What are mutations?
Mutations are irreversible changes in the DNA sequence or chromosome structure. Mutations can be Loss of function or Gain of function mutations.
Loss of function mutations
This is the kind of mutation that causes the gene to be underexpressed or not expressed at all. It can be
- Recessive: both of the alleles have to have a loss of function mutation for it to be non-functional.
If one of the alleles is mutated, the other can retain function e.g in tumour suppressor genes; unlike proto-oncogenes; where a gain of function mutation of one of the alleles is sufficient enough to transform the cell.
- Dominant Negative: in proteins with more than one subunit, if one of the subunits is non-functional, the whole complex is non-functional.
Gain of function mutations
This is when the mutation causes the gene to be expressed more than normal, with a stronger activity or at the wrong time and location.
An example is the conversion of proto-oncogenes to oncogenes. Here, a mutation in one chromosome is enough to be expressed i.e heterozygous organisms will express overexpression unlike in tumour suppressor genes.