The big idea: In eukaryotic cells the first copy of a gene, pre-mRNA, is changed into mature mRNA before it leaves the nucleus.
The points to remember
- Transcription makes pre-mRNA, a copy of the whole gene: exons and introns.
- The introns are removed.
- The exons are spliced (joined) together, in order, to form mature mRNA.
- A 5' cap is added to the 5' end and a 3' poly-A tail (many A nucleotides) to the 3' end.
- The cap and tail stabilise the mRNA, so it is not broken down too soon.
Remember it as: Cut the introns, join the exons, cap the front, tail the back.
Real example: dystrophin. The human dystrophin gene, needed by muscle cells, is about 2.2 million bases long, yet its mature mRNA is only about 14,000 bases: nearly all of the gene is introns, cut out after transcription.
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What each change does: Each change makes the mRNA ready to be translated in the cytoplasm.
| Change | Where | Why it matters |
|---|---|---|
| Introns removed | Inside the gene | Only the coding exons are left |
| Exons spliced | Inside the gene | One continuous coding sequence |
| 5' cap added | 5' end | Stabilises the mRNA and helps the ribosome bind |
| Poly-A tail added | 3' end | Stabilises the mRNA against breakdown |
Cap 5', tail 3': The cap goes on the 5' end and the poly-A tail on the 3' end. Mature mRNA has no introns. Prokaryotes do not do any of this.
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How it is asked: A 1A item picking the diagram of a mature mRNA, or a short Describe of the changes before mRNA leaves the nucleus.
IB-style questionDescribe[3 marks]
Describe how mRNA is modified in a eukaryotic cell before it leaves the nucleus.
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See the answer broken into clear points, with guidance on how each point earns marks.