Protein synthesis & mutations
Big question: How does an error during DNA replication affect protein synthesis and function? Read the notes (10 min), take the quiz alone without looking (10 min), then watch the links for anything you missed.
1. The path: DNA → RNA → protein
- Replication (S phase, nucleus): DNA copies itself. DNA polymerase proofreads, but a few errors slip through. An error that stays = a mutation.
- Transcription (nucleus): the gene's DNA is copied into mRNA. Base pairing: A→U, T→A, C→G, G→C. Example: template DNA TAC → mRNA AUG.
- Translation (ribosome, cytoplasm): the ribosome reads mRNA 3 bases at a time (codons). Each tRNA has an anticodon that matches a codon and carries one amino acid. mRNA AUG ↔ anticodon UAC.
- Protein shape → function. The amino acid order decides how the protein folds. Change the order and the shape (and the job) can change.
Codons used on this page
| mRNA codon | Amino acid | mRNA codon | Amino acid |
|---|---|---|---|
| AUG | Met (START) | GAG | Glu |
| GCA, GCG | Ala | GUG | Val |
| UUU | Phe | UAC | Tyr |
| AAA | Lys | UGG | Trp |
| GGC | Gly | UAA, UAG, UGA | STOP |
2. Kinds of mutations
| Type | What happens | Effect on protein |
|---|---|---|
| Silent (substitution) | One base swapped, codon still means the same amino acid. GCA→GCG (Ala→Ala) | None |
| Missense (substitution) | One base swapped → a different amino acid. GAG→GUG (Glu→Val) = sickle cell | None to severe. Depends where and how different the new amino acid is |
| Nonsense (substitution) | One base swapped → a STOP codon. UAC→UAA (Tyr→STOP) | Protein cut short, usually doesn't work |
| Frameshift (insertion or deletion of 1 or 2 bases) | Reading frame shifts, every codon after it changes | Usually severe: many wrong amino acids, often an early STOP |
| Insertion/deletion of 3 bases | Adds or removes one whole codon; frame stays | One amino acid added or missing; often milder |
Other must-knows
- Causes: random replication errors, or mutagens such as UV light (makes thymine dimers), X-rays, chemicals in tobacco smoke.
- Body cell vs. sex cell: a mutation in a skin cell affects only that person (could lead to cancer). Only mutations in gametes (egg/sperm) pass to offspring.
- Not always bad: most are neutral, some harmful, a few helpful. Mutations are the source of new variation for evolution.
- Chromosome-level mutations (bigger): deletion, duplication, inversion, translocation of a chunk of chromosome.
3. Practice quiz
Tap an answer. Try first, then read why. Do it again tomorrow: questions shuffle each time.
4. Explain it (test-style short answer)
Q1. A DNA replication error changes one base. Explain two different ways this could affect the protein.
Check your answer has: (1) it could be silent: same amino acid because several codons code for the same one, so no change; (2) missense: a different amino acid → protein may fold differently → may not work (e.g., sickle cell hemoglobin); or nonsense: early STOP → short, non-working protein. Use the words codon, amino acid, shape, function.
Q2. Why is a one-base deletion usually worse than a one-base substitution?
Check your answer has: the deletion shifts the reading frame, so every codon after it is read wrong (many amino acids change, often an early STOP). A substitution changes at most one codon.
Q3. Claim-Evidence-Reasoning: "Sickle cell disease is caused by a mutation."
Check your answer has: Claim: one base substitution in the hemoglobin gene causes it. Evidence: the codon GAG becomes GUG, so glutamic acid (Glu) is replaced by valine (Val). Reasoning: the different amino acid changes how hemoglobin folds and sticks together → red blood cells become sickle-shaped → carry oxygen poorly and block vessels.
5. Best resources (free)
- Learn.Genetics: Transcribe and Translate a Gene (interactive; do this first)
- Learn.Genetics: What is Mutation? and The Outcome of Mutation (same site your class used)
- HHMI BioInteractive: Transcription animation and Translation animation (short, clear)
- Amoeba Sisters: Mutations (Updated) (fun, ~7 min)
- Khan Academy: Impact of mutations on amino acids (video) and Khan Academy: Mutations practice (extra questions)
General study notes, not from any class materials or answer keys.