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.

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

Coverage 2017–2025 · Updated 16 Jul 2026

Strand	Relationship to the new mRNA
DNA template	antiparallel and complementary
DNA coding strand	same base order as mRNA, except DNA has T where RNA has U

DNA template: 3′–TAC CTT GCG–5′

mRNA: 5′–AUG GAA CGC–3′

Codon tables are read from mRNA 5′→3′. Do not look up a DNA template triplet directly.

DNA template bases pair with RNA nucleotides according to A–U and C–G complementarity.

gene selected → DNA opens locally → RNA polymerase reads one template strand → complementary RNA nucleotides are joined → mRNA leaves → DNA re-forms unchanged

Question	Correct check
Which DNA strand determines mRNA?	the template strand
Which base replaces thymine in RNA?	uracil
Which direction is mRNA written?	5′→3′
Why can the gene be reused?	DNA sequence and covalent backbones remain intact

The mRNA now carries codons in an order that a ribosome can translate into an amino-acid sequence.

If a DNA template is supplied, write the antiparallel complementary mRNA, replacing DNA A with RNA U.

A DNA template is transcribed into mRNA codons read 5′ to 3′, and a genetic-code table maps those codons to amino acids.

Write the mRNA 5′→3′, locate the stated or biologically relevant start, and divide the sequence into consecutive triplets from that frame.

Look up each mRNA codon, record amino acids in order and stop when a stop codon is reached.

Template 3′–TAC CTT GCG ACT–5′ gives mRNA 5′–AUG GAA CGC UGA–3′, which translates as Met–Glu–Arg–Stop.

DNA template → complementary mRNA → codons read 5′→3′ → charged tRNAs selected → peptide bonds join amino acids → polypeptide primary structure

Checkpoint	Question to ask
transcription	Is the mRNA complementary to the template and written 5′→3′?
code table	Are mRNA codons grouped from the correct start?
tRNA selection	Does the anticodon pair and is the tRNA correctly charged?
elongation	Has the ribosome advanced one codon while the chain remains on the peptidyl-tRNA?
mutation	Did the codon, amino acid or reading frame actually change?

A sequence change matters biologically only through a causal path: altered primary structure → altered interactions or folding → altered protein activity → possible cellular or organism effect.

[ 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 $3^{\prime}$

1st base in codon	2nd base U	2nd base C	2nd base A	2nd base G	3rd base in codon
U	Phe	Ser	Tyr	Cys	U
U	Phe	Ser	Tyr	Cys	C
U	Leu	Ser	STOP	STOP	A
U	Leu	Ser	STOP	Trp	G
C	Leu	Pro	His	Arg	U
C	Leu	Pro	His	Arg	C
C	Leu	Pro	Gln	Arg	A
C	Leu	Pro	Gln	Arg	G
A	Ile	Thr	Asn	Ser	U
A	Ile	Thr	Asn	Ser	C
A	Ile	Thr	Lys	Arg	A
A	Met	Thr	Lys	Arg	G
G	Val	Ala	Asp	Gly	U
G	Val	Ala	Asp	Gly	C
G	Val	Ala	Glu	Gly	A
G	Val	Ala	Glu	Gly	G

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-Ile-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

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.

D1.2.9—Using genetic code table

Exam analysis

Common command terms

Scoring notes

Recent exam appearances

Template DNA Determines the mRNA Sequence

Reconstruct the Route from a Selected Gene to mRNA

Read the mRNA 5′→3′ before Using a Codon Table

Trace Information—and the Places Where It Can Change

Concept essentials