Translation
Translation is the process that builds a protein from an mRNA transcript at the ribosome, using tRNA, amino acids, and GTP.
Translation happens in the cytoplasm, at the ribosome. It requires four main components: the mRNA transcript carrying the genetic code, tRNA molecules to bring in the correct amino acids, the amino acids themselves as building blocks, and GTP as a source of energy to power the process.
Key Takeaways
Translation happens at the ribosome and requires mRNA, tRNA, amino acids, and GTP; prokaryotic ribosomes are 70S (50S + 30S subunits), eukaryotic ribosomes are 80S (60S + 40S subunits) — S values reflect sedimentation, not simple addition.
The A site receives incoming tRNA, the P site holds the growing peptide chain, and the E site is where spent tRNA exits.
Initiation differs by organism (Shine-Dalgarno sequence in prokaryotes, 5' cap scanning in eukaryotes), but in both, the initiator tRNA (carrying methionine) uniquely enters at the P site.
Elongation cycles through A-site entry, peptide bond formation (via peptidyl transferase, an rRNA-catalyzed reaction), and translocation; termination occurs at a stop codon via a release factor that frees the finished protein.
Most proteins require posttranslational modification — structural changes (cleavage, signal sequence removal, subunit assembly) and/or chemical additions (phosphorylation, carboxylation, glycosylation, prenylation) — before becoming fully functional.
Ribosome Structure: Prokaryotes vs. Eukaryotes
The ribosome — the structure where translation happens — is made up of ribosomal RNA and proteins, and it has two subunits: a large subunit and a small subunit. Ribosome size differs between prokaryotes and eukaryotes.
Organism | Full Ribosome | Large Subunit | Small Subunit |
|---|---|---|---|
Prokaryotes | 70S | 50S | 30S |
Eukaryotes | 80S | 60S | 40S |
These "S" values are based on sedimentation rates, not simple addition — that's why 50S + 30S doesn't arithmetically equal 70S. The values reflect a particle's shape and density during centrifugation, not just its size.
The A, P, and E Sites
Within the ribosome, three key sites are where tRNA molecules interact with the mRNA:
Site | Full Name | Function |
|---|---|---|
A site | Aminoacyl-tRNA site | Incoming tRNA carrying a new amino acid first enters here |
P site | Peptidyl site | Holds the growing polypeptide chain |
E site | Exit site | tRNA exits the ribosome here after delivering its amino acid |
The amino acid held in the A site is joined to the chain held in the P site — the core chemistry of every elongation cycle.
With the structure established, translation proceeds through three stages: initiation, elongation, and termination.
Initiation
During initiation, the small ribosomal subunit binds to the mRNA — but exactly how depends on whether the cell is prokaryotic or eukaryotic.
Feature | Prokaryotes | Eukaryotes |
|---|---|---|
Small subunit binding site | Shine-Dalgarno sequence (ribosome binding site upstream of the start codon) | 5' cap, then scans along the mRNA for the start codon |
Start codon | AUG | AUG |
Once the start codon is located, the initiator tRNA — carrying methionine, always the first amino acid in a newly synthesized protein — binds to it. The initiator tRNA binds directly to the P site, which is unique: every other tRNA enters translation at the A site.
A group of proteins called initiation factors guide the ribosome to the mRNA, stabilize the interaction, and ensure everything assembles correctly. Once the initiator tRNA is in place and the start codon is recognized, the large ribosomal subunit joins to complete the full ribosome — marking the end of initiation.
Elongation
During elongation, the ribosome reads the mRNA one codon at a time, building the protein by linking amino acids together. Each cycle follows the same steps:
A new tRNA enters the A site, carrying the amino acid that corresponds to that position's codon.
A peptide bond forms between the amino acid in the A site and the growing peptide chain in the P site. This reaction is catalyzed by peptidyl transferase — an enzymatic function of the ribosome that is actually carried out by the ribosomal RNA itself.
The ribosome undergoes translocation, shifting forward along the mRNA in the 5' to 3' direction: the tRNA that was in the P site moves to the E site and exits; the tRNA that was in the A site — now carrying the growing peptide chain — moves to the P site; the A site opens up for the next incoming tRNA.
This cycle repeats for every codon in the mRNA. Elongation factors help guide tRNAs into the A site and coordinate the ribosome's movement down the mRNA, using GTP as an energy source throughout.
Termination
Eventually, the ribosome reaches a stop codon — UAA, UAG, or UGA. These codons don't correspond to any amino acid, and no tRNA binds to them, so translation cannot continue as usual.
Instead, a protein called a release factor binds to the stop codon. This triggers the ribosome to cut the bond between the final tRNA and the completed polypeptide chain, releasing the finished protein into the cytoplasm. Additional termination factors may assist by helping disassemble the ribosomal subunits and cleanly end translation. Once the protein is released, the ribosomal subunits fall apart, and the mRNA can either be translated again or degraded, depending on the cell's needs.
Many eukaryotic proteins carry signal sequences — short peptide tags that direct the finished protein to its proper location in the cell, such as the endoplasmic reticulum, the nucleus, or the cell membrane.
Posttranslational Modification
Once a protein is made, the job isn't quite finished. Most proteins need posttranslational modification before they become fully functional — changes that help the protein fold correctly, reach the right cellular location, and perform its biological function. These fall into two broad categories.
Structural changes to the protein itself:
Cleavage — part of the protein is cut off, which can activate the protein or allow it to change shape.
Signal sequence removal — trimming off the short peptide tag that directed the protein to its destination.
Subunit assembly — multiple protein chains come together to form a functional complex.
Chemical additions — covalent modifications that add specific groups to the protein:
Modification | What It Adds | Functional Effect |
|---|---|---|
Phosphorylation | Phosphate group | Common on/off regulatory switch; central to cell signaling pathways |
Carboxylation | Carboxyl groups | Important for calcium binding; stabilizes structures in metal-ion-binding proteins |
Glycosylation | Sugar chains (carbohydrates) | Guides proteins to the correct cellular destination; roles in immune recognition and cell signaling |
Prenylation | Lipid groups | Anchors the protein into cellular membranes |
These modifications let the same protein behave differently depending on context — phosphorylation, for example, might activate a protein in one environment and inactivate it in another.
Common MCAT Mistakes
Thinking the initiator tRNA enters at the A site. It's the one exception — the initiator tRNA (carrying methionine) binds directly to the P site, while every other tRNA during elongation enters at the A site.
Forgetting the S values don't add up arithmetically. 50S + 30S makes a 70S ribosome, not 80S — sedimentation coefficients reflect shape and density during centrifugation, not simple mass addition.
Mixing up initiation signals between prokaryotes and eukaryotes. Prokaryotic small subunits find the start codon via the Shine-Dalgarno sequence; eukaryotic small subunits bind the 5' cap and scan along the mRNA. Both still use AUG as the start codon.
Assuming peptidyl transferase is a protein enzyme. The peptide-bond-forming activity at the heart of elongation is actually catalyzed by ribosomal RNA, not a protein — the ribosome is a ribozyme at this step.
MCAT-Style Concept Check
Question: A researcher is studying a ribosome caught in the middle of an elongation cycle. The tRNA in the P site has just transferred the growing polypeptide chain to the amino acid in the A site, and peptidyl transferase has just catalyzed peptide bond formation. What happens next?
A) The ribosome undergoes translocation: the former P-site tRNA exits via the E site, the former A-site tRNA (now holding the chain) shifts to the P site, and the A site opens for the next tRNA
B) A release factor binds the A site because a stop codon has been reached, ending translation
C) The initiator tRNA re-enters the P site to begin a new round of initiation
D) The large and small ribosomal subunits immediately dissociate from the mRNA
Answer: A
Explanation: After peptide bond formation, the ribosome undergoes translocation, moving forward one codon in the 5' to 3' direction. The tRNA that was in the P site (now empty of its amino acid) shifts to the E site and exits; the tRNA that was in the A site (now carrying the growing chain) shifts to the P site; and the A site opens up to accept the next incoming tRNA. Option B is wrong because nothing in the scenario indicates a stop codon has been reached — release factors only act at stop codons during termination. Option C is wrong because initiator tRNA binding is unique to initiation, not a repeating step of elongation. Option D is wrong because subunit dissociation happens at termination, not mid-elongation.
FAQ
Why does the initiator tRNA bind the P site instead of the A site?
Every other tRNA during translation enters at the A site first, but the initiator tRNA (carrying methionine, the first amino acid of every new protein) is the one exception — it binds directly to the P site as part of initiation, before the A site is even in use.
What's the difference between initiation in prokaryotes and eukaryotes?
Prokaryotic small ribosomal subunits locate the start codon using the Shine-Dalgarno sequence upstream of it. Eukaryotic small subunits instead bind the mRNA's 5' cap and scan along the transcript until they find the start codon. Both use AUG as the actual start codon.
What enzyme forms the peptide bond during elongation, and what's unusual about it?
Peptidyl transferase catalyzes peptide bond formation between the amino acid in the A site and the growing chain in the P site. What's unusual is that this catalytic activity is carried out by ribosomal RNA itself, not by a protein — making the ribosome a ribozyme at this step.
What are the main types of posttranslational modification?
They fall into two categories: structural changes (cleavage, signal sequence removal, subunit assembly) and chemical additions (phosphorylation, carboxylation, glycosylation, prenylation). Both help a newly translated protein fold correctly, reach the right location, and become fully functional.
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