Spontaneous mutation: classification, causes, examples
What mutations are called spontaneous? If you translate the term into an accessible language, then these are natural errors that arise during the interaction of genetic material with the internal and / or external environment. Such mutations are usually random. They are observed in the genital and in other cells of the body.
Exogenous causes of mutations
Spontaneous mutation can occur under the influence of chemicals, radiation, under the influence of high or low temperatures, rarefied air or high pressure.
Every year, a person absorbs aboutone tenth of a series of ionizing radiation, which is a natural radiation background. This number includes the gamma radiation of the Earth's core, the solar wind, the radioactivity of elements lying in the thickness of the earth's crust and dissolved in the atmosphere. The dose obtained depends also on where the person is located. A quarter of all spontaneous mutations happen precisely because of this factor.
Ultraviolet radiation, contrary to conventional wisdomopinion, plays an insignificant role in the occurrence of DNA breakdowns, since it can not penetrate deep enough inside the human body. But the skin often suffers from excessive insolation (melanoma and other cancers). However, unicellular organisms and viruses mutate under the influence of sunlight.
Too high or low temperatures can also cause changes in the genetic material.
Endogenous causes of mutations
The main reasons whythere is a spontaneous mutation, there are endogenous factors. These include by-products of metabolism, errors in the process of replication, repair or recombination, and others.
- Replication failures:
- spontaneous transitions and inversions of nitrogenous bases;
- incorrect incorporation of nucleotides due to DNA polymerase errors;
- chemical replacement of nucleotides, for example, guanine-cytosine by adenine-guanine.
- Recovery errors:
- mutations in genes responsible for repairing individual parts of the DNA chain after their rupture under the influence of external factors.
- Problems with recombination:
- failures in the processes of crossing-over under meiosis or mitosis lead to the deposition and completion of the bases.
These are the main factors causing spontaneousmutations. The causes of malfunctions can be the activation of mutator genes, as well as the transformation of safe chemical compounds into more active metabolites that affect the nucleus of the cell. In addition, there are still structural factors. These include repeating the sequence of nucleotides near the site of the chain rearrangement, the presence of additional DNA sections similar in structure to the genome, as well as the mobile elements of the genome.
Pathogenesis of mutation
Spontaneous mutation arises fromeffects of all of the above factors, acting together or separately in a certain period of cell life. There is such a phenomenon as a sliding disruption of the mating of the daughter and maternal DNA strands. As a result, loops of peptides are often formed, which could not be adequately incorporated into the sequence. After removing excess DNA from the daughter chain, loops can both be resected (deletions) and embedded (duplications, insertions). The changes that appear are fixed in the next cycles of cell division.
The rate and number of mutations that arise depend on the primary DNA structure. Some scientists believe that absolutely all DNA sequences have mutagenicity, if they form bends.
The most common spontaneous mutations
What is most often manifested in the geneticmaterial spontaneous mutations? Examples of such states are the loss of nitrogenous bases and the removal of amino acids. Cytosine residues are particularly susceptible to them.
It is proved that today more thanhalf of vertebrates there is a mutation of cytosine residues. After deamination, methylcytosine changes to thymine. Subsequent copying of this section repeats the error or deletes it or doubles and mutates to a new fragment.
Another reason for frequent spontaneous mutationsconsider a large number of pseudogenes. Because of this, in the process of meiosis, unequal homologous recombinations can form. The consequence of this is a rearrangement in the genes, rotations and doubling of the individual nucleotide sequences.
Polymerase model of mutagenesis
According to this model, spontaneous mutationsarise as a result of random errors of molecules synthesizing DNA. For the first time such a model was presented by Bresler. He suggested that mutations are due to the fact that polymerases in some cases incorporate non-complementary nucleotides into the sequence.
Years later, after lengthy inspections andexperiments, this point of view was approved and accepted in the scientific world. Some regularities have been derived, which allow scientists to control and direct mutations by exposing certain regions of DNA to ultraviolet light. For example, it was found that adrenine is most often built in front of a damaged triplet.
Tautomeric model of mutagenesis
Another theory that explains spontaneous andartificial mutations, was proposed by Watson and Crick (pioneers of the structure of DNA). They suggested that the basis of mutagenesis is the ability of some DNA bases to transform into tautomeric forms that alter the way in which the bases are joined.
After publication, the hypothesis is activewas developed. New forms of nucleotides were discovered after irradiation with their ultraviolet. This gave scientists new opportunities for research. Modern science is still debating the role of tautomeric forms in spontaneous mutagenesis and its effect on the number of mutations detected.
Spontaneous mutation is possible in case of violationDNA polymerase recognition of nucleic acids. Poltayev and co-authors clarified the mechanism that ensures compliance with the principle of complementarity in the synthesis of daughter DNA molecules. This model allowed to study the regularities of the appearance of spontaneous mutagenesis. Scientists explained their discovery by the fact that the main reason for changing the structure of DNA is the synthesis of non-canonical pairs of nucleotides.
They suggested that the swelling of the grounds occursdue to deamination of DNA. This leads to a change in cytosine to thymine or uracil. Due to such mutations, pairs of incompatible nucleotides are formed. Therefore, during the next replication, there is a transition (point substitution of the nucleotide bases).
Classification of mutations: spontaneous
There are different classifications of mutations depending on which criterion is the basis of them. There is a separation according to the nature of the change in the function of the gene:
- hypomorphic (mutated alleles synthesize fewer proteins, but they are similar to the original ones);
- amorphous (the gene has completely lost its functions);
- antimorphous (mutated gene completely changes the sign that it represents);
- neomorphic (new signs appear).
But the classification is more common, which divides all mutations in proportion to the mutable structure. Allocate:
1. Genomic mutations. These include polyploidy, that is, the formation of a genome with a triple or more chromosome set, and aneuploidy - the number of chromosomes in the genome is not a multiple of the haploid.
2. Chromosome mutations. There are significant changes in some parts of the chromosomes. Distinguish between the loss of information (deletion), its duplication (duplication), the change in the direction of nucleotide sequences (inversion), and the transfer of chromosome regions to another place (translocation).
3. Gene mutation. The most common mutation. In the DNA chain, several random nitrogenous bases are replaced.
Consequences of mutations
Spontaneous mutations - the causes oftumors, accumulation diseases, dysfunctions of organs and tissues of humans and animals. If the mutated cell is located in a large multicellular organism, then with a high degree of probability it will be destroyed by triggering apoptosis (programmed cell death). The body controls the process of preserving genetic material and by means of the immune system gets rid of all possible damaged cells.
In one case, out of hundreds of thousands of T-lymphocytestime to recognize the affected structure, and it gives a clone of cells that also contain a mutated gene. The conglomerate of cells already has other functions, produces toxic substances and negatively affects the general state of the body.
If the mutation occurred not in the somatic, but in thethe sex cell, then the changes will be observed in the offspring. They are manifested by congenital pathologies of organs, deformities, metabolic disorders and accumulation diseases.
Spontaneous mutations: significance
In some cases, previously seemingly uselessmutations can be useful for adaptation in new conditions of life. This represents a mutation as a measure of natural selection. Animals, birds and insects have a camouflage coloration, corresponding to the area of residence, to protect themselves from predators. But if their habitat varies, then with the help of mutations nature tries to protect the species from extinction. In the new conditions, the fittest survive and transfer this ability to others.
Mutation can occur in inactive areasgenome, and then no visible changes in the phenotype are observed. It is possible to detect a "breakdown" only with the help of specific studies. This is necessary for studying the origin of related animal species and compiling their genetic maps.
The problem of spontaneity of mutations
In the forties of the last century there wasthe theory that mutations are caused solely by the influence of external factors and help them to adapt. In order to test this theory, a special test and a method of repetition was developed.
The procedure was that a smallthe number of bacteria of one species was sown on test tubes and after several inoculations antibiotics were added to them. Some of the microorganisms survived, and they were transferred to a new environment. Comparison of bacteria from different test tubes showed that resistance appeared spontaneously, both before and after antibiotic contact.
The method of repetition was thatthe fleecy tissue was transferred to microorganisms, and then transferred to several pure media simultaneously. The new colonies were cultured and treated with an antibiotic. As a result, bacteria in different test tubes survived on the same parts of the medium.