The Genetic Code
High-Yield Summary
- Central dogma: DNA → (transcription) → RNA → (translation) → protein. A gene is a DNA unit coding for one protein or RNA molecule.
- mRNA carries instructions to the ribosome (monocistronic in eukaryotes = 1 protein/strand; polycistronic in prokaryotes = multiple proteins/strand). tRNA delivers amino acids via its anticodon, cloverleaf-shaped, "charged"/"activated" once bound to its amino acid (via aminoacyl-tRNA synthetases). rRNA is structural AND catalytic — a ribozyme that forms peptide bonds.
- Codon = 3-base mRNA sequence coding one amino acid; anticodon = complementary 3-base tRNA sequence. AUG = start codon (also codes methionine, always first amino acid). UAA/UGA/UAG = stop codons (code no amino acid).
- The code is degenerate — multiple codons can specify the same amino acid (e.g. 6 for leucine) — buffering against mutation. The wobble position (3rd codon base) is the most mutation-tolerant site.
- Point mutations: silent (nucleotide changes, amino acid doesn't — often wobble position), missense (different amino acid, e.g. sickle cell: glutamate→valine), nonsense (codon→stop codon, premature truncation). Frameshift (insertion/deletion, not substitution) shifts the entire downstream reading frame — usually most disruptive.
Key Terms
- Codon
- Three-base mRNA sequence specifying one amino acid (or start/stop signal).
- Anticodon
- Three-base tRNA sequence complementary to an mRNA codon.
- Degenerate code
- Property where multiple codons can specify the same amino acid.
- Wobble position
- The third base of a codon — the most flexible/mutation-tolerant position.
- Ribozyme
- An RNA molecule with catalytic activity (e.g. rRNA catalyzing peptide bond formation).
- Frameshift mutation
- Insertion or deletion of a nucleotide that shifts the reading frame for all downstream codons.
The Three RNA Types
| Type | Primary Role |
|---|---|
| mRNA (messenger) | Carries genetic instructions from DNA to ribosome; codons in triplets |
| tRNA (transfer) | Delivers correct amino acid to ribosome via anticodon; cloverleaf shape |
| rRNA (ribosomal) | Structural + catalytic ribozyme; catalyzes peptide bond formation |
Classifying Point Mutations
| Mutation Type | What Changes / Effect |
|---|---|
| Silent | Nucleotide changes, amino acid doesn't — protein unaffected |
| Missense | Codon specifies a different amino acid — single substitution (e.g. sickle cell) |
| Nonsense | Codon becomes a stop codon — premature termination, shortened protein |
| Frameshift | Insertion/deletion shifts entire downstream frame — usually most disruptive |
Common MCAT Trap
- AUG does double duty: it's both the start codon AND codes for methionine — don't treat it as a signal-only codon like the stop codons.
- A frameshift mutation is categorically different from silent/missense/nonsense — it comes from insertion/deletion (not substitution) and affects everything downstream, not just one codon.
- Silent mutations cluster at the wobble position (3rd base) because of code degeneracy — this is why the third base is described as most "tolerant" of change.
Quick Recall
What are the three stop codons?
What makes rRNA a ribozyme?
What amino acid substitution causes sickle cell disease, and what mutation type is that?
Why does the genetic code's degeneracy protect against mutation?