The mRNA has left the nucleus and found a ribosome. Now the cell has to turn a string of bases into a string of amino acids — two completely different chemical languages. That swap is translation, and it happens three bases at a time.
📘 What you need to know
Translation happens in the cytoplasm, at a ribosome.
A ribosome is made of protein and rRNA, in a large and a small subunit. The mRNA binds to the small subunit.
The mRNA is read in codons — groups of three bases. One codon means one amino acid.
tRNA molecules carry amino acids to the ribosome. Each tRNA has an anticodon: three bases complementary to a codon.
Two tRNAs fit in the ribosome at once, so the amino acids they carry are held side by side.
A peptide bond forms between them by condensation. This is anabolic and needs ATP.
The ribosome moves along one codon at a time until it reaches a stop codon, then the finished polypeptide is released.
The cast list
Four things have to be in the same place for translation to work. If you can name all four and say what each does, you can answer almost any question on this page.
mRNA — carries the message, read in codons from one end to the other.
The ribosome — the workbench. It holds the mRNA against the tRNAs and catalyses the peptide bonds.
tRNA — the delivery molecule. One end grips a specific amino acid, the other end has the anticodon that finds the right spot on the mRNA.
Free amino acids — already floating in the cytoplasm, waiting to be picked up.
Picture a builder following instructions written on a long tape. The ribosome is the builder, the mRNA is the tape, and the tRNAs are the labourers carrying one brick each. The builder never carries bricks — it just checks that the right one has turned up and cements it on.
Codon, anticodon, triplet — which is which?
All three words mean “a group of three bases”, so it is easy to muddle them. The difference is simply which molecule they sit on.
Name
Which molecule it sits on
How it relates to the others
Triplet
DNA
The original three bases in the gene
Codon
mRNA
Complementary to the DNA triplet on the template strand
Anticodon
tRNA
Complementary to the mRNA codon it pairs with
The pairing rule in RNA
A pairs with U • C pairs with G
Quick check. If an mRNA codon is CAG, the tRNA anticodon must be GUC. Flip each base to its partner and remember there is no T anywhere — both molecules are RNA.
The molecule folds back on itself and hydrogen bonds hold the folds in place — that is what gives tRNA its clover shape.
What happens at the ribosome
🧩 The cycle, one codon at a time
Attach. The mRNA binds to the small subunit of the ribosome.
First tRNA arrives. A tRNA whose anticodon matches the first codon pairs with it by hydrogen bonding. The start codon is AUG, so the first amino acid brought in is methionine.
Second tRNA arrives. A second tRNA pairs with the next codon. Now two amino acids are sitting side by side inside the ribosome.
Bond them. A peptide bond forms between the two amino acids by condensation. This needs ATP, supplied by the mitochondria.
Move on. The ribosome shifts one codon along the mRNA. The first tRNA is released (without its amino acid) and can go and collect another one.
Repeat. Steps 3 to 5 happen over and over, and the polypeptide chain grows.
Stop. When a stop codon is reached there is no matching tRNA, so no amino acid is added. The finished polypeptide is released from the ribosome.
Notice the anticodons: UAC pairs with AUG, and CGA pairs with GCU. Every pairing in this diagram follows A–U and C–G.
Why it has to stop
Three of the 64 codons are stop codons. No tRNA carries an anticodon for them, so when the ribosome reaches one, nothing arrives and nothing can be added. That gap is the signal: the chain is finished, and the polypeptide is released.
Without stop codons a ribosome would keep reading straight past the end of the gene and produce a protein of the wrong length, which almost certainly would not fold or work properly.
A neat way to remember the energy point: making bonds between amino acids builds something up, so it is anabolic, and anabolic reactions cost energy. That energy is ATP, and ATP comes from respiration in the mitochondria. Examiners like that whole chain in one sentence.
Worked examples
WORKED EXAMPLE
From codons to anticodons
An mRNA strand reads AUG GCC UUA CAG. Write the anticodons of the tRNA molecules that will bind to it, in order.
Step 1: Remember both molecules are RNA
So pair A with U and C with G. No T anywhere.
Step 2: Flip each codon base by baseAUG → UACGCC → CGGUUA → AAUCAG → GUCUAC, CGG, AAU, GUCKeep the order the same as the mRNA — the tRNAs arrive in that sequence.
WORKED EXAMPLE
Counting bonds and bases
A polypeptide is 8 amino acids long. How many peptide bonds does it contain, and what is the shortest possible length of the mRNA that coded for it (including the stop codon)?
Step 1: Peptide bonds
Bonds sit between amino acids, so there is always one fewer bond than amino acid.
8 − 1 = 7 peptide bondsStep 2: Codons needed
8 amino acids = 8 codons, plus 1 stop codon = 9 codonsStep 3: Turn codons into bases9 × 3 = 27 bases7 peptide bonds; at least 27 bases of mRNA“At least”, because real mRNA also has untranslated regions at each end.
WORKED EXAMPLE
Explaining a result
A drug stops mitochondria from producing ATP. Explain why protein synthesis in the cell slows down almost immediately.
Step 1: Find the step that needs energy
Forming a peptide bond is a condensation reaction. It is anabolic, so it requires energy.
Step 2: Link the energy source
That energy comes from ATP, which is supplied by the mitochondria.
Step 3: Join the chain of reasoning
No ATP → peptide bonds cannot form → polypeptide chains cannot be extended.
Translation stalls because peptide bond formation needs ATP“Explain” questions want the chain, not just the last link.
💡 Exam tip
Say “complementary base pairing” out loud in your answer. It is the phrase that earns the mark when describing how a tRNA finds its codon.
Two tRNAs at once is the detail most students forget, and it is the reason a peptide bond can form at all.
If a question says “describe the role of tRNA”, give both ends: it carries a specific amino acid and its anticodon pairs with the codon.
Name the bond. Amino acids are joined by peptide bonds, formed by condensation. Do not just write “they join together”.
Ribosomes are made of protein and rRNA — a favourite one-mark question.
Watch the direction: the ribosome reads the mRNA in the 5′ to 3′ direction, the same direction it was made in.
⚠ Common mix-up
Writing T in an anticodon. tRNA is RNA. If you have written a T, it is wrong.
Saying the tRNA “makes” the amino acid. It does not. The amino acids are already in the cytoplasm; the tRNA only collects and delivers one.
Mixing up codon and anticodon. Codon is on the mRNA, anticodon is on the tRNA. If you cannot remember, “anti” means opposite — the anticodon is the opposite one.
Thinking a stop codon codes for a “stop amino acid”. There is no such thing. It codes for nothing, and that absence is the signal.
Putting translation in the nucleus. Transcription is the nuclear one. Translation happens in the cytoplasm.
Forgetting ATP. Questions about energy in protein synthesis are almost always about peptide bond formation.
Up next: The Genetic Code — we have been using codons all the way through this page, so now let us look at how 64 codons manage to describe only 20 amino acids, and why that turns out to be very good news.
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