Nucleotide Mutation
High-Yield Summary
- Mutation = a change in DNA sequence; the ultimate source of new genetic variation in a population's gene pool (all alleles present in that population).
- Point mutations (single nucleotide substitution): silent (same amino acid, code redundancy), missense (different amino acid), nonsense (creates a premature stop codon → truncated protein).
- Frameshift mutations: insertion/deletion NOT a multiple of 3 nucleotides → shifts entire downstream reading frame → generally MORE severe than point mutations (point mutations affect at most 1 amino acid; frameshifts affect everything downstream).
- Chromosomal mutations (larger scale): duplication (extra copy, gene dosage effect), inversion (segment flips, order changes but no material lost), deletion (segment removed, often harmful), insertion (extra DNA added), translocation (segment moves to a nonhomologous chromosome — linked to certain cancers).
- Mutations fall on a spectrum: beneficial (↑ fitness, natural selection may increase frequency), neutral (no survival/reproduction effect, spreads by chance), harmful (disrupts function/regulation → disease).
- Population-level allele redistribution (separate from mutation): gene flow = migration moves alleles between populations (↑ within-population variation, ↓ between-population differences). Genetic drift = random chance changes allele frequencies, strongest in SMALL populations.
- Founder effect = small group starts a new population with only a subset of original variation. Bottleneck effect = existing population crashes in size, survivors don't represent full original diversity. Both are forms of genetic drift.
Point Mutations
| Type | Effect |
|---|---|
| Silent | Codon still codes for the same amino acid — no change (genetic code redundancy) |
| Missense | Codon codes for a different amino acid — severity depends on the substitution and location |
| Nonsense | Codon becomes a stop codon — premature termination, usually truncated/nonfunctional protein |
Mutation vs. Gene Flow vs. Genetic Drift
| Mechanism | What It Does |
|---|---|
| Mutation | INTRODUCES new variation into the gene pool |
| Gene flow | REDISTRIBUTES existing variation between populations (migration/interbreeding) |
| Genetic drift | CHANGES allele frequencies purely by chance; strongest in small populations (founder + bottleneck effects) |
Key Terms
- Gene pool
- The complete set of alleles present in a population.
- Frameshift mutation
- Insertion/deletion of nucleotides not in multiples of 3, shifting the entire downstream reading frame.
- Translocation
- A chromosome segment breaks off and attaches to a different, nonhomologous chromosome.
- Founder effect
- A small group establishing a new population carries only a subset of the original population's genetic variation.
- Bottleneck effect
- A population's genetic diversity is reduced after a drastic size crash, regardless of allele benefit.
Common MCAT Trap
- Frameshift mutations are generally MORE severe than point mutations — a point mutation affects at most one codon/amino acid, a frameshift alters every codon downstream of the mutation site.
- An insertion/deletion of 3 nucleotides (or a multiple of 3) does NOT cause a frameshift — it adds/removes whole codons, keeping the reading frame intact (though it can still add/remove amino acids).
- Genetic drift is NOT driven by fitness (unlike natural selection) — it's random sampling error, and its effect is inversely related to population size (bigger population = less drift).
- Gene flow increases variation WITHIN populations but decreases differences BETWEEN populations — the opposite of what intuition about 'flow adding diversity everywhere' might suggest.
Quick Recall
Why are frameshift mutations generally more damaging than point mutations?
What's the difference between the founder effect and the bottleneck effect?
A chromosomal segment flips orientation but no genetic material is lost — what mutation type is this?
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