3.9B Describe how genetic variants in the non-coding DNA of a gene can…

The idea

Non-coding DNA lies within or beside a gene but is not translated into amino acids. Some of it acts as a control region where RNA polymerase binds to begin transcription. A genetic variant in that region can change the base sequence so the polymerase binds more or less effectively. Better binding raises the rate of transcription, so more mRNA is made and more protein is produced; weaker binding lowers transcription, so less mRNA and less protein result. Because the amino acid sequence is unchanged, the effect on phenotype comes from the quantity of protein, not its structure. For example, a variant that reduces polymerase binding in a pigment gene could give paler colour because less enzyme is made. Describe the chain: variant in non-coding DNA → changed polymerase binding → altered transcription → altered quantity of protein → altered phenotype.

What earns the marks

• Identifies that the variant lies in non-coding DNA, which is not translated into amino acids.
• States that the non-coding region can act as a control region where RNA polymerase binds to start transcription.
• Explains that the variant changes how well RNA polymerase binds to the DNA.
• Links stronger binding to more transcription, more mRNA and a greater quantity of protein.
• Links weaker binding to less transcription and a smaller quantity of protein.
• Recognises that the amino acid sequence is unchanged, so the effect on phenotype is through protein quantity rather than protein structure.

Common mistakes and how to put them right

• Saying the variant changes the amino acid sequence.

  Put it right by noting the variant is in non-coding DNA, so the amino acid sequence is not altered and the effect is on quantity of protein.

• Confusing RNA polymerase with DNA polymerase.

  Put it right by stating RNA polymerase binds the control region to carry out transcription.

• Claiming the protein is made faster or slower in a way that changes its shape.

  Put it right by focusing on how much protein is produced, not its structure.

• Ignoring the phenotype link.

  Put it right by finishing the chain with the observable effect of more or less protein.

Exam tips

• Write the causal chain in order: variant → polymerase binding → transcription → quantity of protein → phenotype.
• Use the phrase 'quantity of protein' rather than 'different protein' to show the amino acid sequence is unchanged.
• Check the option names RNA polymerase and links binding to transcription, not to translation.