(HT only) Not all parts of DNA code for proteins.

The idea

DNA contains many base sequences, but only some of them code for proteins. A gene is a sequence of DNA that codes for a particular protein, but between genes there are non-coding regions. Even within a gene, not all bases necessarily code for amino acids; there are non-coding sections such as introns in eukaryotes. These non-coding parts of DNA do not directly determine the amino acid sequence of a protein. They can have roles in regulating gene expression, for example controlling whether a gene is switched on or off, and some non-coding DNA is involved in the structure of chromosomes. Mutations in non-coding DNA may therefore have no effect on a protein, or may affect how genes are expressed.

What earns the marks

• Only some parts of DNA code for proteins; these coding regions are genes.
• Non-coding regions of DNA lie between genes and may also occur within genes.
• Non-coding DNA does not directly code for the amino acid sequence of a protein.
• Non-coding DNA can have roles in regulating gene expression and in chromosome structure.
• Mutations in non-coding DNA may have no effect on a protein or may affect gene expression.

Common mistakes and how to put them right

• Thinking that all DNA is made of genes.

  Put it right by stating that genes are only the coding regions and much DNA is non-coding.

• Assuming non-coding DNA has no function at all.

  Put it right by explaining that it can regulate gene expression and affect chromosome structure.

• Confusing non-coding DNA with mutations.

  Put it right by distinguishing between DNA that does not code for protein and changes in the base sequence.

Exam tips

• Use the term 'non-coding DNA' and state that it does not code for amino acids.
• Give a role of non-coding DNA, such as regulating gene expression, to show understanding beyond the definition.
• When discussing mutations, consider whether the mutation is in a coding or non-coding region.