HSC · Topic explainer
How to read a codon chart in HSC Biology
During transcription, RNA polymerase uses the DNA template strand to make complementary RNA. During translation, a ribosome reads the mRNA in groups of three bases. Each codon specifies an amino acid or a stop signal. Read a standard codon chart using mRNA in the 5′ to 3′ direction.
Worked example
For the DNA template fragment 3′–TAC GGA ATT–5′, the corresponding mRNA is 5′–AUG CCU UAA–3′. The bases pair in opposite strand directions, and RNA uses uracil in place of thymine.
On an mRNA codon chart, AUG specifies methionine, CCU specifies proline and UAA signals stop. If a mutation changes CCU to CCC, the second amino acid remains proline. In that example the substitution is synonymous.
Official exam practice
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