The big idea: A codon table turns each mRNA codon into its amino acid. Split the mRNA into codons, then look each one up in order.
The points to remember
- Read the mRNA in codons of three, from the first base, without overlapping.
- First base: the row block on the left.
- Second base: the column along the top.
- Third base: the line inside the box, read from the right-hand side.
- STOP means the chain ends there: no amino acid.
Remember it as: Row, column, line: first, second, third.
Real example: insulin. The insulin B chain begins Phe-Val-Asn. In the human insulin gene those amino acids come from the codons UUU GUG AAC: check each one in the table.
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From DNA to amino acids: The table is for mRNA. If you are given DNA, make the mRNA first.
Four steps
mRNA
Pair the template strand: A with U, T with A, C with G, G with C.
Codons
Split the mRNA into threes from the first base.
Look up
Find each codon in the table: row, column, line.
Stop
End at a STOP codon; write the amino acids in order.
Worked example. Template strand T A C G G T C T T gives mRNA A U G C C A G A A. The codons AUG, CCA and GAA code for Met-Pro-Glu.
Three traps: Use the template strand, not the coding strand. Never look up a codon with T in it. And do not read past a STOP codon.
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How it is asked: A multiple-choice question with the codon table printed: pick the mRNA for a set of amino acids, or the mRNA and amino acids made from a DNA strand.
Using the codon table, deduce the amino acids coded for by the mRNA A U G C C A G A A U A A.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.