Using the codon table at Higher Level: The same statement as SL. At HL the DNA is often given as two strands with their 3' and 5' ends, so you must pick the template strand first.
Practise this as you read
- Deduce the amino acids coded by an mRNA strand.
- Deduce the mRNA and amino acids made from a DNA molecule.
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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: haemoglobin. Human beta-globin mRNA begins AUG GUG CAC CUG. The table gives Met-Val-His-Leu, the start of the beta chain of haemoglobin.
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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.
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.
The template strand of part of a gene reads 3' T A C G G T C T T 5'. Using the codon table, deduce the amino acids it codes for.
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