Point mutation

As a point mutation, a mutation is known in biology, if only a single nucleobase is affected by the change.

Of the point mutation

Reading frame is maintained

Due to the degenerate property (redundancy) of the genetic code, such a point mutation without consequences. Degeneracy here means that an amino acid can be encoded by multiple codons. The 20 amino acids are 61 base triplets coding available ( 3 base triplet combinations are non- coding ). If the point mutation is a codon in the original codon encoding the same amino acid is produced the same protein as the original codon.

Substitution

Is a point mutation at a base of the DNA replaced with another, so called this procedure substitution. This exchange takes in a coding region, an altered mRNA is transcribed and causes the installation of a different amino acid, resulting in an altered protein that possibly his duties - not more not perform, or completely - depending on the location of the mutation and substituted amino acid can. A known example of a point mutation in humans is sickle cell anemia. An improvement of the polypeptide is also possible, though unlikely. For example, increased conversion of an enzyme substrate.

The point mutation there are two types of substitution:

• Transition: substitution of a purine by another purine or a pyrimidine for another pyrimidine.
• Transversion: substitution of a purine for a pyrimidine or vice versa.

For coding sequences can be divided substitution into the following categories:

• " Nonsense" mutation ( mutation paraphrased ): encodes for a stop of translation
• " Missense " mutation or non- synonymous mutation ( sense mutation changing ) codes for a different amino acid
• " Silent" mutation or synonymous ( silent mutation ): encodes the same amino acid
• "Read through" mutation: a stop codon is encoded as amino acid

Shifts the reading frame

May extend Worse, a point mutation, when a base is eliminated completely or added a new one. Here, the mRNA strand loses its original meaning behind the mutation because it is shifted to the left or right ( frameshift ). This means that the translated protein has a completely different structure later. Its original function is lost in most cases.

Deletion

When a deletion is the loss of a base. The following bases move on to the reading direction, which shifts the reading frame of subsequent codons in that direction.

Insertion

When an insertion is the gain of a base. The following bases move up in the reading direction, which shifts the reading frame of subsequent codons in that direction.

Example

The frameshift ( frameshift ) can be explained by a small example. If the sentence

Considered, this makes sense. Each word consists of three letters ( triplets ). Similarly, the information is also available on a gene, here are the bases always so-called base triplets are summarized. Now, if a letter is added - but will still be taken to ensure that a three-letter - word sentence is to be formed - this information will disappear. In this example, the letter X is added after the word "I". This corresponds to an insertion.

If, after the word "I " is the letter M is deleted - what a deletion equivalent - but the three-letter - word - rule remains valid, the theorem follows also from the word I no longer make sense.

Similarly, if in a gene, a base is added. The genetic information can not be read meaningful.

Experimental benefits

With the understanding of point mutations, it was possible to develop the Ames test, which detects the presence of mutagens.