Source: Chapter 7 Review Sheet, Cell Biology, University of Florida
Difficulty: Intermediate | Prerequisites: Parts 1–3 (Transcription, RNA processing, the genetic code and tRNA)
Tags: ribosome, translation, peptide bond, peptidyl transferase, large subunit, small subunit, A site, P site, E site, initiation, elongation, termination, release factors, initiator tRNA, polysome, polyribosome, PCB 3023
This is the final step of the central dogma: translating the mRNA's nucleotide sequence into a polypeptide. The ribosome is the molecular machine at the centre of it all, and it coordinates three jobs simultaneously: decoding the mRNA, catalysing peptide bond formation, and translocating along the message. You need to be solid on tRNA structure and the genetic code (Part 3) before this section will make sense. Translation is one of the most heavily tested topics in cell biology courses, so give this part serious attention.
Ribosomes are made of rRNA and protein, assembled in the nucleolus, and have binding sites for mRNA and tRNAs. Translation proceeds in three stages: initiation (ribosome assembles at the start codon with initiator tRNA), elongation (amino acids are added one by one as tRNAs cycle through the A, P, and E sites), and termination (a stop codon triggers release of the polypeptide). Polysomes are clusters of ribosomes translating the same mRNA simultaneously, boosting protein output.
Ribosome
A large molecular machine composed of ribosomal RNA (rRNA) and ribosomal proteins, organised into a large subunit and a small subunit. It catalyses the assembly of amino acids into polypeptides during translation. In eukaryotes, the ribosome is an 80S particle (60S large + 40S small).
Large ribosomal subunit
Contains the peptidyl transferase activity (the catalytic centre that forms peptide bonds). In eukaryotes, it is the 60S subunit. The catalytic component is the rRNA itself (a ribozyme), not a protein.
Small ribosomal subunit
Responsible for binding the mRNA and ensuring correct codon-anticodon pairing. In eukaryotes, it is the 40S subunit.
Peptidyl transferase
The catalytic activity that forms peptide bonds between amino acids during translation. It is carried out by the rRNA of the large subunit (specifically the 23S rRNA in bacteria, or the 28S rRNA in eukaryotes). This makes the ribosome a ribozyme.
A site (aminoacyl site)
The ribosomal binding site where each new aminoacyl-tRNA (charged tRNA) enters and its anticodon is matched to the mRNA codon.
P site (peptidyl site)
The ribosomal binding site that holds the tRNA carrying the growing polypeptide chain.
E site (exit site)
The ribosomal binding site from which the now-empty (uncharged) tRNA exits the ribosome after transferring its amino acid.
mRNA binding site
Located on the small subunit. The mRNA threads through a groove in the small subunit so that its codons are presented sequentially at the A and P sites.
Initiator tRNA
A special tRNA that carries methionine (Met) and is used specifically during translation initiation. It binds directly to the P site (not the A site) as part of the initiation complex. In bacteria, the initiator tRNA carries a modified methionine called formyl-methionine (fMet).
Initiation factors
Proteins that assist in the assembly of the ribosome at the start codon. They help the small subunit, initiator tRNA, and mRNA come together, and are released once the large subunit joins. Eukaryotic and bacterial initiation factors differ.
Release factors
Proteins that recognise stop codons in the A site and trigger the release of the finished polypeptide from the ribosome. They mimic the shape of a tRNA but do not carry an amino acid.
Polysome (polyribosome)
A cluster of multiple ribosomes simultaneously translating the same mRNA molecule. Each ribosome is at a different position along the mRNA, producing its own copy of the polypeptide. This allows the cell to produce many copies of a protein rapidly from a single mRNA.
Translocation
The movement of the ribosome along the mRNA by one codon (three nucleotides) after each peptide bond is formed. This shifts the tRNAs from A to P and from P to E, and brings the next codon into the A site. Translocation requires GTP hydrolysis (via elongation factor EF-G in bacteria, or eEF-2 in eukaryotes).
Each ribosome has two subunits: a large subunit and a small subunit, which come together on the mRNA to form the functional ribosome.
Composition: both subunits are made of ribosomal RNA (rRNA) and ribosomal proteins. The rRNA makes up the majority of the ribosome by mass and is responsible for its catalytic activity.
Assembly location: ribosomal subunits are constructed in the nucleolus (in eukaryotes), then exported to the cytoplasm.
Four key binding sites:
mRNA binding site (on the small subunit)
A site (aminoacyl site, accepts incoming charged tRNA)
P site (peptidyl site, holds the tRNA with the growing polypeptide)
E site (exit site, releases the empty tRNA)
Catalytic activity: the ribosome catalyses peptide bond formation. The catalytic component is the rRNA of the large subunit (peptidyl transferase activity). This means the ribosome is a ribozyme, an RNA molecule with enzymatic activity.
Decoding: the small subunit ensures correct codon-anticodon base pairing at the A site, checking that the right aminoacyl-tRNA has entered.
Peptide bond formation: the large subunit's rRNA (peptidyl transferase) catalyses the transfer of the polypeptide chain from the tRNA in the P site to the amino acid on the tRNA in the A site, forming a new peptide bond.
Translocation: the ribosome moves one codon along the mRNA, shifting the tRNAs from A → P and P → E, opening the A site for the next aminoacyl-tRNA.
A charged (aminoacyl) tRNA enters the A site, where its anticodon is matched to the mRNA codon.
Once verified, peptide bond formation occurs: the growing polypeptide chain is transferred from the P-site tRNA to the amino acid on the A-site tRNA.
The ribosome translocates: the now-peptidyl tRNA moves from A to P, and the empty tRNA moves from P to E.
The empty tRNA exits from the E site and is recycled (recharged with a new amino acid by its aminoacyl-tRNA synthetase).
A new charged tRNA enters the now-empty A site, and the cycle repeats.
The mRNA is translated in the 5' to 3' direction (which is also the direction the ribosome moves along the mRNA).
The polypeptide is synthesised from the N-terminus (amino end) to the C-terminus (carboxyl end).
Peptide bond formation is thermodynamically unfavourable on its own. It is driven by the prior hydrolysis of ATP during tRNA charging (the aminoacyl-tRNA synthetase reaction, which spends ATP → AMP + PPi) and by GTP hydrolysis during elongation (codon recognition and translocation each consume one GTP).
The small ribosomal subunit binds to the mRNA.
In eukaryotes: the small subunit (with initiation factors) binds to the 5' cap of the mRNA and scans along the 5'-UTR until it reaches the first AUG start codon. The initiator tRNA (carrying Met) is positioned in the P site. The large subunit then joins, and initiation factors are released.
In bacteria: there is no 5' cap. Instead, the small subunit binds directly to a specific sequence upstream of the start codon called the Shine-Dalgarno sequence (a ribosome binding site). The initiator tRNA carries formyl-methionine (fMet).
Why the start codon is more important than the stop codon: the start codon sets the reading frame for the entire mRNA. If the ribosome starts at the wrong AUG (or skips it), every subsequent codon will be misread, producing a completely wrong protein. The stop codon merely ends translation, and even if a stop codon is missed (e.g., by a read-through event), the consequences are limited to a longer-than-normal polypeptide rather than a wholesale change in amino acid sequence.
When a stop codon (UAA, UAG, or UGA) enters the A site, no aminoacyl-tRNA can bind to it (no tRNA has a complementary anticodon for stop codons).
Instead, a release factor binds to the A site. Release factors recognise the stop codon and trigger hydrolysis of the bond between the polypeptide and the tRNA in the P site.
The finished polypeptide is released.
The ribosomal subunits dissociate from the mRNA and from each other, ready to be recycled for a new round of translation.
A polysome is formed when multiple ribosomes are translating the same mRNA molecule at the same time.
Each ribosome begins at the 5' end of the mRNA (at the start codon) and moves toward the 3' end. As one ribosome moves along, another can initiate behind it.
The result is a string of ribosomes on a single mRNA, each at a progressively later stage of translation. The ribosome nearest the 5' end has just started, and the one nearest the 3' end is almost finished.
Significance: polysomes allow the cell to produce many copies of the same protein simultaneously and efficiently from a single mRNA, greatly increasing the rate of protein synthesis.
Many antibiotics work by targeting bacterial ribosomes. Because bacterial ribosomes (70S) differ in structure from eukaryotic ribosomes (80S), drugs like tetracycline (blocks the A site), chloramphenicol (inhibits peptidyl transferase), and erythromycin (blocks translocation) can kill bacteria without harming human cells. Understanding ribosome structure is why we have selective antibiotics that exploit this difference.
Students often think a protein enzyme catalyses peptide bond formation. It is the rRNA (a ribozyme) in the large subunit that does this, not a protein component.
Students sometimes confuse the A, P, and E sites or get the order of tRNA movement wrong. The flow is always A → P → E. New tRNAs enter at A; the growing chain is held at P; empty tRNAs leave from E.
Some students assume translation begins at the A site. It does not. The initiator tRNA goes directly to the P site during initiation. All subsequent tRNAs enter at the A site.
Students frequently forget that the start codon sets the reading frame. Losing or shifting the start codon is far more catastrophic than a change to a stop codon.
⚠️ Know the composition and assembly location of ribosomal subunits (rRNA + protein, assembled in the nucleolus).
⚠️ Know the four binding sites and what occupies each (mRNA on the small subunit; A, P, and E sites for tRNAs).
⚠️ Know that peptidyl transferase is rRNA (a ribozyme) in the large subunit.
⚠️ Be able to walk through the three stages of translation (initiation, elongation, termination) with the roles of initiation factors, elongation factors, and release factors.
⚠️ Know the difference between eukaryotic initiation (5' cap scanning) and bacterial initiation (Shine-Dalgarno sequence).
⚠️ Understand why the start codon is more important than the stop codon.
⚠️ Be able to explain what a polysome is and why it matters for protein production.
True or False: Peptide bond formation is catalysed by a ribosomal protein. → False. It is catalysed by the rRNA of the large subunit (peptidyl transferase, a ribozyme).
Fill in the blank: The tRNA carrying the growing polypeptide chain sits in the ______ site. → P site (peptidyl site).
True or False: In eukaryotes, the small ribosomal subunit scans from the 5' cap to find the first AUG. → True.
Fill in the blank: When a stop codon enters the A site, a ______ ______ binds instead of a tRNA. → Release factor.
True or False: In a polysome, the ribosome nearest the 3' end of the mRNA has the shortest polypeptide. → False. The ribosome nearest the 3' end is furthest along in translation and has the longest (nearly complete) polypeptide.
Q: What are the four binding sites on the ribosome, and what does each bind?
A: (1) The mRNA binding site on the small subunit holds the mRNA. (2) The A site accepts incoming aminoacyl-tRNAs. (3) The P site holds the tRNA carrying the growing polypeptide. (4) The E site is where empty tRNAs exit after donating their amino acid.
Q: What is the catalytic component of the ribosome, and where is it located?
A: The catalytic component is rRNA (not protein), specifically the peptidyl transferase activity of the large ribosomal subunit's rRNA. This makes the ribosome a ribozyme.
Q: Describe the key difference between translation initiation in eukaryotes and bacteria.
A: In eukaryotes, the small subunit binds the 5' cap and scans along the mRNA until it finds the first AUG start codon. In bacteria, there is no 5' cap; instead, the small subunit binds directly to the Shine-Dalgarno sequence upstream of the start codon. Bacteria also use formyl-methionine (fMet) on their initiator tRNA, rather than unmodified methionine.
Q: Why is the start codon more important than the stop codon?
A: The start codon establishes the correct reading frame for the entire mRNA. If the start codon is missed or shifted, every downstream codon is misread, producing a completely incorrect protein. A stop codon only terminates translation; if it is altered, the result is merely an extended polypeptide, not a wholesale amino acid sequence change.
Q: What are polysomes, and why are they significant?
A: Polysomes are multiple ribosomes translating the same mRNA simultaneously, each at a different stage of translation. They are significant because they allow the cell to produce many copies of the same protein rapidly from a single mRNA, greatly increasing the efficiency of protein synthesis.
Q: What drives the thermodynamically unfavourable process of peptide bond formation?
A: The energy comes from GTP hydrolysis during elongation steps (codon recognition and translocation) and from the prior ATP hydrolysis during aminoacyl-tRNA charging (the aminoacyl-tRNA synthetase reaction: ATP → AMP + PPi). The overall energy input from these hydrolysis reactions makes the net process energetically favourable.
Translation connects back to transcription and mRNA processing (Parts 1–2), since only a fully processed mRNA is translated. It connects forward to post-translational modification, protein folding, and protein targeting (how the cell directs newly made proteins to the correct location). The difference between bacterial and eukaryotic ribosomes is central to pharmacology and antibiotic design. Polysomes connect to topics in cell signalling and growth, as cells that need to produce large quantities of protein (such as secretory cells) have abundant polysomes and rough endoplasmic reticulum.
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