Gene Mutations

This lesson covers:

  1. What a gene mutation is 
  2. The different types of gene mutations 
  3. How gene mutations affect protein structure

Gene mutations

A gene mutation is a change in the DNA base sequence. 


Mutations occur spontaneously if DNA is misread during DNA replication. Ultraviolet or ionising radiation, chemicals and viruses can increase the rate of mutations. These are known as mutagens.

The three types of gene mutations you need to know are: 

  1. Substitution - One base is replaced by another. 
  2. Deletion - One base is removed from the sequence. 
  3. Insertion - One base is added into the sequence.


These mutations may change the amino acid sequence coded for by the gene. This will lead to a different polypeptide and potentially a different tertiary structure. 


If the protein is an enzyme, this could change the shape of the active site, and stop it from forming an enzyme-substrate complex. 

Substitutions

Not all substitution mutations will result in a change to the amino acid sequence of a protein.

Diagram showing the effect of substitution mutations on amino acid sequences with examples of no change and a change in protein structure.

In the example above, the base C has been substituted for a T. However, this triplet codes for the same amino acid so has no effect on the amino acid sequence. 


This is due to the degenerate nature of the genetic code. Many amino acids are coded for by more than one triplet. 

Diagram showing substitution mutation where base G is changed to C, affecting the amino acid sequence.

In the example above, the base G has been substituted for a C. However, this triplet does code for a different amino acid so the structure of the resulting protein is changed.

Deletions

Deletions are much more likely to change the amino acid sequence because they cause a frameshift.

Diagram showing how a deletion causes a frameshift mutation changing the amino acid sequence.

In the example above, the base T has been deleted from the base sequence. This causes a frameshift where the entire sequence after the deletion shifts to the left by one base.


This changes all the triplets after the deletion and so also changes the amino acid sequence.

Insertions

Insertions are likely to change the amino acid sequence because they also cause a frameshift.

Diagram showing an insertion mutation causing a frameshift in the DNA sequence and changing the amino acid sequence.

In the example above, the base A has been inserted into the base sequence. This causes a frameshift where the entire sequence after the insertion shifts to the right by one base.


This changes all the triplets after the insertion and so also changes the amino acid sequence.