Grade 7-9 Mutations and Protein Function

Grade 7-9 Mutations and Protein Function

A mutation is a change in the DNA base sequence. Mutations can happen spontaneously during DNA replication or be caused by mutagens such as UV radiation or certain chemicals.

How a mutation can change protein function:

1. A DNA base is changed, inserted, or deleted.
2. During transcription, the altered sequence is copied into mRNA — the codon for that position is changed.
3. During translation, a different amino acid is inserted at that position in the chain.
4. The amino acid sequence is different, so the protein folds into a different 3D shape.
5. If the shape of the active site (in an enzyme) or binding site (in another protein) is changed, the protein may no longer function correctly.

Three Outcomes of a Mutation

• Neutral: Most mutations do not change the amino acid. Because the genetic code is redundant — multiple codons code for the same amino acid — many base changes produce no effect. These are called silent mutations.
• Harmful: Some mutations change a critical amino acid and disrupt protein function. For example, sickle cell anaemia is caused by a single base change in the haemoglobin gene. One wrong amino acid causes haemoglobin molecules to clump together, distorting red blood cells into a sickle shape that cannot carry oxygen efficiently.
• Beneficial (rare): Occasionally a mutation produces a protein with a new or improved function. For example, a mutation in a bacterial gene can alter an enzyme that an antibiotic targets, making the bacterium resistant to that antibiotic. This mutation is beneficial to the bacterium and will be selected for.

Key principle: The effect of a mutation depends on whether the changed amino acid alters the shape of a functionally important region of the protein.