The big idea: Translation needs three things: mRNA carries the code, the ribosome reads it and joins the amino acids, and tRNA brings each amino acid.
The points to remember
- mRNA carries the code from the nucleus to the cytoplasm, as codons of three bases.
- A ribosome has two parts: a small subunit and a large subunit.
- The mRNA binds to the small subunit.
- Each tRNA carries one amino acid and has an anticodon that pairs with a codon.
- Two tRNAs can bind to the large subunit at the same time, so their amino acids can be joined.
Remember it as: mRNA brings the message, tRNA brings the amino acids, the ribosome does the joining.
Real example: streptomycin. The antibiotic streptomycin binds to the small subunit of bacterial ribosomes. The mRNA codons are then misread, so the bacteria make faulty proteins. Human ribosomes are a different shape, so ours keep working.
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Who does what: Learn one job for each molecule, and which ribosome subunit each one binds.
| Molecule | What it does | Where it binds |
|---|---|---|
| mRNA | Carries the code as codons; sets the order of amino acids | Small subunit of the ribosome |
| tRNA | Brings one specific amino acid; its anticodon pairs with a codon | Large subunit (two tRNAs at a time) |
| Ribosome | Holds mRNA and tRNA together and joins amino acids by peptide bonds | Made of a small and a large subunit |
Small or large?: The mRNA binds the small subunit. The tRNAs bind the large subunit, and there is room for two at once.
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How it is asked: Naming the ribosome and mRNA on a diagram, a multiple-choice question on what binds where, or the role of mRNA or tRNA in a few marks.
IB-style questionOutline[3 marks]
Outline the role of mRNA in translation.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.