The big idea: In a eukaryotic cell transcription happens in the nucleus and translation in the cytoplasm, so the mRNA can be modified before any ribosome reads it.
The points to remember
- The nuclear membrane keeps the DNA and transcription inside the nucleus.
- Translation by ribosomes happens outside, in the cytoplasm.
- So there is time to modify the mRNA after transcription (for example, cutting out introns).
- Only the finished mRNA leaves through the nuclear pores to the ribosomes.
- Prokaryotes have no nucleus: ribosomes can join the mRNA at once, so it cannot be modified first.
Remember it as: Copy in the nucleus, edit, then read outside.
Real example: the human beta-globin gene: The gene for part of haemoglobin is copied into pre-mRNA with two introns in it. Both are cut out in the nucleus before the mRNA reaches a ribosome.
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How it is asked: Expect it inside a question on compartments: an Explain [2] about the nucleus, or one point of a longer answer on why eukaryotic cells are divided up.
IB-style questionExplain[2 marks]
Explain one advantage to a eukaryotic cell of having its DNA inside a nucleus, separate from the cytoplasm.
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