Point mutations occur when they are free from Weegy
As Point mutation In biology, a gene mutation is called when only a single nucleobase is affected by the change. It is therefore to be regarded as a special case of gene mutation and thus also of structural chromosome aberration.
Forms of point mutation
Reading frame is retained
However, due to the degenerate property (redundancy) of the genetic code, such a point mutation can be comparatively harmless. Degeneracy here means that an amino acid can be coded for by several codons. 61 coding base triplets are available for the 20 amino acids.
If one base of the DNA is exchanged for another in a point mutation, this process is called substitution. If this exchange takes place in a coding region, a modified mRNA is transcribed and causes the incorporation of a different amino acid, which leads to a modified protein which, depending on the location of the mutation and substituted amino acid, may not or no longer be able to carry out its tasks completely . A well-known example of a point mutation in humans is sickle cell anemia. Improvement in the polypeptide is just as possible, although less likely. For example, an increased substrate conversion of an enzyme.
There are two types of substitution in point mutation:
- Transition: substitution of a purine for a purine base or a pyrimidine for a pyrimidine base.
- Transversion: substitution of a purine for a pyrimidine base or vice versa.
For coding sequences, the substitution can be divided into the following categories:
- "Nonsense"-Mutation (meaning distorting mutation): codes for a stop in translation
- "Missense"-Mutation or non-synonymous mutation (meaning-changing mutation): codes for a different amino acid
- "Silent"-Mutation or synonym (silent mutation): codes for the same amino acid
- "Readthrough"-Mutation: a stop codon is encoded as an amino acid
It can be either a "Missense"-Mutation (meaning-changing mutation), i.e. the exchange of an amino acid during translation, or a "Nonsense"-Mutation (mutation that disrupts the meaning), in which a stop codon is formed. Also can "Nonsense"-Mutations in introns cause splicing errors. As a third possibility, it can also be a silent (silent) mutation or neutral mutation in which a base has been changed, but the same amino acid is still encoded. This is entirely possible because, due to the degeneracy of the genetic code, there are several coding options for some amino acids.
- see also: polymorphism
Shifts in the reading frame
A point mutation can be worse if a base is completely removed or a new one is added. Here, the mRNA strand behind the mutation loses its original meaning because it is shifted to the left or right (frameshift). As a result, the translated protein later has a completely different structure. Most of the time, its original function is lost.
A deletion is the loss of a base. The following bases move against the reading direction, which shifts the reading frame of the following codon in this same direction.
An insertion is the gain of a base. The following bases move up in the reading direction, which shifts the reading frame of the following codon in this same direction.
The frameshift can be explained using a small example. When you get the sentence
considered, it makes sense. Each word consists of three letters (triplets). Similarly, the information is also available on a gene, here the bases are always combined into so-called base triplets. If a letter is now added - but attention is still paid to the fact that a three-letter word sentence is to be formed - this information disappears. In this example, the letter X is added after the word “I”. This corresponds to a Insertion.
If the letter M is deleted after the word "I" - what one Deletion equals - but the three-letter word rule remains valid, the sentence also no longer makes sense from the word ICH.
The situation is similar when a base is added to a gene. The genetic information can no longer be read off meaningfully.
With an understanding of point mutations, it was possible to develop the Ames test, which detects the presence of mutagens.
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