Protein synthesis and gene variants — Edexcel GCSE Biology
Explain how the order of bases decides protein shape, describe transcription and translation, and how gene variants change phenotype.
Revision notes
4 short notes, in the order of the specification. Each one in short:
The order of bases in a gene decides the order of amino acids in a protein. The chain of amino acids then folds up in a way that depends on that order, giving a specifically shaped protein. The shape decides what the protein can do; for example, an enzyme's active site must have a shape that fits its substrate.
Protein synthesis has two stages. In transcription, in the nucleus, RNA polymerase binds to non-coding DNA in front of a gene and makes a complementary mRNA strand. In translation, the mRNA attaches to a ribosome and is read in codons, triplets of bases. tRNA brings the matching amino acids, which are linked to form a polypeptide.
A genetic variant is a difference in the DNA sequence. A variant in the non-coding DNA in front of a gene can change how well RNA polymerase binds there. More or less mRNA is then made, so the quantity of protein produced is altered, which can change the phenotype. The protein's amino acid sequence is not changed.
A genetic variant in the coding DNA of a gene can alter the sequence of amino acids in the protein. The polypeptide may then fold into a different shape, so the activity of the protein changes; for example, an enzyme's active site may no longer fit its substrate. This change in activity can affect the phenotype.
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Quick check questions
What decides the order of amino acids in a protein?
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The order of bases in a section of DNA (the gene).
What does RNA polymerase do?
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It binds to non-coding DNA in front of a gene and produces a complementary mRNA strand from the coding DNA.
What is a codon?
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A triplet of bases in mRNA that codes for a specific amino acid.
What carries amino acids to the ribosome?
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tRNA.
How can a variant in non-coding DNA affect phenotype?
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It can change the binding of RNA polymerase and so alter the quantity of protein produced.
Frequently asked questions
What happens in transcription and translation?
In transcription, in the nucleus, RNA polymerase makes a complementary mRNA copy of a gene, and the mRNA moves out into the cytoplasm. In translation, the mRNA attaches to a ribosome and is read in codons. tRNA molecules bring the matching amino acids, which are linked together to form a polypeptide.
How does the order of bases determine a protein's shape?
The order of bases in a gene decides the order of amino acids in the protein. The amino acid chain then folds in a way that depends on that order, which gives the protein its specific shape. A different order of bases could give a differently folded protein that cannot do the same job.
How can a mutation change a protein?
A genetic variant in the coding DNA of a gene can change the sequence of amino acids in the protein. The polypeptide may then fold into a different shape, so its activity changes. For example, the active site of an enzyme may no longer fit its substrate, and this change can affect the phenotype.
What is the role of tRNA in protein synthesis?
tRNA transfers amino acids to the ribosome during translation. Each tRNA carries the amino acid that matches a particular codon, a triplet of bases on the mRNA. As the mRNA is read at the ribosome, the amino acids brought by tRNA are linked together to form a polypeptide, which folds into a protein.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026