At HL you need translation in more detail: not just “tRNAs bring amino acids”, but exactly where they dock, in what order, and what happens to the finished chain afterwards. A polypeptide coming off a ribosome is usually not yet a working protein — and when it is finally worn out, the cell takes it apart and uses the pieces again.
📘 What you need to know
Translation has three stages: initiation, elongation and termination, and happens in the cytoplasm.
Ribosomes have three tRNA binding sites: A (aminoacyl, where the tRNA arrives), P (peptidyl, where peptide bonds form) and E (exit, where the tRNA exits).
The small subunit binds the 5′ end of the mRNA; the initiator tRNA carrying methionine occupies the P site; the ribosome finds AUG and the large subunit joins.
tRNA is about 80 nucleotides long, folds into a clover shape held by hydrogen bonds, and there are 20 types, one per amino acid.
After translation, polypeptides are modified: folding into secondary, tertiary and quaternary structures, disulfide bonds, and cutting by enzymes. Molecular chaperones prevent incorrect folding.
Unwanted or damaged proteins are tagged with ubiquitin and broken down by proteases in the proteasome. This is proteolysis, and it recycles amino acids.
The proteome is the full set of proteins a cell or organism can produce.
Initiation, step by step
🧩 Getting translation started
The mature mRNA leaves the nucleus through a nuclear pore and enters the cytoplasm.
The small ribosomal subunit attaches to the 5′ end of the mRNA.
An initiator tRNA carrying methionine binds to the small subunit and sits in the P site.
The ribosome moves along the mRNA until it locates the start codon, AUG.
The large subunit joins the small subunit, completing the ribosome. Elongation can now begin.
The A, P and E sites
The three sites are simply three slots in the large subunit, and the letters tell you what happens in each one. A tRNA always travels through them in the same order: A, then P, then E.
Site
Full name
What happens there
A
Aminoacyl
The next tRNA arrives, and its anticodon pairs with the mRNA codon
P
Peptidyl
The growing chain is held here and the peptide bond is formed
E
Exit
The empty tRNA exits the ribosome and goes to collect another amino acid
The first letters do the work for you: arrives at A, peptide bond at P, exits at E.
The elongation cycle
🧩 The repeating loop
The initiator tRNA sits in the P site. The next codon exposes the A site, and a tRNA with the complementary anticodon binds there.
A peptide bond forms between the two amino acids, producing a dipeptide.
The ribosome shifts along the mRNA by one codon. The tRNA in the P site moves to the E site and is released; the tRNA carrying the chain moves from the A site into the P site.
The next codon is exposed at the A site, a new tRNA arrives, and its amino acid is added to the chain.
The cycle repeats, and the polypeptide gets one amino acid longer each time.
Termination comes when a stop codon reaches the A site. No tRNA can bind, no amino acid is added, and the completed polypeptide is released from the ribosome.
Free tRNAs are constantly reloading in the cytoplasm: a tRNA that has just left the E site picks up another molecule of its own amino acid and rejoins the queue. Nothing is wasted.
A polypeptide is not yet a protein
When a chain is released from the ribosome, all the cell has made is the primary structure — the amino acid sequence. It usually cannot do its job yet. Before it becomes a functional protein it may need to:
Fold into secondary, tertiary and, if it has several chains, quaternary structure;
form disulfide bonds that lock the folded shape in place;
be helped by molecular chaperones, proteins that stop it folding the wrong way while it is still forming;
be cut by enzymes, so that unwanted sections are removed.
Insulin: modification you can follow
Insulin is the standard example because every stage is visible.
It is first made as pre-proinsulin, a single chain of about 110 amino acids attached to the wall of the endoplasmic reticulum.
An enzyme removes a signal peptide from the end. The molecule detaches from the ER and is now proinsulin.
The chain folds and disulfide bonds form between different parts of it. The proinsulin is packaged into vesicles at the Golgi apparatus.
A section from the middle, the C peptide, is cut out. What is left is two chains — the A chain and B chain — held together by two disulfide bonds.
That is mature insulin, ready to be secreted.
Three named molecules in order — pre-proinsulin, proinsulin, insulin — is often all a question asks for.
Recycling: taking proteins apart again
Proteins do not last forever. Some are damaged, some fold incorrectly, and some are simply no longer needed. Rather than waste them, the cell breaks them down and reuses the amino acids in new proteins.
Enzymes called proteases break the peptide bonds. The process is called proteolysis.
It takes place inside the proteasome, an organelle found in eukaryotic cells.
Keeping the protease enzymes shut inside an organelle stops them digesting useful proteins by accident.
Proteins to be destroyed are first tagged with a chemical called ubiquitin, which marks them for the proteasome.
The released amino acids go back into the cytoplasm and are picked up by tRNAs for the next round of translation.
The proteome. The proteome is the whole set of proteins a cell can make, and unlike the genome it changes constantly — with the cell type, with the time of day, and with what the cell is doing. Constant synthesis and constant proteolysis are what keep it balanced.
Worked examples
WORKED EXAMPLE
Following one tRNA through the ribosome
A tRNA carrying leucine arrives at a ribosome. State the order of the sites it passes through and what happens at each.
Step 1: Where it arrivesA site — its anticodon pairs with the codon on the mRNA.
Step 2: Where the bond formsP site — the ribosome shifts along and the leucine is joined to the chain by a peptide bond.
Step 3: Where it leavesE site — now empty, the tRNA exits and collects another leucine.
A → P → EThe tRNA never moves backwards, and it always leaves without its amino acid.
WORKED EXAMPLE
How much of the chain is thrown away?
A pre-proinsulin molecule contains 110 amino acids. A signal peptide of 24 amino acids is removed, and later a C peptide of 35 amino acids is cut out. How many amino acids are in the mature insulin molecule?
Step 1: Remove the signal peptide110 − 24 = 86 amino acids (proinsulin)Step 2: Remove the C peptide86 − 35 = 51 amino acidsStep 3: Say what is left
Those 51 amino acids form the A chain and B chain, held together by disulfide bonds.
51 amino acidsOver half the original chain is cut away. Modification is not a small tidy-up.
WORKED EXAMPLE
Why keep proteases in an organelle?
Explain the advantage of proteolysis taking place inside the proteasome rather than freely in the cytoplasm.
Step 1: State what proteases do
They break peptide bonds, so they can digest any protein they meet.
Step 2: Identify the risk
Loose in the cytoplasm they would destroy useful proteins, including enzymes the cell needs.
Step 3: Explain the solution
Enclosing them means only proteins tagged with ubiquitin and delivered to the proteasome are broken down.
It confines digestion to proteins that are meant to be destroyedSame logic as lysosomes: dangerous enzymes are kept behind a membrane.
💡 Exam tip
Learn the site letters through their meanings:Arrives, Peptide bond, Exits. Then you can never put them in the wrong order.
In initiation, be specific: small subunit first, then initiator tRNA with methionine, then the large subunit joins.
Give the tRNA numbers if you can: about 80 nucleotides, 20 types, clover shape held by hydrogen bonds.
Name the modifications rather than saying “the protein changes”: folding, disulfide bonds, chaperones, cutting by enzymes.
For insulin, learn the three named forms in order and what is removed at each step.
For recycling, the four key words are protease, proteolysis, proteasome and ubiquitin. Use all four.
⚠ Common mix-up
Getting the site order wrong. It is A, then P, then E — not the alphabetical order they are usually drawn in.
Saying the large subunit binds first. The small subunit attaches to the mRNA first.
Proteasome and ribosome. The ribosome builds proteins; the proteasome breaks them down. One letter, opposite jobs.
Thinking modification happens during translation. The chain is completed and released first, and modified afterwards.
Calling ubiquitin an enzyme. It is a tag that marks a protein for destruction; the proteases do the digesting.
Confusing genome and proteome. The genome is the set of genes; the proteome is the set of proteins that can be made, and it is much larger.
That is the end of Making Proteins. Before you move on, try telling the whole story out loud in one go: gene → transcription → modification → translation → folding → working protein. If you can do that without notes, this chapter is finished.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
