MCAT topics → Gene Expression — Gene to Protein → Translation (Initiation, Elongation, Termination)

Translation (Initiation, Elongation, Termination)

MCAT trap: Mislocates peptide bond formation to the A site rather than the peptidyl transferase center. Peptide bond formation (peptidyl transferase activity) occurs at the peptidyl transferase center of the large subunit, transferring the growing chain from the P-site tRNA to the aminoacyl-tRNA in the A site.

Translation is one of the most detail-heavy mechanisms the MCAT tests — and the most common ribosome confusion is swapping prokaryotic and eukaryotic sizes. Prokaryotic ribosomes are 70S (30S + 50S); eukaryotic ribosomes are 80S (40S + 60S). This distinction is directly testable because antibiotics like streptomycin and erythromycin target bacterial 30S and 50S subunits specifically, leaving eukaryotic 80S ribosomes unharmed. Get the sizes backwards and every antibiotic selectivity question goes wrong. The exam also tests that peptidyl transferase activity is carried out by rRNA, not ribosomal protein — the ribosome is a ribozyme.

What makes translation tricky is that students memorize facts in isolation — A, P, E sites; 70S vs 80S; AUG start codon — without understanding how these facts connect mechanistically. The MCAT rewards the connected model. For example, knowing tRNA moves A→P→E is not enough; you need to know that peptide bond formation happens before translocation, so the tRNA arriving at the P site already carries the full growing chain. Similarly, knowing that peptidyl transferase activity resides in the large subunit's rRNA (not a protein!) matters because the exam can ask about catalytic RNA or use antibiotics that target this activity.

Two other traps show up constantly: swapping prokaryotic and eukaryotic ribosome sizes (70S is prokaryotic, 80S is eukaryotic), and not being able to explain why AUG loss kills initiation beyond just saying 'it's the start codon.' If you can trace a ribosome from small subunit recruitment through peptide release with chemistry attached to each step, you're in the range the MCAT rewards.

Common misconceptions

Common mistake
Wrong: Peptide bond formation occurs at the A site of the ribosome.
Right: Peptide bond formation (peptidyl transferase activity) occurs at the peptidyl transferase center of the large subunit, transferring the growing chain from the P-site tRNA to the aminoacyl-tRNA in the A site.
Peptide bond formation does not happen at the A site — it happens at the peptidyl transferase center (PTC) of the large ribosomal subunit, which sits at the interface between the A and P sites. The reaction transfers the growing polypeptide from the P-site tRNA onto the amino acid carried by the A-site tRNA, creating a new peptide bond. The A site is where the aminoacyl-tRNA is delivered and decoded, but the chemistry happens in the large subunit's active site, not at the decoding center.
Common mistake
Wrong: tRNA moves A → P → E through the ribosome without any chemical event between A and P site occupancy.
Right: When tRNA moves from the A site to the P site, peptide bond formation has already occurred — the growing polypeptide is transferred onto the incoming tRNA before translocation.
The A→P→E movement is not just physical shuffling — each transition is coupled to a chemical event. Specifically, peptide bond formation must occur before translocation: the polypeptide chain is transferred onto the A-site tRNA first, making it the new peptidyl-tRNA, and then EF-G (with GTP) drives translocation so that tRNA moves to the P site. By the time a tRNA occupies the P site, it is already carrying the elongated chain. Omitting this means you'll misattribute what's happening chemically during elongation.
Common mistake
Wrong: Eukaryotic ribosomes are 70S and prokaryotic ribosomes are 80S.
Right: Prokaryotic ribosomes are 70S (30S + 50S subunits) and eukaryotic ribosomes are 80S (40S + 60S subunits).
Prokaryotic ribosomes are 70S (composed of 30S and 50S subunits) and eukaryotic ribosomes are 80S (40S and 60S subunits) — not the reverse. A memory anchor: Pro-70, Eu-80 in alphabetical/size order. This distinction is clinically important because antibiotics like streptomycin and erythromycin target bacterial 30S and 50S subunits specifically, leaving eukaryotic 80S ribosomes unharmed — that selectivity is a direct consequence of the size difference.
Common mistake
Gap: Understands AUG is required for initiation but cannot explain the assembly mechanism or role of initiation factors
Translation initiation requires assembly of the ribosomal subunits at the start codon (AUG) with the help of initiation factors; loss of AUG prevents this assembly entirely.
AUG isn't just a signal the ribosome 'recognizes' passively — it's the anchor for a specific assembly process. In eukaryotes, the 43S preinitiation complex (small subunit + initiator Met-tRNA + initiation factors) scans the mRNA from the 5' cap until it finds AUG, where base pairing between the AUG and the anticodon of the initiator tRNA triggers large subunit joining. In prokaryotes, the Shine-Dalgarno sequence near the AUG positions the small subunit. Loss of AUG means the assembly signal is gone, so the full 80S (or 70S) ribosome never forms and elongation never begins.
Free Deck audit

See if your Anki deck covers this topic.

Upload your deck →
Guided session

Stuck on this? An AI tutor that probes your understanding.

Start a session →

What the exam tests

  1. Know the three ribosomal sites (A, P, E) and the direction tRNA flows through them during elongation — and what chemical state the tRNA is in at each site.
  2. Be able to walk through initiation, elongation, and termination identifying the required factors (initiation factors, EF-Tu, EF-G, release factors) and energy sources (GTP) at each phase.
  3. Understand that peptide bond formation is catalyzed by the peptidyl transferase center of the large ribosomal subunit — and that this activity is carried out by rRNA (a ribozyme), not a protein enzyme.
  4. Distinguish prokaryotic (70S = 30S + 50S) from eukaryotic (80S = 40S + 60S) ribosomes and explain why this structural difference is the basis for antibiotic selectivity against bacterial but not human cells.

Can you avoid these mistakes?

A drug blocks EF-G (elongation factor G) function. Which specific step of elongation is halted, and what would you observe about the tRNA occupancy of the ribosomal sites just before the block takes effect?
An mRNA has its AUG start codon mutated to AUC. Predict the consequence for translation and explain mechanistically — not just 'no start codon' — why initiation fails.
Linezolid is an antibiotic that inhibits formation of the 70S initiation complex. Why does this drug selectively kill bacteria without harming human cells, and which ribosomal subunits does it target?
At the moment peptide bond formation occurs, what molecule is attached to the P-site tRNA before the reaction, and what is attached to it after the reaction? What happens to the now-empty tRNA, and where does it go next?

Related topics

Genetic Code, Codons, and Wobble Eukaryotic RNA Processing (Cap, Splicing, PolyA) Nucleotide and Nucleic Acid Structure DNA Double Helix and Base Pairing DNA Replication (Semiconservative, Enzymes, Fork) DNA Repair Pathways

See how your Anki deck covers this topic.

Upload your deck for a free audit →