D1.2.9—Using genetic code table
Genetic code tables are used by reading mRNA codons in the correct frame and order to determine amino acid sequence.
- Syllabus
- First assessment 2025
- Objective
- D1.2.9
- Level
- HL
Exam analysis
Chance of appearing3%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1
Typical marks1
Common command terms
- Determine
Scoring notes
- Common mistake
- Using DNA triplets directly in the mRNA code table without converting thymine to uracil.
Recent exam appearances
May 2025Paper1A ["HL"] · TZ210[ 1 ]D1.2.9—Using genetic code table
November 2019Paper1 ["HL"] · TZ07[ 1 ]D1.2.9—Using genetic code table
November 2017Paper1 ["HL"] · TZ08[ 1 ]D1.2.9—Using genetic code table
Practice this objective
Coverage 2017–2025 · Updated 16 Jul 2026
A Code Table Converts Codons into a Sequence
A genetic-code table is used by reading mRNA codons 5' to 3' and matching each to its amino acid.
The first, second and third bases select positions in the table. Reading-frame errors shift every later codon, so strand labels and grouping matter.
Use: locate start; split triplets; read each codon; stop at a stop codon; report amino-acid order.
For 5'-AUG-GCU-UAA-3', AUG starts, GCU adds alanine and UAA terminates. If one base is omitted before AUG, every later triplet may be regrouped, so the table must be used from the correct start frame.
Do not use the DNA template directly as if it were mRNA; transcribe first.
Using genetic code table
Assessment in practice
1 marksHow it is assessed
This objective is assessed through multiple choice, commonly using Determine.
Command terms
Determine
What earns marks
Build the answer around this relationship: Genetic code tables use mRNA codons.
Watch for
Using DNA triplets directly in the mRNA code table without converting thymine to uracil.
Representative question
Question 1
Which sequence of mRNA bases and amino acids could be produced by transcription and translation of the DNA molecule shown?
3' AAAGTGGCACGTATATTT 5'
5' TTTCACCGTGCATATAAA
\begin{tabular}{|l|l|l|l|l|l|l|l|}
\hline \multirow{6}{*}{} & \multicolumn{6}{|c|}{2nd base in codon} & \multirow{14}{*}{3rd base in codon} \\
\hline & & U & C & A & G & & \\
\hline & \multirow{4}{*}{U} & Phe & Ser & Tyr & Cys & U & \\
\hline & & Phe & Ser & Tyr & Cys & C & \\
\hline & & Leu & Ser & STOP & STOP & A & \\
\hline & & Leu & Ser & STOP & Trp & G & \\
\hline \multirow[t]{8}{*}{} & C & Leu & Pro & His & Arg & U & \\
\hline & \multirow{3}{*}{A} & lle & Thr & Asn & Ser & U & \\
\hline & & lle & Thr & Lys & Arg & A & \\
\hline & & Met & Thr & Lys & Arg & G & \\
\hline & \multirow{4}{*}{G} & Val & Ala & Asp & Gly & U & \\
\hline & & Val & Ala & Asp & Gly & C & \\
\hline & & Val & Ala & Glu & Gly & A & \\
\hline & & Val & Ala & Glu & Gly & G & \\
\hline
\end{tabular}
Sequence of mRNA bases
Sequence of amino acids
UUU-GAG-GCU-CGA-UAU-UUU
Phe-Glu-Ala-Arg-Tyr-Phe
AAA-CUC-CGA-GCU-AUA-UUU
Lys-Leu-Arg-Ala-lle-Phe
UUU-CAC-CGU-GCA-UAU-AAA
Phe-His-Arg-Ala-Tyr-Lys
AAA-GUG-GCA-CGU-AUA-UUU
Lys-Val-Ser-Arg-Ile-Phe
Core Protein Synthesis
- Transcription: RNA polymerase builds complementary mRNA from the DNA template; A pairs with U and C with G.
- Translation: ribosomes read mRNA codons 5′ to 3′ while tRNA anticodons deliver specific amino acids for peptide-bond formation.
- Genetic code: codons are triplets; the code is degenerate and almost universal, with start and stop signals. Use mRNA—not DNA—when reading a code table.
- Information flow: codon order determines amino-acid sequence, which determines protein folding and function.
- Expression and variation: cells regulate which genes are transcribed. A mutation may change a codon, primary structure and phenotype, as in sickle-cell haemoglobin.
Concept essentials
- Genetic code tables use mRNA codons.
- Template DNA must be converted to complementary mRNA before translation.
- Codons must be read in the correct reading frame and direction.
- The amino acid sequence follows the order of codons on mRNA.