3.10B Describe how genetic variants in the coding DNA of a gene can affect…

The idea

Coding DNA is the part of a gene that is translated into amino acids. The order of bases in the coding DNA determines the order of amino acids in the protein. A genetic variant changes the base sequence, so the sequence of amino acids in the protein may change. Because a protein's activity depends on its shape, and shape depends on the amino acid sequence, the altered protein may work differently. For example, a variant could change one amino acid in the active site of an enzyme, so the substrate no longer fits and the enzyme's activity falls. The chain to describe is: variant in coding DNA → altered base sequence → altered amino acid sequence → changed protein shape → altered activity of the protein → altered phenotype.

What earns the marks

• Identifies that the variant lies in coding DNA, which is translated into amino acids.
• States that the order of bases in coding DNA determines the order of amino acids.
• Explains that the variant alters the sequence of amino acids in the protein.
• Links the altered amino acid sequence to a change in the shape of the protein.
• Explains that the changed shape alters the activity of the protein, for example an enzyme's active site no longer fitting its substrate.
• Connects the altered protein activity to a change in phenotype.

Common mistakes and how to put them right

• Saying the variant changes how much protein is made.

  Put it right by noting that coding-DNA variants change the amino acid sequence and so the protein's activity, not primarily its quantity.

• Claiming the amino acid sequence stays the same.

  Put it right by stating that a change in coding base sequence can change the amino acids coded for.

• Skipping the shape step.

  Put it right by explaining that altered amino acids change protein shape, which changes activity.

• Stopping at the protein.

  Put it right by linking altered protein activity to the observable phenotype.

Exam tips

• Write the chain in order: variant → amino acid sequence → protein shape → protein activity → phenotype.
• Use a concrete example such as an enzyme active site no longer fitting its substrate.
• Distinguish coding from non-coding DNA: coding DNA is translated, so it affects the amino acid sequence.