Edexcel IGCSE Biology · Spec 3.18B
Translation: tRNA, Codons and Anticodons
Covers define codons and anticodons, Explain how tRNA delivers specific amino acids and Describe translation at a ribosome.
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Translation: tRNA, Codons and Anticodons
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Explained
Codons, anticodons, and how the ribosome reads them
Translation is the second stage of protein synthesis. Transcription has already produced a messenger RNA copy of the gene, and that mRNA has left the nucleus through a pore and attached to a ribosome in the cytoplasm.
The problem now is that a sequence of bases has to become a sequence of amino acids, which are chemically nothing like bases. Transfer RNA is what bridges that gap.
Codons and anticodons
A codon is a sequence of three bases on the mRNA. Each codon specifies one amino acid, or acts as a stop signal to end the chain.
An anticodon is a sequence of three bases on a tRNA molecule, complementary to a particular codon. Each tRNA carries the specific amino acid that its codon calls for.
The two words are easy to swap. Codon is on the messenger, anticodon is on the transfer molecule, and the anti part tells you it is the complementary one.
What happens at the ribosome
The ribosome holds the mRNA and reads it three bases at a time, moving along in one direction.
For each codon, a tRNA with the complementary anticodon binds, bringing its amino acid with it. The ribosome joins that amino acid to the growing chain with a peptide bond, the tRNA is released to collect another amino acid, and the ribosome moves on to the next codon.
When a stop codon is reached, the chain is complete. It then folds into a specific three dimensional shape, and that shape is what allows the protein to work, which is why an enzyme's active site fits only one substrate.
Why the order matters
The order of bases determines the order of codons, which determines the order of amino acids, which determines how the chain folds, which determines the shape and therefore the function.
That chain is why a single base change can matter. Change one base and you may change one codon, which may change one amino acid, which may change the folding, which may stop the protein working. It may also do nothing at all, because several codons code for the same amino acid.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme for a question about a drug that blocks protein production sets out the whole process as a sequence of separate marks: transcription makes the mRNA, the RNA leaves the nucleus and enters the cytoplasm or moves to the ribosome, tRNA brings or carries or transports the amino acids, tRNA binds the mRNA or anticodons bind codons, and the amino acids join to make a polypeptide or peptide bonds form.
Five distinct steps, each credited separately. The one most often left out is the third, because students describe the ribosome reading the message without saying what actually delivers the amino acids.
The guidance accepts a number of alternatives, including that DNA unzips and mRNA is made, that polymerase makes the mRNA, and that anticodons and codons cannot bind if a complementary RNA is present.
It also accepts, for the consequence, that a different amino acid sequence is made or that the amino acids are not joined. Saying that the protein is not made is the conclusion; saying which step failed is the explanation.
Keeping the molecules straight
mRNA carries the message from the nucleus to the ribosome. tRNA carries the amino acids to the ribosome. The ribosome does the joining.
Three molecules, three jobs, and every question on this topic is asking you to attach the right job to the right name.
Spec 3.18B
What you need to know
- Define codons and anticodons
- Explain how tRNA delivers specific amino acids
- Describe translation at a ribosome
Active recall
Quick check
Answer each question before opening the answer.
What is a codon?
A sequence of three mRNA bases specifying an amino acid or stop signal
What is the function of a tRNA anticodon?
It pairs with a complementary mRNA codon
Where are amino acids joined together?
At a ribosome
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