A mutation is a change in a gene or a chromosome.
The majority of mutations are changes in the DNA of a single gene. A gene mutation changes the DNA base sequence of a gene or some part may be omitted altogether. The gene will then produce a different type of protein from the one the gene normally codes for.
There are different forms of gene mutations involving the adding, subtraction or rearrangement of the DNA base sequence in a gene. These mutations are referred to as duplication, insertion, deletion and substitution.
Sickle cell anaemia, an inherited disease in humans, is an example of a gene mutation involving DNA base substitution. The DNA base to the gene responsible for the code to produce haemoglobin is substituted resulting in the incorrect amino acid being added in the protein chain.
Chromosome mutations affect all or part of a chromosome. This may result in a change to the number or structure of the chromosomes. Certain mutations may involve part of the chromosome breaking off, or sections may break off and re-join in a different way or become attached to another chromosome. In some cases a whole chromosome is either lost or gained. Changes to the number of chromosomes are usually due to errors that occur during meiosis but they can also occur during mitosis.
An example of chromosome mutation is non-disjunction. This occurs during meiosis when the spindle fibres fail to separate two homologous chromosomes resulting in gametes with one extra chromosome and other gametes lacking a chromosome. If non-disjunction occurs in chromosome 21 of the human egg cell and this egg gets fertilised by a normal sperm cell then the resulting cell has 3 copies of chromosome 21. Then the individual formed would have 47 chromosomes instead of 46. Such individuals have a condition called Down’s syndrome.
Causes of mutations
Mutations occur randomly and naturally i.e. any gene can undergo mutation at any time and the rates at which mutations occur vary between organisms.
Some factors increase the rate of mutations. These include high energy electromagnetic radiations such as gamma rays, X-rays and ultra-violet light. The greater the size of the dose of radiation, the greater the chance of mutation.
Some chemicals also cause mutations and are referred to as mutagens. These include formaldehyde, mustard gas and certain components of tobacco.