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Genetic variants in coding DNA

Genetics · Protein synthesis and gene variants · note 4 of 4

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In short

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.

Variants in the coding DNA of a gene can change the base sequence that codes for the protein.

  1. A variant in the coding DNA can alter the sequence of amino acids.
  2. The polypeptide may then fold into a different shape.
  3. A protein with a different shape may have different activity. For example, an enzyme's active site may no longer fit its substrate.
  4. The change in the activity of the protein can affect the phenotype.
Where a variant is and what it changes
Where the variant isWhat changesEffect on phenotype
Non-coding DNA in front of the geneBinding of RNA polymeraseQuantity of protein produced is altered
Coding DNA of the geneSequence of amino acidsActivity of the protein produced is altered

Quick check

  1. What decides the order of amino acids in a protein?

    Show answer

    The order of bases in a section of DNA (the gene).

  2. What does RNA polymerase do?

    Show answer

    It binds to non-coding DNA in front of a gene and produces a complementary mRNA strand from the coding DNA.

  3. What is a codon?

    Show answer

    A triplet of bases in mRNA that codes for a specific amino acid.

  4. What carries amino acids to the ribosome?

    Show answer

    tRNA.

  5. How can a variant in non-coding DNA affect phenotype?

    Show answer

    It can change the binding of RNA polymerase and so alter the quantity of protein produced.

Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026

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.

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