The big idea: A codon is three bases on mRNA. An anticodon is three bases on tRNA. They pair by complementary base pairing, so each tRNA lands on the right codon.
The points to remember
- A codon is a triplet of bases on mRNA.
- An anticodon is a triplet of bases on tRNA.
- They pair by complementary base pairing: A with U, G with C.
- The pairs are held by hydrogen bonds.
- Each tRNA carries the amino acid that matches its anticodon, so the codon order sets the amino acid order.
Remember it as: Codon on mRNA, anticodon on tRNA: pair them like a zip, A with U and G with C.
Real example: the start of every protein. Every polypeptide your cells make starts at the codon AUG. The tRNA that pairs with it has the anticodon UAC and carries the amino acid methionine.
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Working out an anticodon: Pair each base of the codon in turn. Both are RNA, so the anticodon has U, never T.
| Base in the codon (mRNA) | Base in the anticodon (tRNA) |
|---|---|
| A | U |
| U | A |
| G | C |
| C | G |
Worked example. The codon G C A pairs with the anticodon C G U. That tRNA carries alanine.
Two traps: An anticodon is complementary to the codon, not the same as it. And it never contains T: tRNA is RNA.
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How it is asked: Picking the anticodon that pairs with a codon from a short table, or explaining the pairing in two marks.
Part of an mRNA reads A U G U U C. Deduce the anticodons of the two tRNAs that pair with it.
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