A gene mutation is a change in the genetic material ( mutation ) in just one gene . If it has harmful effects on the organism, it is also referred to as a genetic defect . If, on the other hand, larger sections of a chromosome are affected by the change , it is not a gene mutation, but a structural chromosome aberration .
The mutation can affect sections of a gene of different sizes. A gene mutation is already present if only one base in the DNA is changed ( point mutation ), removed or added (possible grid mutation ). It is the most common and important type of mutation and occurs randomly. It is either triggered by a faulty replication of the DNA or a mutagen causes a base exchange in the genome . Usually mutations with a recessive effect arise . The gene mutation can be unaffected, disadvantageous, sometimes fatal (lethal) or even beneficial.
Gene mutation types
There are two types of gene mutation:
A distinction is also made between the following types of gene mutations:
- Point mutation by substitution
- Deletion mutation by deletion
- Insertion mutation by insertion
- Duplication mutation due to duplication
- Inversion mutation by inversion , but the inversion is usually as chromosome mutation viewed
In the case of substitution, an individual base is exchanged for another. Substitution here means the process of changing the point mutation as a variant of the gene mutation.
According to the bases, a distinction is made between two types of substitution:
- Transition : substitution of a purine - for another purine base or a pyrimidine - for another pyrimidine base.
- Transversion : substitution of a purine for a pyrimidine base or vice versa.
A point mutation through substitution can have different effects. A missense mutation creates a codon that codes for a different amino acid . A nonsense mutation results in a codon that does not code for any of the amino acids; it can act as a stop codon , and it can also cause faulty splicing in introns . A silent or silent mutation , on the other hand, is one that creates a codon that codes for the same amino acid. This is entirely possible as there are several coding options for many amino acids (see genetic code ). The neutral mutation is similar to a silent mutation , in which the newly created codon encodes a different amino acid, but one that is chemically very similar to the original amino acid. For example , if the codon AUU arises with a ( missense ) mutation from CUU with a transversion of C → A , the amino acid isoleucine is coded instead of leucine , which is so chemically similar that the mode of action of the protein formed u. U. remains unaffected.
There are two types of deletion:
- out of frame -Deletion: The reading frame is shifted. A modified protein is created , starting at the deletion point.
- in frame deletion: The reading frame is retained. The result is a modified protein with deleted amino acids.
The reading frame can be maintained if the deletion of base pairs contains a number divisible by three. If, for example, six base pairs are deleted in a row, then two amino acids are also missing in the protein (which may or may not result in a defective protein), but the reading frame has been retained.
Insertion or addition
There are two types of gene duplication:
- partial (partial) gene duplications
- complete gene duplications
A well-known example of a gene mutation is sickle cell anemia . Those affected produce abnormal hemoglobin and the altered red blood cells are broken down by leukocytes , leading to anemia. There are two variants:
- Homozygous (pure breeding): Severe anemia resulting in death, those affected usually do not have fertile offspring
- Heterozygous (mixed breed): Malaria resistant , red blood cells are only attacked at high altitudes
Other examples are:
- z. B. Lubert Stryer: Biochemistry , 4th edition, spectrum, Heidelberg - Berlin - Oxford, 1996, p. 872