The universal genetic code at Higher Level: The same statement as SL. At HL it joins with the evidence for the last universal common ancestor, LUCA: shared genes and a shared code across all living things.
Practise this as you read
- Explain why a shared code points to one ancestor.
- Say near-universal: there are small variations.
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The big idea: Almost all living things use the same genetic code. The best explanation is that they all inherited it from one common ancestor.
The points to remember
- The genetic code is the set of rules for which three-base codon codes for which amino acid.
- Almost every organism uses the same code: GGC means glycine in a bacterium, an oak tree and you.
- There are only a few small variations, so the code is called near-universal.
- A shared code is evidence that all living things are descended from one common ancestor.
- The code was inherited from that ancestor; the same arbitrary code arising again and again by chance is very unlikely.
Remember it as: One code, one family tree.
Real example: a glowing tobacco plant: In 1986 scientists put the gene for the firefly's light-making enzyme into tobacco plants. The plants read the animal gene with their own code and made the enzyme, so they glowed.
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How it is asked: A Paper 1A item on what counts as evidence of a common ancestor, or a short Explain.
IB-style questionExplain[2 marks]
Explain how the genetic code provides evidence that all living organisms share a common ancestor.
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See the answer broken into clear points, with guidance on how each point earns marks.