromosome; e.g., Down syndrome (47, +21) |
| Klinefelter syndrome | 47, XXY; male, tall, infertile |
| Patau syndrome | Trisomy 13; cleft lip/palate, polydactyly |
| Edwards syndrome | Trisomy 18; clenched fists, rocker-bottom feet |
| Deletion | Loss of chromosome segment; Cri-du-chat (5p deletion) |
| Duplication | Segment repeated; Bar eye in Drosophila |
| Inversion | Segment reversed 180°; paracentric (excludes centromere) or pericentric (includes) |
| Translocation | Segment moved to non-homologous chromosome; Philadelphia chromosome t(9;22) → CML |
| Transition | Purine↔Purine (A↔G) or Pyrimidine↔Pyrimidine (C↔T); more common |
| Transversion | Purine↔Pyrimidine; less common |
| Silent mutation | New codon = same amino acid (due to degeneracy) |
| Missense mutation | New codon = different amino acid; e.g., sickle cell: GAG→GUG (Glu→Val, position 6) |
| Nonsense mutation | New codon = stop codon → truncated protein |
| Frame-shift mutations | Insertion/deletion not in multiples of 3 → shifts reading frame; usually non-functional protein |
| UV radiation | Causes thymine dimers |
| EMS | Most commonly used chemical mutagen in plant breeding; causes transitions via alkylation |
| Acridine dyes | Intercalate between bases → insertions/deletions (frame-shifts) |
| Nitrous acid | Causes deamination → transitions |
| Photoreactivation | Photolyase uses light to break thymine dimers |
| Excision repair (dark repair) | Endonuclease cuts → exonuclease removes → DNA Pol I fills → ligase seals; most common |
| Mismatch repair | Corrects mismatched bases after replication; distinguishes new (unmethylated) from old strand |
| SOS repair | Error-prone; last resort under severe damage; introduces mutations |
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