Translation
High-Yield Summary
- Translation needs mRNA, tRNA, amino acids, and GTP, and happens at the ribosome. Prokaryotic ribosome: 70S (50S + 30S). Eukaryotic: 80S (60S + 40S). "S" = sedimentation rate, not simple addition.
- A site: incoming aminoacyl-tRNA enters here. P site: holds the growing polypeptide chain. E site: spent tRNA exits here.
- Initiation: small subunit binds mRNA (Shine-Dalgarno sequence in prokaryotes; 5' cap + scanning in eukaryotes) → finds AUG → initiator tRNA (carrying methionine) binds directly at the P site (unique — every other tRNA enters via A site) → large subunit joins.
- Elongation cycle: new tRNA enters A site → peptidyl transferase (an rRNA-catalyzed, ribozyme reaction) forms peptide bond between A-site amino acid and P-site chain → translocation shifts ribosome forward (P→E site tRNA exits, A→P site tRNA moves, A site opens). Elongation factors + GTP drive this.
- Termination: ribosome reaches a stop codon (UAA/UAG/UGA) — no tRNA binds — release factor binds instead, cleaving the finished polypeptide free. Signal sequences then direct many eukaryotic proteins to their destination.
- Posttranslational modification: structural (cleavage, signal sequence removal, subunit assembly) and chemical additions (phosphorylation = signaling switch, carboxylation = Ca²⁺ binding, glycosylation = targeting/immune recognition, prenylation = membrane anchoring).
Key Terms
- A site (aminoacyl)
- Ribosome site where incoming charged tRNA first enters.
- P site (peptidyl)
- Ribosome site holding the growing polypeptide chain.
- E site (exit)
- Ribosome site tRNA passes through before leaving.
- Peptidyl transferase
- rRNA-catalyzed activity forming the peptide bond between A-site and P-site residues.
- Translocation
- Ribosome's forward shift along mRNA after each peptide bond forms, moving tRNAs P→E and A→P.
- Signal sequence
- Short peptide tag directing a finished protein to its correct cellular location.
Ribosome Size and Initiation: Prokaryotes vs. Eukaryotes
| Feature | Prokaryotes vs. Eukaryotes |
|---|---|
| Full ribosome / subunits | 70S (50S + 30S) / 80S (60S + 40S) |
| Small subunit binding site | Shine-Dalgarno sequence / 5' cap, then scans for AUG |
| Start codon | AUG / AUG |
Elongation Cycle (Repeats per Codon)
- 1A new tRNA enters the A site, carrying the amino acid matching that codon.
- 2Peptidyl transferase (rRNA) catalyzes a peptide bond between the A-site amino acid and the P-site chain.
- 3Translocation: P-site tRNA moves to the E site and exits; A-site tRNA (now holding the chain) moves to the P site.
- 4The A site opens for the next incoming tRNA — cycle repeats until a stop codon is reached.
Posttranslational Chemical Additions
| Modification | Functional Effect |
|---|---|
| Phosphorylation | Common on/off regulatory switch; central to signaling |
| Carboxylation | Important for calcium binding; stabilizes metal-ion-binding proteins |
| Glycosylation | Guides proteins to correct destination; immune recognition, signaling |
| Prenylation | Anchors the protein into cellular membranes |
Common MCAT Trap
- The initiator tRNA is the one exception that binds the P site directly during initiation — every other tRNA throughout elongation enters via the A site first.
- Peptidyl transferase activity comes from rRNA, not a protein enzyme — it's a classic ribozyme example, reinforcing that RNA can be catalytic.
- 70S ≠ 50S + 30S arithmetically — Svedberg (S) units reflect sedimentation behavior (shape + density), not additive mass.
Quick Recall
Which ribosomal site does the initiator tRNA bind directly, unlike every other tRNA?
What catalyzes peptide bond formation, and what kind of molecule is it?
What stops translation, and what protein carries this out?
Which posttranslational modification anchors a protein into a membrane?