The mutation is a change in base pair or a change in the chromosome, which alters the reading of the DNA base sequence. If a mutation occurs at the base level, we speak of gene mutations, and when it involves only one base, we speak of point mutations. Mutations are hereditary changes in genetic material and are rare, random, and sudden events.
They can be:
Dominant: if they affect a dominant allele;
Recessive: if they affect a recessive allele.
There are still other classifications of mutations, according to:
- Type of cell in which they occur
- Type of mutation that occurs at the basic level.
- The effects that mutations cause.
- The effects on the genotype.
- The cause of the mutation.
- The effects on the genotype, according to the cause of the mutation.
Mutations can, therefore, be distinguished into somatic mutations, when the cell in which they occur, called the mutated cell, gives rise to somatic cells. The characteristic of this type of mutations is that they occur only in the individual where they occur but are not transmitted to subsequent generations. Mutations in the germline of organisms that reproduce sexually can be transmitted, through gametes, to subsequent generations. We can then speak of mutations for substitution of bases, if we have, in fact, a substitution of the base and to be precise we speak of mutation by transition when a pair of purine bases (A, G) is replaced with another purine (G, A) and a pyrimidine (C, T) with another pyrimidine (T, C), and mutation by transversion, when a purine (A, G) is replaced with a pyrimidine pair (C, T) and the other way around.
If, one or more base pairs are added (insertion) or deleted (deletion), we can say it frameshift mutations (phase slip), since this type of mutations leads to a change of reading in the translation and then in the transcription which renders the protein produced.
On the other hand, when mutations concern the change of the chromosome structure we speak of chromosome mutations, they are divided into four main types:
- deletions
- duplication
- inversions
- translocations
The first two involve a change in the amount of DNA.
Deletions are mutations in which a chromosome trait is missing. These mutations are produced by breaks in chromosomes, which in turn can be caused by temperature, radiation (especially ionic ones), viruses, chemicals, transposable elements, or by errors in recombination.
The consequences of deletions can be different and depend on the genes or parts of genes that are removed. In heterozygous organisms for deletion can be normal. On the other hand, if the counterpart contains recessive genes with deleterious effects, the consequences can be serious.
If the deletion implies the loss of the centromere, the result is an acentric chromosome that is generally lost during meiosis. This can lead to the loss of entire chromosomes, which, depending on the organism, can have very serious consequences, even lethal.
Furthermore, since a chromosomal segment is missing, the deletion cannot "revert" to the wild.
When the order of the genes in the duplicated section is the opposite of the original order, it is a reverse tandem duplication when, however, the duplicated parts are arranged at the end of the chromosome; it is a terminal tandem duplication.
Duplications of certain genetic regions can have very specific phenotypic effects.
Reversals are chromosomal mutations that occur when a chromosome segment is excised and then reintegrated into the chromosome after 180 ° rotation with respect to the original orientation.
There are two types of inversions: a pericentric inversion, which includes the centromere, and a paracentric inversion, which does not include the centromere.
Usually, genetic material is not lost when inversion occurs, although there can be phenotypic consequences when the breakpoints (ends of the inversions) are within a gene or in regions that control its expression. They are chromosomal mutations that provoke a change in the position and, consequently, in the localization in the genome of chromosome segments and of the gene sequences contained in them.
In translocations, there is no increase or loss of genetic material.
There are two types of simple translocation:
1) intra-chromosomal translocation, which implies a change of position of a chromosomal tract within the same chromosome.
2) Inter-chromosomal translocation involves the transfer of a chromosomal tract from a chromosome to a non-homologous chromosome.
If the latter type of translocation involves the transfer of a segment from one chromosome to another, there is a non-reciprocal translocation. If it involves the exchange of segments between chromosomes, it is a reciprocal translocation.
In homozygous organisms for translocations (when both copies of the chromosome in the diploid genome have translocations), the genetic consequence of translocations is a change in the association relationship between genes.
Missense and nonsense mutations
Missense Mutation: insertion of a wrong amino acid into a polypeptide for which a defective protein is produced. They determine a reduction in function such as thermosensitive coli (ts) at 42 ° C
Nonsense mutation: the modified triplet does not code for any amino acid for which the production of truncated proteins occurs. It is a complete mutant phenotype.
The mutations are then divided into missense mutations when they cause a coding change for a different amino acid; the phenotype can change according to the type of amino acid inserted and in nonsense mutations, which determine a coding change for an amino acid to a STOP codon. This leads to the synthesis of truncated proteins.
Neutral mutations are distinguished among missense mutations, although while substituting a base pair. The resulting triplet codes for an amino acid that does not determine any variation in the functionality of the synthesized protein; and silent mutations, if after the replacement of the base pair, the triplet always codes for the same amino acid.
The effects of a mutation can be diminished by a suppressor mutation, which is a mutation in a site other than the original mutation. The suppressor mutation masks or compensates for the effects of a mutation but does not reverse it.
These mutations can take place within the same gene of the first mutation (intragenic suppressors) or occur in a different gene (intergenic suppressors).
Intragenic suppressants work by altering a different nucleotide within the same codon in which the original mutation took place or by altering a nucleotide in a different codon. Intergenic suppressants occur as a result of a second mutation in a different gene. The genes that cause mutation suppression in other genes are called suppressor genes.
Mutations can be spontaneous mutations, which occur naturally, and induced mutations, which occur when an organism is deliberately or casually exposed to physical or chemical agents, called mutagens, which interact with DNA causing mutations. Induced mutations occur much more frequently than spontaneous mutations.
A frameshift mutation is due to the insertion or deletion of one or a few base pairs (never in the number of three or multiples of three) in coding or non-coding regions. The result is scrolling off the reading frame from the changed site onwards. If the inserted base or sequence is identical to the previous one, this is called duplication.
Insertions, duplications, or deletions almost always lead to a complete mutant phenotype because they are responsible for the synthesis of severely altered proteins.
The deletion (or addition) of three pairs of nucleotides leads to the loss of an amino acid (or more) in the protein sequence without overall being seriously modified. The mutation is called silent when it leads to the formation of a new triplet that code for the same amino acid due to the degeneration of the genetic code
, so there is no phenotypic variation. The mutation is said to be neutral when it has no effect on the phenotype and goes unnoticed.
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