How does a gene code for a protein inside the cell?
How the base sequence of a gene codes for the order of amino acids in a protein, the roles of transcription and translation, the part played by mRNA and ribosomes, and how mutations can change a protein.
A focused CCEA GCSE Biology answer on protein synthesis, covering how the base sequence of a gene codes for amino acids, the roles of transcription and translation, mRNA and ribosomes, and how mutations can change a protein.
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What this dot point is asking
CCEA wants you to explain that the base sequence of a gene codes for the order of amino acids in a protein, describe the roles of transcription and translation, explain the part played by mRNA and ribosomes, and explain how mutations can change a protein.
How a gene codes for a protein
Transcription and translation
The mRNA carries the code out of the nucleus (DNA is too large to leave) to the ribosome, where the protein is actually assembled.
The genetic code in triplets
The ribosome reads the mRNA in triplets (groups of three bases). Each triplet codes for one amino acid. By reading the triplets in order, the ribosome adds the amino acids in the right sequence, so the protein is built correctly.
Examples in context
- Example 1. Why a mutation can cause a faulty enzyme
- An enzyme is a protein with a precise active site. If a mutation changes the base sequence of the gene, the order of amino acids can change, so the protein folds differently and the active site changes shape. The substrate no longer fits, so the enzyme does not work. This is how a single change in DNA can lead to a disorder, linking protein synthesis to enzymes and to genetic disease.
- Example 2. Why the same DNA makes different proteins in different cells
- Every cell has the same full set of genes, but a muscle cell and a nerve cell look and behave very differently. This is because only some genes are switched on in each cell type, so only some proteins are made. A muscle cell transcribes and translates the genes for muscle proteins; a nerve cell transcribes different genes. This shows that protein synthesis, controlled by which genes are active, is what makes specialised cells different.
- Example 3. From gene to a useful protein, insulin
- The body makes the hormone insulin from a gene in the pancreas. The base sequence of that gene codes for the exact order of amino acids in the insulin protein. In transcription the gene is copied into mRNA, and in translation a ribosome reads the mRNA in triplets and joins the amino acids in that order, building insulin. Because the order of amino acids is set by the order of bases, the insulin is made correctly every time. This is also why genetic engineering works: if the insulin gene is put into a bacterium, the bacterium reads it in the same way and makes human insulin. Following the same flow of information from gene to protein in a real example helps the steps stick.
Try this
Q1. Name the molecule that carries the code from the nucleus to a ribosome. [1 mark]
- Cue. Messenger RNA (mRNA).
Q2. How many bases code for one amino acid? [1 mark]
- Cue. Three (a triplet).
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20215 marksDescribe how the information in a gene is used to make a protein.Show worked answer →
Five marks for the two stages and the molecules involved.
The base sequence of a gene codes for the order of amino acids in a protein.
In transcription, the DNA strands separate and a copy of the gene is made as a molecule of messenger RNA (mRNA).
The mRNA leaves the nucleus and travels to a ribosome.
In translation, the ribosome reads the mRNA in groups of three bases, and each triplet codes for a particular amino acid.
The amino acids are joined in the correct order to build the protein, which then folds into its shape.
Markers reward base sequence coding for amino acid order, mRNA copy made (transcription), mRNA to a ribosome, triplets read, amino acids joined in order.
CCEA 20193 marksExplain how a mutation in a gene could change the protein it makes.Show worked answer →
Three marks for the change in bases leading to a change in the protein.
A mutation is a change in the base sequence (the DNA) of a gene.
Because the base sequence codes for the order of amino acids, a change can alter which amino acid is added.
This can change the order of amino acids and so the shape of the protein, which may stop it working properly (for example a faulty enzyme).
Markers reward a change in the base sequence, a change in the amino acid order, and a change in the protein or its shape and function.
Related dot points
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A focused CCEA GCSE Biology answer on chromosomes, genes and DNA, covering how they relate to the nucleus, the double helix and complementary base pairing, the human chromosome number, and how genes control characteristics.
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- Enzymes as biological catalysts, the lock and key model and the active site, how temperature and pH affect enzyme activity including denaturing, and investigating enzyme activity experimentally.
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Sources & how we know this
- CCEA GCSE Biology specification — CCEA (2017)