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Spontaneous mutation process and its causes. Spontaneous and induced mutations

Induced mutations are those that occur after treatment of cells (organisms) with mutagenic factors. There are physical, chemical and biological mutagenic factors. Most of these factors either directly react with nitrogenous bases in DNA molecules or are included in nucleotide sequences.[...]

Induced mutagenesis makes it possible to significantly increase the frequency of mutations, that is, to increase the hereditary variability of the selected material. The main purpose of its use in fish breeding is to increase genetic variability due to new (induced), including beneficial, mutations.[...]

Mutations are sudden, natural (spontaneous) or artificial (induced) heritable changes in genetic material, leading to changes in certain characteristics of the organism.[...]

Induced mutations arise as a result of disruption of the normal processes of reduplication, recombination, repair or divergence of carriers of genetic information caused by the action of mutagens.[...]

Mutations are changes in the gene apparatus of a cell, which are accompanied by changes in the characteristics controlled by these genes. There are macro- and microdamage to DNA, leading to changes in the properties of the cell. Macro changes, namely: loss of a section of DNA (division), movement of a separate section (translocation) or rotation of a certain section of the molecule by 180° (inversion) are observed relatively rarely in bacteria. Microdamages, or point mutations, i.e., are much more typical for them. qualitative changes in individual genes, for example, replacement of a pair of nitrogenous bases. Mutations can be direct and inverse, or reverse. Direct mutations are mutations in wild-type organisms, for example, loss of the ability to independently synthesize growth factors, i.e., a transition from proto- to auxotrophy. Back mutations represent a return, or reversion, to the wild type. The ability to revert is characteristic of point mutations. As a result of mutations, such important characteristics as the ability to independently synthesize amino acids and vitamins (auxotrophic mutants) and the ability to form enzymes change. These mutations are called biochemical. Mutations leading to changes in sensitivity to antibiotics and other antimicrobial substances are also well known. Based on their origin, mutations are divided into spontaneous and induced. Spontaneous occur spontaneously without human intervention and are random in nature. The frequency of such mutations is very low and ranges from 1 X 10-10 to 1 X 10-10. Induced ones occur when microorganisms are exposed to physical or chemical mutagenic factors. Physical factors that have a mutagenic effect include ultraviolet and ionizing radiation, as well as temperature. A number of compounds are chemical mutagens, and among them the most active are the so-called supermutagens. Under natural conditions and in experiments, changes in the composition of bacterial populations can occur as a result of the action of two factors - mutations and autoselection, which occurs as a result of the adaptation of some mutants to environmental conditions. This process is obviously observed in an environment where the predominant food source is a synthetic substance, for example, a surfactant or caprolactam.[...]

The frequency of induced mutations is determined by comparing cells or populations of organisms treated and untreated with the mutagen. If the frequency of mutation in a population increases 100 times as a result of treatment with a mutagen, then it is believed that only one mutant in the population will be spontaneous, the rest will be induced.[...]

The productivity of induced mutants also varies widely, always, however, remaining at a lower level than the productivity of a typical TMV strain. Some of the induced mutants cause severe forms of the disease, but no correlation has been established between the severity of symptoms and the productivity of the virus. The intensity of reproduction of this mutant during successive passages is quite constant, on the basis of which we can conclude that productivity is a genetically stable trait of the strain. Mutations induced by exposure to chemicals often resulted in the virus being able to cause more severe forms of disease and very rarely (if at all) in increasing productivity. Kassais (personal communication) has isolated strains from a typical TMV culture that cause slowly spreading bright yellow local lesions (usually without subsequent systemic infection) on the leaves of White Barley tobacco plants (photo 73). Such strains are very difficult to maintain in the laboratory, and they never persist in the wild.[...]

The method of chemically induced mutagenesis was used, for example, when working with Kazakhstani carp. These compounds, selectively affecting the DNA of chromosomes, damage it, which can lead to mutations. [...]

Spontaneous mutations are those that occur in organisms under normal (natural) conditions for no apparent reason at first glance, while induced mutations are those that arise as a result of the treatment of cells (organisms) with mutagenic factors. The main difference between spontaneous mutations and induced ones is that a mutation can occur at any period of individual development. As for the random nature of mutations in space, this means that a spontaneous mutation can arbitrarily affect any chromosome or gene.[...]

For a long time it was believed that spontaneous mutations are causeless, but now there are other ideas on this issue, which boil down to the fact that spontaneous mutations are not causeless, that they are the result of natural processes occurring in cells. They arise in the natural radioactive background of the Earth in the form of cosmic radiation, radioactive elements on the surface of the Earth, radionuclides incorporated into the cells of organisms that cause these mutations or as a result of DNA replication errors. Factors in the natural radioactive background of the Earth cause changes in the sequence of bases or damage to bases, similar to what occurs in the case of induced mutations (see below). [...]

Almost all of the chemically induced TMV mutants mentioned in this chapter can be considered defective in the sense that they produce fewer viral particles during reproduction than the parental strain. Yati mutants were taken for study because the structural protein could be isolated from the resulting particles to study amino acid substitutions. About two-thirds of the mutants identified from disease symptoms did not contain any changes in the structural protein. The reason for the reduced productivity of many mutant strains is not known. It is quite possible that a substitution of one or another amino acid occurs in RNA codimerase or some other virus-specific enzyme, which leads to a decrease in the functional activity of the enzyme and, as a result, to a decrease in virus yield. A mutation leading to the synthesis of peffective TLI i-polymerase must be lethal, since in this case it is viral RNA. is not synthesized, and a mutation leading to disruption of the function of the structural protein may not be lethal if the safety of the viral RNA inside the cell is somehow ensured. Several mutants of this kind have been isolated.[...]

Depending on the origin, spontaneous and induced gene and chromosomal mutations are distinguished, which occur in organisms regardless of their level of organization.[...]

The nature and mechanisms of damage repair have been most fully studied in the case of damage induced by UV radiation. Cells react to UV radiation by causing damage to their DNA, the main of which are photochemical changes in pyrimidine bases, transforming into pyrimidine dimers, in particular thymine ones. The latter are formed due to the covalent binding of neighboring thymine bases in the same chain of the molecule through the addition of the carbon of one thymine to the carbon of another thymine. Dimerization of flanking bases in a gene is accompanied by inhibition of transcription and DNA replication. It also leads to mutations. As a result, the cell may die or undergo malignancy.[...]

Further, G. A. Nadson notes that in 1920 he discovered the variability of microbes under the influence of radium and X-rays, which occurs spasmodically. These abrupt changes are hereditary, and to distinguish them from mutations in plants and animals, the author proposed calling them saltations (from the Latin saltus - jump). This term did not survive in the literature, and the phenomenon of sudden hereditary variability of microorganisms is considered mutational variability. Mutants that arose under the influence of treatment of a culture with radiation or chemical reagents belong to the category of induced mutants, in contrast to those that arise naturally under the influence of the environment that is not taken into account. [...]

Much new and interesting information is given, in particular, about the mechanism of photoperiodism and its practical use, about the peculiarities of the action and use of endogenous and synthetic regulators of growth and fruiting, about theoretical issues of genetics and selection and the practical use of heterosis, polyploidy, induced mutations.[...]

The maximum amount of ozone (concentration of about 7 million “) is located at a distance of 20-25 km from. surface of the Earth. The absorption of energy by the ozone layer significantly affects the energy reserve in the atmosphere below and significantly impedes vertical air convection. Thus, the ozone layer is a very active inversion region. The importance of the ozone layer in the life of the Earth is even greater due to the fact that for the ozone layer the maximum absorption of UV radiation (254 nm) is very close to that for DNA (260 nm). Note that DNA is the carrier of genetic information of all living things. Ozone protects DNA from UV-induced biochemical changes that cause mutations. During the evolution of the Earth, living beings were able to emerge from the seas (also absorbing UV radiation) onto land only when the first ozone layer appeared above the earth. Thus, the stratospheric ozone layer above the Earth should be considered as a necessary prerequisite for the existence of all living things on land.[...]

In other words, primary radiation damage dramatically changes (reduces) the genetic resistance of the organism. Consequently, it is inappropriate to estimate the extent of genetic damage to organisms at low irradiation intensities based on linear extrapolation from the region of high doses, since the yield of genetic changes per dose unit has a complex dependence on the irradiation intensity. It has been established that in areas with an increased level of ionizing radiation, formed by the release of radioactive elements on the surface of the earth, many representatives of the biocenosis are affected at relatively low dose rates of chronic irradiation. The increased radioresistance of chronically irradiated populations with additional acute irradiation indicates that radioadaptation is occurring in the populations living in these places. At the same time, the yield of mutations induced by ionizing radiation is influenced by genotypic differences in organisms, which necessarily take place both within populations and, especially, between populations, therefore the hereditary changes that arise in natural conditions will be subject to natural selection. Ultimately, there must be a balance between the mutagenic pressure of ionizing radiation and the selection pressure. The mechanism of the “dose-effect” process in any case is the result of two largely oppositely directed processes: the formation of primary damage and their repair (restoration), while the latter process can be greatly modified by both environmental conditions and the physiological state of the body, and their various thrusts.

Organism.

Induced mutations are heritable changes in the genome that arise as a result of certain mutagenic effects in artificial (experimental) conditions or under adverse environmental influences.

Mutations appear constantly during processes occurring in a living cell. The main processes leading to the occurrence of mutations are DNA replication, DNA repair disorders, transcription and genetic recombination.

Relationship between mutations and DNA replication[ | ]

Many spontaneous chemical changes in nucleotides result in mutations that occur during replication. For example, due to the deamination of cytosine opposite guanine, uracil can be included in the DNA chain (a U-G pair is formed instead of the canonical C-G pair). During DNA replication, opposite uracil, adenine is included in the new chain, a U-A pair is formed, and during the next replication it is replaced by a T-A pair, that is, a transition occurs (a point replacement of a pyrimidine with another pyrimidine or a purine with another purine).

Relationship between mutations and DNA recombination[ | ]

Of the processes associated with recombination, unequal crossing over most often leads to mutations. It usually occurs in cases where there are several duplicated copies of the original gene on the chromosome that have retained a similar nucleotide sequence. As a result of unequal crossing over, duplication occurs in one of the recombinant chromosomes, and deletion occurs in the other.

Relationship between mutations and DNA repair[ | ]

Tautomeric model of mutagenesis[ | ]

It is assumed that one of the reasons for the formation of base substitution mutations is deamination of 5-methylcytosine, which can cause transitions from cytosine to thymine. Due to the deamination of the cytosine opposite it, uracil can be included in the DNA chain (a U-G pair is formed instead of the canonical C-G pair). During DNA replication opposite uracil, adenine is included in the new chain, a U-A pair is formed, and during the next replication it is replaced by a T-A pair, that is, a transition occurs (a point replacement of a pyrimidine with another pyrimidine or a purine with another purine).

Mutation classifications[ | ]

There are several classifications of mutations according to various criteria. Möller proposed dividing mutations according to the nature of the change in the functioning of the gene into hypomorphic(altered alleles act in the same direction as wild-type alleles; only less protein product is synthesized), amorphous(a mutation looks like a complete loss of gene function, e.g. white in Drosophila), antimorphic(the mutant trait changes, for example, the color of the corn grain changes from purple to brown) and neomorphic.

Modern educational literature also uses a more formal classification based on the nature of changes in the structure of individual genes, chromosomes and the genome as a whole. Within this classification, the following types of mutations are distinguished:

genomic;
chromosomal;
genetic.

A point mutation, or single base substitution, is a type of mutation in DNA or RNA that is characterized by the replacement of one nitrogenous base with another. The term also applies to pairwise nucleotide substitutions. The term point mutation also includes insertions and deletions of one or more nucleotides. There are several types of point mutations.

Complex mutations also occur. These are changes in DNA when one section of it is replaced by a section of a different length and a different nucleotide composition.

Point mutations can appear opposite damage to the DNA molecule that can stop DNA synthesis. For example, opposite cyclobutane pyrimidine dimers. Such mutations are called target mutations (from the word “target”). Cyclobutane pyrimidine dimers cause both targeted base substitution mutations and targeted frameshift mutations.

Sometimes point mutations occur in so-called undamaged regions of DNA, often in a small vicinity of photodimers. Such mutations are called untargeted base substitution mutations or untargeted frameshift mutations.

Point mutations do not always form immediately after exposure to a mutagen. Sometimes they appear after dozens of replication cycles. This phenomenon is called delayed mutations. With genomic instability, the main cause of the formation of malignant tumors, the number of untargeted and delayed mutations increases sharply.

There are four possible genetic consequences of point mutations: 1) preservation of the meaning of it due to the degeneracy of the genetic a (synonymous nucleotide substitution), 2) change in the meaning of it, leading to the replacement of an amino acid in the corresponding place of the polypeptide chain (missense mutation), 3) formation of a meaningless it with premature termination (nonsense mutation). In genetic e there are three meaningless ona: amber - UAG, ocher - UAA and opal - UGA (in accordance with this, mutations leading to the formation of meaningless triplets are also named - for example, amber mutation), 4) reverse replacement (stop-on it is semantic).

By influence on gene expression mutations are divided into two categories: mutations such as base pair substitutions And reading frame shift type. The latter are deletions or insertions of nucleotides, the number of which is not a multiple of three, which is associated with the triplicity of the genetic a.

The primary mutation is sometimes called direct mutation, and a mutation that restores the original structure of the gene is reverse mutation, or reversion. A return to the original phenotype in a mutant organism due to restoration of the function of the mutant gene often occurs not due to true reversion, but due to a mutation in another part of the same gene or even another non-allelic gene. In this case, the recurrent mutation is called a suppressor mutation. The genetic mechanisms due to which the mutant phenotype is suppressed are very diverse.

Kidney mutations(sports) - persistent somatic mutations that occur in the cells of plant growth points. Lead to clonal variability. They are preserved during vegetative propagation. Many varieties of cultivated plants are bud mutants.

Consequences of mutations for cells and organisms[ | ]

Mutations that impair cell activity in a multicellular organism often lead to cell destruction (in particular, programmed cell death - apoptosis). If intra- and extracellular protective mechanisms do not recognize the mutation, and the cell undergoes division, then the mutant gene will be passed on to all the descendants of the cell and, most often, leads to the fact that all these cells begin to function differently.

In addition, the frequency of mutations of different genes and different regions within one gene naturally varies. It is also known that higher organisms use “targeted” (that is, occurring in certain sections of DNA) mutations in the mechanisms of immunity [ ] . With their help, a variety of lymphocyte clones is created, among which, as a result, there are always cells capable of giving an immune response to a new disease unknown to the body. Suitable lymphocytes undergo positive

Spontaneous mutagenesis, i.e. the process of mutations occurring in the body in the absence of intentional exposure to mutagens is the final result of the total influence of various factors leading to damage to genetic structures during the life of the body.

Causes of spontaneous mutations can be divided into:
exogenous (natural radiation, extreme temperatures, etc.);
endogenous (chemical compounds-metabolites spontaneously occurring in the body that cause a mutagenic effect; errors in replication, repair, recombination; the action of mutator and antimutator genes; transposition of mobile genetic elements, etc.).

Main source of spontaneous mutations are endogenous factors that lead to damage to genes and chromosomes during normal cellular metabolism. The result of their action is errors in the genetic processes of replication, repair and recombination.

To endogenous factors of spontaneous mutagenesis This also includes the mutagenic activity of special genome elements: mutator genes and endogenous metabolites.

Occurrence of mutations depends on the characteristics of the primary DNA structure at the site of rearrangement, and a number of researchers believe that all DNA sequences in a state of bend have increased endogenous mutagenicity. It is this conformational structure of DNA that is characteristic of: promoter parts of genes, places of origin of replication, places of contact of chromosomes with the nuclear matrix, i.e. those sections of DNA that are affected by topoisomerases involved in the processes of replication, transcription, recombination, including non-homologous (illegal). The result of the latter can be not only intragene mutations, but also large structural rearrangements of chromosomes (translocations, inversions, etc.).

Gene mutations. The concept of gene diseases.

Gene mutations– change in the structure of one gene. This is a change in the nucleotide sequence: deletion, insertion, substitution, etc. For example, replacing A with T. Causes: violations during DNA doubling (replication). Examples: sickle cell anemia, phenylketonuria.

Gene diseases is a large group of diseases that occur as a result of DNA damage at the gene level. The term is used in relation to monogenic diseases, in contrast to the broader group - Hereditary diseases

Causes of gene pathologies

Most gene pathologies are caused by mutations in structural genes that perform their function through the synthesis of polypeptides - proteins. Any gene mutation leads to a change in the structure or quantity of the protein.

The onset of any gene disease is associated with the primary effect of the mutant allele.

The basic scheme of gene diseases includes a number of links:

mutant allele → altered primary product → chain of biochemical processes in the cell → organs → organism

As a result of a gene mutation at the molecular level, the following options are possible:

synthesis abnormal protein;

production excess amount of gene product;

absence development of a primary product;

production reduced amount of normal primary product.

Without ending at the molecular level in the primary links, the pathogenesis of gene diseases continues at the cellular level. In various diseases, the point of application of the action of the mutant gene can be either individual cell structures - lysosomes, membranes, mitochondria, peroxisomes, or human organs.

Clinical manifestations of gene diseases, the severity and speed of their development depend on the characteristics of the body’s genotype, the patient’s age, environmental conditions (nutrition, cooling, stress, overwork) and other factors.

A feature of genetic (as in general all hereditary) diseases is their heterogeneity. This means that the same phenotypic manifestation of a disease can be caused by mutations in different genes or by different mutations within the same gene. The heterogeneity of hereditary diseases was first identified by S. N. Davidenkov in 1934.

The overall frequency of gene diseases in the population is 1-2%. Conventionally, the frequency of gene diseases is considered high if it occurs with a frequency of 1 case per 10,000 newborns, average - 1 per 10,000 - 40,000, and then low.

Monogenic forms of gene diseases are inherited in accordance with the laws of G. Mendel. According to the type of inheritance, they are divided into autosomal dominant, autosomal recessive and linked to the X or Y chromosomes.

Spontaneous - These are mutations that occur spontaneously, without intervention from the experimenter.

Induced – these are those mutations that are caused artificially, using various mutagenesis factors.

The process of mutation formation is called mutagenesis, and the factors causing mutations are mutagens.

Mutagenic factors are divided into:

physical,
chemical,
biological.

Causes of spontaneous mutations not entirely clear. Previously, it was believed that they were caused by a natural background of ionizing radiation. However, it turned out that this was not the case. For example, in Drosophila, the natural background radiation causes no more than 0.1% of spontaneous mutations. With age, the effects of exposure to natural background radiation can accumulate, and in humans, 10 to 25% of spontaneous mutations are associated with this.

The second reason for spontaneous mutations is accidental damage to chromosomes and genes during cell division and DNA replication due to random errors in the functioning of molecular mechanisms.

The third reason for spontaneous mutations is movement of transposable elements throughout the genome, which can invade any gene and cause a mutation in it.

American geneticist M. Green showed that about 80% of mutations that were discovered as spontaneous arose as a result of the movement of mobile elements.

Induced mutations first discovered in 1925 by G.A. Nadson and G.S. Filippov in the USSR. They irradiated cultures of the mold fungi Mucor genevensis with X-rays and obtained a splitting of the culture “into two forms or races, differing not only from each other, but also from the original (normal) form.” The mutants turned out to be stable, since after eight successive subcultures they retained their acquired properties.

In 1927, G. Möller reported on the effect of X-rays on the mutation process in Drosophila and proposed a quantitative method for accounting for recessive lethal mutations in the X chromosome (ClB), which became a classic method.

In 1946, Möller was awarded the Nobel Prize for the discovery of radiation mutagenesis.

It has now been established that almost all types of radiation (including ionizing radiation of all types - a, b, g; UV rays, infrared rays) cause mutations. They are called physical mutagens.

The main mechanisms of their action:

disruption of the structure of genes and chromosomes due to a direct effect on DNA and protein molecules;
the formation of free radicals that interact chemically with DNA;
ruptures of the spindle filaments;
formation of dimers (thymine).

TO chemical mutagens include:

natural organic and inorganic substances;
products of industrial processing of natural compounds - coal, oil;
synthetic substances not previously found in nature (pesticides, insecticides, etc.);
some metabolites of the human and animal body.

Chemical mutagens cause predominantly gene mutations and act during DNA replication.

Their mechanisms of action:

modification of base structure (hydroxylation, deamination, alkylation);
replacement of nitrogenous bases with their analogues;
inhibition of the synthesis of nucleic acid precursors.

TO biological mutagens include:

viruses (rubella, measles, etc.);
non-viral infectious agents (bacteria, rickettsia, protozoa, helminths);
mobile genetic elements.

Their mechanisms of action:

Induced mutagenesis, starting from the late 20s of the 20th century, have been used for the selection of new strains, breeds and varieties. The greatest success has been achieved in the selection of strains of bacteria and fungi that produce antibiotics and other biologically active substances.

Thus, it was possible to increase the activity of antibiotic producers by 10-20 times, which made it possible to significantly increase the production of corresponding antibiotics and sharply reduce their cost.

The use of dwarf mutations in wheat made it possible in the 60-70s to dramatically increase the yield of grain crops, which was called the “green revolution”. Dwarf wheat varieties have a shortened thick stem that is resistant to lodging; it can withstand increased load from a larger ear. The use of these varieties has made it possible to significantly increase yields (several times in some countries).

Gene (point) mutations associated with relatively minor changes in nucleotide sequences. Gene mutations are divided into changes in structural genes and changes in regulatory genes.

Mutational variability is the result of mutations.

Mutation(from Latin “mutazio” - change, change) – a hereditary change in the genotype (this is a change in the hereditary material, leading to the appearance of new characteristics of the organism that can be transmitted to the next generation. The term “mutation” was introduced into science in 1901 by the Dutch geneticist G. de Frieze, who described spontaneous mutations in plants. Mutations are persistent changes affecting entire chromosomes, their parts, and individual genes. Most often, mutations are small, barely noticeable deviations from the norm.

Darwin called hereditary variation indeterminate (individual), emphasizing its random and relatively rare nature.

Mutations are a source of genetic diversity, constituting a reserve of hereditary variability.

Classification of mutations

1. By nature of manifestation:

there are manifestations dominant and recessive. Mutations often reduce viability or fertility. Mutations that sharply reduce viability, partially or completely stop development, are called semi-lethal and incompatible with life - lethal.

2. By place of origin:

A mutation that occurs in germ cells does not affect the characteristics of a given organism, but appears only in the next generation. Such mutations are called generative. If genes change in somatic cells, such mutations appear in this organism and are not transmitted to offspring during sexual reproduction. But with asexual reproduction, if an organism develops from a cell or group of cells that has a changed - mutated - gene, mutations can be transmitted to offspring. Such mutations are called somatic.

3. By level of occurrence:

Chromosomal mutations– change in the structure of chromosomes: loss of a section, doubling of a section, rotation of a section by 180 degrees, transfer of a section to another (non-homologous) chromosome, etc. The reasons are violations during crossing over. Example: Cry Cat Syndrome.

Genomic mutations– change in the number of chromosomes. The causes are disturbances in the divergence of chromosomes. Depending on the nature of the change in the number of chromosomes, there are:

Polyploidy– multiple changes (several times, for example, 12 → 24). It does not occur in animals; in plants it leads to an increase in size.
Aneuploidy– changes on one or two chromosomes. For example, one extra twenty-first chromosome leads to Down syndrome (the total number of chromosomes is 47).

Depending on the nature of the change in the number of chromosomes, there are:

Spontaneous mutations- arise under normal living conditions, depend on external and internal factors, arise in somatic and generative cells .

Induced mutations - This is the artificial production of mutations using mutagens of various natures. The ability of ionizing radiation to cause mutations was first discovered by G.A. Nadson and G.S. Fillipov. In 1927, American scientist Joseph Muller proved that the frequency of mutations increases with increasing exposure dose. Scientists believe that the fact that mutations are inherited is of some concern because it may increase the risk of developing cancer. A mutant gene protects Asians from alcoholism. Why is the percentage of alcoholics in Asian countries much lower than in countries where the majority of the population is the so-called white population.

Environmental factors that cause mutations are called – mutagens.

There are:

Physical mutagens

- ionizing and ultraviolet radiation;

Excessively high or low temperature;

Chemical mutagens

Nitrates, nitrites, pesticides, nicotine, methanol, benzopyrene.

Some food additives, such as aromatic hydrocarbons;

Petroleum products;

Organic solvents;

Medicines, mercury preparations, immunosuppressants.

Biological mutagens

Some viruses (measles, rubella, influenza virus)

Metabolic products (lipid oxidation products );

Mutation properties:

mutations are hereditary, i.e. are passed on from generation to generation.
mutations occur suddenly (spontaneously), undirectedly.
mutations are not directed - any locus can mutate, causing changes in both minor and vital signs in any direction.
the same mutations can occur repeatedly.
mutations are individual, i.e. occur in individuals.
mutations can be beneficial, harmful, neutral; dominant and recessive.

The meaning of mutations

They serve as a reserve of hereditary variability (preserved in the population in a hidden-recessive form) and are material for evolution.

The cause of many hereditary diseases and deformities.

Induced mutations “supply” material for artificial selection and selection.

MUTAGENESIS- reaction processes in the genetic apparatus of a biological object, during which changes occur in the structure of genes that are inherited. Such changes can affect individual nucleotides or groups of them, accompanied in some cases by changes in chromosome morphology. Changes in just one nucleotide that is part of the triplet lead to the formation of another amino acid that is part of the protein and can lead to a change in the corresponding feature.

Mutagenesis can be divided into spontaneous, when mutations occur under "normal" growth conditions, and induced due to the use of physical or chemical mutagens.

Spontaneous mutagenesis depends on external and internal factors (biological, chemical, physical). Spontaneous mutations occur in humans in somatic and generative tissues. The method for determining spontaneous mutations is based on the fact that children develop a dominant trait, although their parents do not have it. During spontaneous mutagenesis, all types of hereditary changes that are observed during induced mutagenesis can occur: replacement of adenine-thymine pairs or, more often, guanine-cytosine, erroneous pairing of two purines or two pyrimidines, deletions, inclusions and other changes. Each biological object is characterized by a certain background of spontaneous mutations, which affect certain genetic characteristics with different frequencies.

Induced mutagenesis is the artificial production of mutations using mutagens of various natures. The ability of ionizing radiation to cause mutations was first discovered by G.A. Nadson and G.S. Fillipov. Then, through extensive research, the radiobiological dependence of the mutations was established. In 1927, American scientist Joseph Muller proved that the frequency of mutations increases with increasing exposure dose. At the end of the forties, the existence of powerful chemical mutagens was discovered that caused serious damage to human DNA for a number of viruses. One example of the effect of mutagens on humans is endomitosis - chromosome duplication followed by centromere division, but without chromosome divergence.