AP Biology · Unit 6 · Topic 6.4
The genetic code is 64 codons for 20 amino acids, and where the spare capacity sits is not an accident. Read a message in all three frames and see why the frame matters more than any single letter.
Learning objective 6.4.A Explain how the sequence of an mRNA is translated into a polypeptide.
A ribosome divides the mRNA into codons and reads them in order. Where it starts dividing is the reading frame — and the same bases read from a different starting point are a different message entirely.
Compare the three peptides. They share no amino acid in the same position, because shifting the start by one base regroups every letter after it. This is why an insertion or deletion of one or two bases is so much worse than a substitution — it moves the frame, and Topic 6.7 measures exactly how much damage that does.
There is no separate “start” instruction. The ribosome finds an AUG and begins there, and because AUG is a methionine codon, methionine is inserted. The signal and the amino acid are the same thing.
Notice on frame 0 that the mRNA does not run out — there are bases after the stop codon that are simply never translated. A stop codon is a signal, not the end of the molecule.
The same pairing rule as every other step in this unit: A with U, G with C. Nothing new is needed to explain how the right amino acid arrives.
The whole table, in the standard layout: first base down the side, second base across the top, third base on the right. Pick an amino acid to see every codon that specifies it.
Two bases would give 4 × 4 = 16 combinations, which cannot name 20 amino acids however cleverly they are assigned. Three gives 64, which is more than enough — and the surplus is what produces the redundancy.
Reading is unambiguous: given a codon, the amino acid is certain. Writing is not: given an amino acid, the codon could be any of several. So you can always translate mRNA into protein, and you can never translate protein back into a unique mRNA — which is worth remembering when a question asks you to do the second thing.
Two amino acids have a single codon each and three have six. Nothing about an amino acid's chemistry required this — but the pattern is not random either, as the next tab shows.
This is usually stated and rarely measured. Here it is measured: every possible single-base change was tried at every sense codon, and the share that left the amino acid unchanged was counted for each position.
| Position in the codon | Changes tried | Amino acid changed | Silent | % silent |
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Pick a codon and a position, and see all three alternatives at once.
| New codon | Amino acid | Changed? |
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A single-base substitution is therefore not one kind of event with one kind of consequence. Where it lands inside the codon changes the odds dramatically: at the second position it always changes the amino acid, at the third position it usually does not. Topic 6.7 puts numbers on what each outcome does to the finished protein.
Answer these in your own words before you open the answers.
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| Stage | What happens | What ends it |
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Every stage here is triggered by a sequence in the mRNA rather than by a timer. The ribosome does not know how long the protein should be; it finds out when it reaches a stop codon.