Gene Mutations (Cambridge (CIE) AS Biology): Revision Note

Gene mutations & their effect on polypeptides

• A gene mutation is a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide

• As the DNA base sequence determines the sequence of amino acids that make up a protein, mutations in a gene can sometimes lead to a change in the polypeptide that the gene codes for

• Most mutations do not alter the polypeptide or only alter it slightly so that its structure or function is not changed

  • This is because the genetic code is degenerate

• There are different ways that a mutation in the DNA base sequence can occur:

Insertion of nucleotides

• A mutation that occurs when a nucleotide (with a new base) is randomly inserted into the DNA sequence is known as an insertion mutation

• An insertion mutation changes the amino acid that would have been coded for by the original base triplet, as it creates a new, different triplet of bases

• An insertion mutation also has a knock-on effect by changing the triplets (groups of three bases) further on in the DNA sequence

  • This is sometimes known as a frameshift mutation

• This may dramatically change the amino acid sequence produced from this gene and therefore the ability of the polypeptide to function

Deletion of nucleotides

• A mutation that occurs when a nucleotide (and therefore its base) is randomly deleted from the DNA sequence

• Like an insertion mutation, a deletion mutation:

  • Changes the amino acid that would have been coded for

  • Has a knock-on effect by changing the groups of three bases further on in the DNA sequence

    • This is another type of frameshift mutation

• This may dramatically change the amino acid sequence produced from this gene and therefore the ability of the polypeptide to function

Substitution of nucleotides 

• A mutation that occurs when a base in the DNA sequence is randomly swapped for a different base

• Unlike an insertion or deletion mutation, a substitution mutation will only change the amino acid for the triplet (a group of three bases) in which the mutation occurs

  • It will not have a knock-on effect (it is not a frameshift mutation)

 The effect of gene mutations on polypeptides 

• Most mutations do not alter the polypeptide or only alter it slightly so that its appearance or function is not changed

• However, a small number of mutations code for a significantly altered polypeptide with a different shape

• This may affect the ability of the protein to perform its function

  • For example:

    • If the shape of the active site on an enzyme changes, the substrate may no longer be able to bind to the active site

    • A structural protein (like collagen) may lose its strength if its shape changes