Short on time? This page is the recap. For the full code table, Figure 1 and worked examples, use the full study guide.
Recap
- Translation turns an mRNA base sequence into a polypeptide’s amino acid sequence. This is how genotype shapes phenotype.
- It happens on ribosomes free in the cytoplasm (all cells) and on the cytoplasmic surface of the rough ER (eukaryotes).
- Prokaryotes translate mRNA while it is still being transcribed, because there is no nucleus.
- mRNA is read in codons (triplets). 64 codons: 61 code for the 20 amino acids, 3 are stop codons. Many amino acids have more than one codon.
- Nearly all organisms share the same code, which is evidence for the common ancestry of life.
Key relationships
| Stage | What happens |
|---|---|
| Initiation | ribosomal rRNA lines up the mRNA at the start codon AUG; tRNA carrying Met pairs with it; ribosome assembles; reading frame set |
| Elongation | tRNA with matching anticodon brings its amino acid; chain transferred to it (peptide bond); ribosome moves one codon towards 3′ |
| Termination | stop codon reached; no tRNA pairs; polypeptide released |
| Molecule | Role |
|---|---|
| mRNA | the message, read 5′ → 3′ |
| tRNA | adaptor: one amino acid, one anticodon |
| rRNA (+ proteins) | ribosome structure; interacts with mRNA at the start codon |
| Reverse transcriptase | retroviral enzyme: copies RNA genome into DNA, which integrates into host DNA |
| Counting rule | Use |
|---|---|
| amino acids = (translated nucleotides including stop) ÷ 3 − 1 | the stop codon adds no amino acid |
| coding nucleotides = (amino acids + 1) × 3 | include the stop codon |
Assumptions behind the numbers
- Counting starts at the A of AUG and ends with the last base of the stop codon.
- Untranslated bases before AUG and after the stop codon are not counted as codons.
Mistakes to avoid
- Starting translation at the first base instead of at AUG.
- Writing anticodons identical to codons, or forgetting they run 3′ → 5′.
- Giving the stop codon an amino acid.
- Saying all ribosomes are on the rough ER.
- Describing retroviruses as making protein directly from RNA without a DNA step.
Quick self-check
- Translate 5′-AUGCCCUGA-3′. (Met–Pro, then stop)
- Anticodon that pairs with the codon 5′-UGG-3′? (3′-ACC-5′)
- Coding nucleotides, including the stop codon, for a 300-amino-acid polypeptide? ((300 + 1) × 3 = 903 nt)
- Why does a shared code support common ancestry? (The same codon meanings in nearly all organisms are best explained by inheritance from one common ancestor.)
Next: practice questions.