IB Biology SLTopic 4 — Making ProteinsPaper 1 & 2Core idea~9 min read
Translation: Building the Polypeptide
The mRNA has left the nucleus and reached a ribosome. Now the message has to be read three bases at a time and turned into a chain of amino acids. This is the stage where the language really changes — from bases to amino acids.
📘 What you need to know
Translation happens in the cytoplasm, at a ribosome. Transcription happened in the nucleus — do not swap these.
A ribosome is made of a large and a small subunit, built from protein and ribosomal RNA (rRNA).
The mRNA binds to the small subunit. Two tRNA molecules can sit in the large subunit at the same time.
Each tRNA carries one specific amino acid and has an anticodon that pairs with a matching codon on the mRNA.
A peptide bond forms between neighbouring amino acids by condensation. It needs energy, supplied as ATP from respiration.
The ribosome moves along one codon at a time until it reaches a stop codon, then the finished polypeptide is released.
The big picture first
Translation is a matching game. The mRNA arrives carrying a long line of three-letter words. Each word has exactly one tRNA that fits it, and each tRNA is carrying one particular amino acid. Match the words in order, join up the amino acids they deliver, and you have built a protein.
Translation in one line
mRNA codons → matched by tRNA anticodons → amino acids joined into a polypeptide
Useful way to picture it: the mRNA is a delivery list, the tRNA molecules are the delivery vans, and the ribosome is the loading bay where two vans can pull in side by side. Nothing gets joined together until two amino acids are sitting next to each other.
The ribosome
A ribosome is not a single blob. It has two parts that clamp around the mRNA:
The small subunit is where the mRNA binds and slides through.
The large subunit has binding sites that hold two tRNA molecules at once, so their two amino acids end up right next to each other.
Part of the large subunit’s surface is what actually catalyses the peptide bond between them.
Ribosomes are themselves made of protein plus rRNA, which is a nice detail to drop into a longer answer.
Only three bases are being read at any moment. Everything to the left has already been built into the chain; everything to the right is still waiting.
tRNA: the delivery van
Transfer RNA is a short RNA molecule that folds back on itself, held in shape by hydrogen bonds between complementary bases in its own strand. Two ends matter for the exam:
One end is an attachment site for one specific amino acid. A tRNA that carries lysine only ever carries lysine.
The other end carries the anticodon — three exposed bases that pair with a matching codon on the mRNA.
That pairing is what makes the whole system accurate. The anticodon can only hydrogen bond to one codon, so only the correct amino acid is delivered to that point in the chain.
The anticodon sits at the bottom, pointing down at the mRNA. Three bases, and they decide which amino acid gets delivered.
How the chain grows
🧩 The elongation cycle
The mRNA attaches to the small subunit of a ribosome in the cytoplasm.
The first tRNA arrives. Its anticodon pairs with the start codon AUG, so the first amino acid is always methionine.
A second tRNA arrives and pairs with the next codon. Now two amino acids are sitting side by side in the large subunit.
A peptide bond forms between them by condensation. This is an anabolic reaction and needs energy from ATP.
The first tRNA leaves, now empty, and is free to pick up another amino acid from the cytoplasm.
The ribosome moves along by one codon and the cycle repeats, adding one amino acid at a time.
A stop codon appears. No tRNA matches it, so translation ends and the finished polypeptide is released.
Where does the ATP come from? Respiration, in the mitochondria. That is a neat link to make in a longer answer — a cell that makes a lot of protein needs a lot of mitochondria.
Codons and anticodons
Three words that get muddled constantly, so learn them as a set:
Term
Where it is
What it means
Triplet
On the DNA
Three DNA bases coding for one amino acid.
Codon
On the mRNA
Three mRNA bases coding for one amino acid. Complementary to the DNA triplet it was copied from.
Anticodon
On the tRNA
Three tRNA bases that pair with one codon, bringing the right amino acid with them.
Because the anticodon pairs with the codon, it ends up with the same bases as the original DNA triplet — with U wherever the DNA had T. Worth noticing, because it makes checking your answers much quicker.
Worked examples
WORKED EXAMPLE
Work out the anticodons
An mRNA molecule reads A U G G C U A A G. Give the anticodon of each tRNA that binds to it, in order.
Step 1: Remember anticodons are complementary to codons
A pairs with U, C pairs with G.
Step 2: Take each codon in turnAUG → UACGCU → CGAAAG → UUCUAC, CGA, UUCKeep the order. The anticodons arrive in the same order as the codons are read.
WORKED EXAMPLE
Counting amino acids and peptide bonds
A length of mRNA that codes for a polypeptide contains 45 bases, ending with a stop codon. How many amino acids are in the finished polypeptide, and how many peptide bonds hold it together?
Step 1: Turn bases into codons45 ÷ 3 = 15 codonsStep 2: Take off the stop codon
The stop codon does not code for an amino acid.
15 − 1 = 14 amino acidsStep 3: Count the bonds between them14 amino acids in a chain → 14 − 1 = 13 peptide bonds14 amino acids, 13 peptide bondsBonds are always one fewer than the beads on the string.
WORKED EXAMPLE
Explain why translation stops
Explain what happens when the ribosome reaches a stop codon.
The key point
There is no tRNA with an anticodon that pairs with a stop codon.
So what follows
No further amino acid can be delivered, so no new peptide bond forms.
And the result
Translation stops and the completed polypeptide is released from the ribosome.
Chain complete and releasedDo not write that the stop codon “codes for stop” — explain that nothing can pair with it.
💡 Exam tip
State the location. Translation is in the cytoplasm, at a ribosome. Free marks if you say it.
Use the word complementary when describing codon and anticodon pairing. Mark schemes reward it.
Remember the first amino acid is always methionine, because the start codon is AUG.
Divide by three, then subtract the stop codon. Almost every “how many amino acids” question is that calculation.
If the question mentions energy, name ATP and say the peptide bond forms by condensation.
Draw a quick sketch of the ribosome with two tRNA in it. Six-mark description questions get much easier once the picture is on the page.
⚠ Common mix-up
Codon and anticodon. Codon is on mRNA, anticodon is on tRNA. Writing the wrong one turns a right answer into a wrong one.
Saying tRNA “makes” the amino acid. It does not. It picks up an amino acid that is already in the cytoplasm and carries it over.
Putting translation in the nucleus. Very common under time pressure. Transcription is in the nucleus; translation is in the cytoplasm.
Thinking one tRNA fits at a time. Two must be in the ribosome together, otherwise the amino acids could not be joined.
Forgetting the stop codon in a calculation. Divide by three and then remember it does not produce an amino acid.
Calling the product a protein straight away. It comes off as a polypeptide; it becomes a functional protein once it has folded, and sometimes only after joining other chains.
Up next: The Genetic Code — why three bases, why 64 codons for only 20 amino acids, and how to read the codon table without panicking.
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