Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- A mutation is a change in the DNA sequence. It can change the type of protein, the amount of protein (for example, a promoter change), or nothing detectable.
- Mutations are caused by replication errors, failed repair, radiation and reactive chemicals. They are random: a mutagen raises the rate, not the direction.
- A mutation is beneficial, detrimental or neutral depending on its effect and the environment. Mutations are the original source of new alleles.
- Nondisjunction in meiosis gives n + 1 or n − 1 gametes, leading to aneuploidy. Whole extra sets are polyploidy. Structural changes (deletion, duplication, inversion, translocation) can cause disorders.
- Transformation, transduction, conjugation (horizontal gene transfer) and transposition add variation in prokaryotes; related viruses in one host cell can recombine.
Key relationships
| Change | Frame | Effect on polypeptide |
|---|---|---|
| Substitution → silent | kept | same amino acid |
| Substitution → missense | kept | one different amino acid |
| Substitution → nonsense | kept | early stop: shortened chain |
| Insert/delete 1, 2, 4, 5… bases | shifted | most later amino acids change; stop usually moves |
| Insert/delete 3, 6, 9… bases | kept | whole amino acids gained or lost |
| Nondisjunction in… | Gametes from one meiosis |
|---|---|
| Meiosis I | n+1, n+1, n−1, n−1 (all 4 abnormal) |
| Meiosis II (one cell) | n+1, n−1, n, n (2 of 4 abnormal) |
Assumptions behind the reasoning
- Codon tables apply to the mRNA. The coding strand matches the mRNA (T for U); the template strand is complementary.
- “Neutral” means no effect on fitness in that environment; it may matter elsewhere.
- A missense prediction depends on where the amino acid is and how different the new R group is.
Mistakes to avoid
- Saying a mutation “denatures” a protein.
- Saying a point mutation causes a frameshift.
- Calling every mutation harmful, or saying organisms mutate “in order to” adapt.
- Reading the template strand as if it were the mRNA.
- Forgetting that a promoter mutation changes the amount of protein, not its sequence.
- Naming a specific disorder instead of explaining the mechanism (specific disorders are not assessed).
Quick self-check
- mRNA codon UGG (Trp) becomes UGA. Type? (Nonsense: UGA is a stop codon.)
- Six adjacent bases are deleted from a coding sequence. Is the frame shifted? (No: 6 is a multiple of 3. Two amino acids are lost; if the gap spans codon boundaries, one amino acid at the join may also change.)
- Why can a mutation that helps in one place harm in another? (Its effect on survival and reproduction depends on the environment, for example a coat colour that hides on dark rock but stands out on pale sand.)
Next: practice questions.