Non-coding parts of DNA can switch genes on and off, so variations in…

The idea

Not all parts of a DNA molecule code for proteins. The non-coding parts of DNA play a crucial role in regulating gene expression by switching genes on and off. This means they control whether a specific gene is used to produce a protein. Because these switches are made of DNA bases, variations or mutations in these non-coding areas can affect how genes are expressed. For example, a mutation in a non-coding region might prevent a gene from being switched on, reducing the amount of a specific protein produced, even though the gene's actual coding sequence remains completely unchanged. This explains why variations in non-coding DNA can significantly alter an organism's phenotype without changing the structure of the proteins themselves.

What earns the marks

• Non-coding parts of DNA do not code for the sequence of amino acids in a protein.
• Non-coding DNA can switch genes on and off, controlling gene expression.
• Variations or mutations in non-coding DNA can alter how genes are expressed.
• Variations in non-coding regions can affect the phenotype without altering the protein's actual structure.

Common mistakes and how to put them right

• Thinking that all DNA codes for proteins.

  Put it right: Only some regions are coding genes; other non-coding regions are involved in switching genes on and off.

• Assuming mutations in non-coding regions have no effect on the organism.

  Put it right: Variations in non-coding DNA can alter gene expression and change the phenotype.

• Confusing a change in gene expression with a change in protein structure.

  Put it right: Non-coding mutations change how much protein is made, not the shape of the protein itself.

Exam tips

• Use the exact phrase 'switches genes on and off' when describing the function of non-coding DNA.
• If asked about mutations in non-coding DNA, explain that it affects gene expression rather than the amino acid sequence.