Mutation theory, or, more correctly, mutation theory, forms one of the foundations of genetics. It originated shortly after the rediscovery of G. Mendel's laws in the works of G. de Vries (1901-1903). Even earlier, the Russian botanist S.I. came to the idea of abrupt changes in hereditary properties. Korzhinsky (1899) in his work “Heterogenesis and Evolution”. So it's fair to talk about mutation theory Korzhinsky-de Vries. The mutation theory is set out in much more detail in the works of G. de Vries, who dedicated most life study of the problem (nutational variability of plants.
At first, mutation theory focused entirely on the phenotypic manifestation of hereditary changes, with virtually no attention to the mechanism of their occurrence. In accordance with the definition of G. de Vries, a mutation is a phenomenon of spasmodic, intermittent changes in a hereditary characteristic. Defining the concept of “mutation” causes difficulties. Until now, despite numerous attempts, there is no brief definition mutation, better than that given by G. de Vries, although it is not free from shortcomings.
The main provisions of the mutation theory of G. de Vries boil down to the following:
- 1. Mutations arise suddenly as discrete changes in characteristics.
- 2. New forms are stable.
- 3. Unlike non-hereditary changes, mutations do not form continuous rows, do not cluster around any middle type. They represent qualitative changes.
- 4. Mutations manifest themselves in different ways and can be either beneficial or harmful.
- 5. The probability of detecting mutations depends on the number of individuals studied.
- 6. Similar mutations can occur repeatedly.
Like many geneticists early period, G. de Vries mistakenly believed that mutations can immediately give rise to new species, that is, bypassing natural selection. G. de Vries created his mutation theory based on experiments with various types Oenothera. The paradox was that in reality he did not receive mutations, but observed the result of combinative variability, since the forms with which he worked turned out to be complex heterozygotes for translocations.
Rigorous proof of the occurrence of mutations belongs to V. Johannsen, who studied inheritance in clean(self-pollinating) lines beans and barley. The result he obtained concerned quantitative characteristic - seed mass. The dimensional values of such features necessarily vary, being distributed around a certain average size. Mutational changes in such characteristics were discovered by V. Johannsen (1908-1913).
One way or another, the hypothesis about the possibility of abrupt hereditary changes - mutations, which many geneticists discussed at the turn of the century, has received experimental confirmation.
The largest generalization of work on the study of variability at the beginning of the 20th century. became law homologous series in hereditary variability N.I. Vavilova (1920). According to this law, similar species and genera of organisms are characterized by similar series of hereditary variability. The closer the taxonomically considered organisms are, the greater the similarity is observed in the series (spectrum) of their variability. The fairness of this law N.I. Vavilov illustrated it using a huge amount of botanical material.
Law N.I. Vavilova finds confirmation in the study of the variability of animals and microorganisms not only at the level of whole organisms, but also their individual structures. Suffice it to recall the evolutionary parallelism principle in tissue development, formulated by A.A. Zavarzin.
Law N.I. Vavilova has great importance for breeding practice, since it predicts the search certain forms cultivated plants and animals. Knowing the nature of variability of one or several closely related species, one can purposefully search for forms not yet known in of a given organism, but already discovered in its taxonomic relatives. With his law of homological series N.I. Vavilov actually laid the foundations of a new direction - comparative genetics.
Mutation theory or mutation theory- a branch of genetics that lays the foundations of genetic variability and evolution.
Emergence
Mutation theory is one of the foundations of genetics. It originated shortly after Mendel's laws at the beginning of the 20th century. It can be considered that it almost simultaneously arose in the minds of the Dutchman Hugo de Vries (1903) and the domestic botanist S.I. Korzhinsky (1899). However, the priority in primacy and in greater coincidence of the original provisions belongs to the Russian scientist. Recognition of the main evolutionary significance behind discrete variability and denial of the role natural selection in the theories of Korzhinsky and De Vries was associated with the unsolvability at that time of the contradiction in evolutionary teaching C. Darwin between important role minor deviations and their “absorption” during crossings (see Jenkin’s nightmare and the History of evolutionary teaching#The Crisis of Darwinism).
Basic provisions
The main provisions of the Korzhinsky-De Vries mutation theory can be reduced to the following points:
- Mutations are sudden, like discrete changes in traits.
- New forms are stable.
- Unlike non-hereditary changes, mutations do not form continuous series and are not grouped around any average type. They represent qualitative leaps in change.
- Mutations manifest themselves in different ways and can be either beneficial or harmful.
- The probability of detecting mutations depends on the number of individuals studied.
- Similar mutations can occur repeatedly.
Research by H. De Vries was carried out on various species of Oslinnik ( Oenothera), which during the experiment did not produce mutations, but showed complex combinative variability, since these forms were complex heterozygotes for translocations.
Strict proof of the occurrence of mutations belongs to V. Johansen based on experiments on self-pollinating lines of beans and barley - seed masses were studied, mutational changes in this trait were discovered by V. Johansen (1908-1913). It is noteworthy that, even having a mutational nature, the mass of seeds was distributed relative to certain average values, thereby casting doubt on the third point of the mutation theory.
The term "mutation" (from lat. mutatio- change) for a long time used in biology to refer to any discontinuous changes. For example, the German paleontologist W. Waagen called the transition from one fossil form to another a mutation. Mutation was also called the appearance of rare characters, in particular, melanistic forms among butterflies.
Modern ideas about mutations developed by the beginning of the 20th century. For example, Russian botanist Sergei Ivanovich Korzhinsky developed in 1899 evolutionary theory heterogenesis, based on ideas about the leader evolutionary role discrete (intermittent) changes.
However, the most famous was mutation theory of the Dutch botanist Hugo (Hugo) De Vries(1901), which introduced modern, genetic concept mutations to indicate rare variants of a trait in the offspring of parents who did not have the trait.
De Vries developed a mutation theory based on observations of a widespread weed - biennial primrose, or evening primrose ( Oenothera biennis). This plant has several forms: large-flowered and small-flowered, dwarf and giant. De Vries collected seeds from a plant of a certain shape, sowed them and received 1...2% of plants of a different shape in the offspring. It was later established that the appearance of rare variants of the trait in evening primrose is not a mutation; This effect is due to the peculiarities of the organization of the chromosomal apparatus of this plant. In addition, rare variants of traits may be due to rare combinations of alleles (for example, white plumage in budgies determined by a rare combination aabb).
Basic provisions of De Vries mutation theory remain true to this day:
- Mutations occur suddenly, without any transitions.
- Success in detecting mutations depends on the number of individuals analyzed.
- Mutant forms are quite stable.
- Mutations are characterized by discreteness (discontinuity); These are qualitative changes that do not form continuous series and are not grouped around an average type (fashion).
- The same mutations can occur repeatedly.
- Mutations occur in different directions, they can be harmful and beneficial
Currently accepted following definition mutations:
Mutations are qualitative changes in genetic material that lead to changes in certain characteristics of the organism.
Mutation is random phenomenon , i.e. It is impossible to predict where, when and what change will occur. One can only estimate the probability of mutation in populations by knowing the actual frequencies of certain mutations.
Gene mutations are expressed in changes in the structure of individual sections of DNA. According to their consequences, gene mutations are divided into two groups:
- mutations without frameshift,
- frameshift mutations.
Mutations without frameshift reading occur as a result of replacement of nucleotide pairs, while the total length of DNA does not change. As a result, amino acid substitution is possible, but due to degeneracy genetic code It is also possible to preserve the protein structure.
Frameshift mutations readings (frameshifts) occur as a result of the insertion or loss of nucleotide pairs, thereby changing the overall length of DNA. As a result, a complete change in the structure of the protein occurs.
However, if after insertion of a nucleotide pair there is a loss of a nucleotide pair (or vice versa), then amino acid composition proteins can be restored. Then the two mutations at least partially compensate each other. This phenomenon is called intragenic suppression.
An organism in which a mutation is found in all cells is called mutant. This occurs if the organism develops from a mutant cell (gametes, zygotes, spores). In some cases, the mutation is not found in all somatic cells body; such an organism is called genetic mosaic. This happens if mutations appear during ontogenesis - individual development. And finally, mutations can only occur in generative cells(in gametes, spores and in germinal cells - precursor cells of spores and gametes). IN the latter case organism is not a mutant, but some of its descendants will be mutants.
The term " mutation" was first proposed G. De Vries in his classic work "Mutation Theory" (1901-1903).
Basic provisions of mutation theory:
1. Mutation occurs spasmodically , i.e. suddenly, without transition.
2. The new forms formed are inherited, i.e. are persistent .
3. Mutations not directed (i.e. can be beneficial, harmful or neutral).
4. Mutations – rare events.
5. The same mutations can occur again .
Mutation – This is an abrupt, persistent, non-directional change in genetic material.
3. The law of homological series in hereditary variability
After the De Vries mutation theory, the next serious study of mutations was the work of N.I. Vavilov on hereditary variability in plants.
Studying morphology various plants, N.I. Vavilov V 1920. came to the conclusion that, despite the pronounced diversity(polymorphism) of many species, you can see and clear patternsin their variability. If we take the family of cereals as an example, it turns out that the same deviations in characteristics are inherent in all species (dwarfism in wheat, rye, corn; spikelets are awnless, non-shattering, etc.).
Law of N. I. Vavilov reads: “Species and genera, genetically close, are characterized similar series of hereditary variability with such accuracy that, knowing a number of forms within one species, one can anticipate finding parallel forms in other species and genera."
His law N.I. Vavilov expressed it with the formula:
Where G 1 , G 2 , G 3 , – species, and a , b , c – various varying signs.
This law is important primarily for breeding practice , because it gives direction to the search for unknown forms in plants (in general, in organisms) of a given species, if they are already known in other species.
Under the leadership of N.I. Vavilov, numerous expeditions were organized around the world. From different countries Hundreds of thousands of samples of seeds of cultivated and wild plants were brought for the collection of the All-Union Institute of Plant Growing (VIR). It is still the most important source of starting materials for the creation of new varieties.
Theoretical value this law Now does not seem as large as it was thought in 1920. In the law N.I. Vavilov contained the foresight that closely related species should have homologous , i.e. genes similar in structure. At that time, when nothing was known about the structure of the gene, this was, of course, a step forward in the knowledge of living things (N.I. Vavilov’s law was compared in importance to periodic law D.I. Mendeleev). Molecular genetics and gene sequencing confirmed the correctness of N.I.’s guess. Vavilov, his idea has become an obvious fact and is no longer the key to understanding the living.
4. Classification of mutations
The most complete classification of mutations was proposed in 1989. S. G. Inge-Vechtomov. We present it with some changes and additions.
I. According to the nature of the genotype change:
Gene mutations, or point mutations.
Chromosomal rearrangements.
Genomic mutations.
II. According to the nature of the phenotypic change:
Morphological.
Physiological.
Biochemical.
Behavioral
III. By manifestation in heterozygote:
Dominant.
Recessive.
IV. According to the conditions of occurrence:
Spontaneous.
Induced.
V. By localization in the cell:
1. Nuclear.
2. Cytoplasmic (mutations of extranuclear genes).
VI. Possible inheritance (by localization in the body):
1. Generative (arisen in germ cells).
2. Somatic (arising in somatic cells).
VII. By adaptive value:
Useful.
Neutral.
Harmful (lethal and semi-lethal).
8. Straight And reverse.
Now let's explain some types of mutations.
Mutational variability
Mutations are hereditary changes in genotypic material. They are characterized as rare, random, undirected events. Most mutations lead to various disorders of normal development, some of them are lethal, but at the same time, many mutations are the starting material for natural selection and biological evolution.
The frequency of mutations increases under the influence of certain factors - mutagens, which can change the material of heredity. Depending on their nature, mutagens are divided into physical ( ionizing radiation, UV radiation, etc.), chemical ( big number various compounds), biological (viruses, mobile genetic elements, some enzymes). The division of mutagens into endogenous and exogenous is very arbitrary. Thus, ionizing radiation, in addition to primary DNA damage, forms unstable ions (free radicals) in the cell that can secondary cause damage to genetic material. Many physical and chemical mutagens are also carcinogens, i.e. induce malignant cell growth.
The mutation rate follows the Poisson distribution, which is used in biometrics when the probability of an individual event is very small and the sample in which the event can occur is large. The probability of mutations in a single gene is quite low, but the number of genes in the body is large, and in the gene pool of the population it is huge.
In the literature you can find various mutations: by manifestation in a heterozygote (dominant, recessive), by ionizing factor (spontaneous, induced), by localization (generative, somatic), by phenotypic manifestation (biochemical, morphological, behavioral, lethal, etc.).
Mutations are classified according to the nature of the genome change. Based on this indicator, 4 groups of mutations are distinguished.
Genetic - changes in the nucleotide composition of the DNA of individual genes.
Chromosomal (aberrations) – changes in the structure of chromosomes.
Genomic – changes in the number of chromosomes.
Cytoplasmic – changes in non-nuclear genes.
Mutation theory
Mutation theory, or more correctly, the theory of mutations, is one of the foundations of genetics. It originated shortly after the first discovery of G. Mendel's laws in the works of G. De Vries (1901-1903). Even earlier, the Russian botanist S.I. came to the idea of abrupt changes in hereditary properties. Korzhinsky (1899) in his work “Heterogenesis and Evolution”. It is fair to talk about the mutation theory of Korzhenevsky - De Vries, who devoted most of his life to studying the problem of mutational variability in plants.
At first, mutation theory focused entirely on the phenotypic manifestation of hereditary changes, with virtually no attention to the mechanism of their manifestation. In accordance with the definition of G. De Vries, a mutation is the phenomenon of spasmodic, intermittent changes in a hereditary trait. Until now, despite numerous attempts, there is no concise definition of mutation better than that given by G. De Vries, although it is not free from shortcomings.
The main provisions of the mutation theory of G. De Vries boil down to the following:
1. Mutations occur suddenly as discrete changes in characteristics.
2. New forms are stable.
3. Unlike non-hereditary changes, mutations do not form continuous series and are not grouped around any average type. They represent qualitative changes.
4. Mutations manifest themselves in different ways and can be either beneficial or harmful.
5. The probability of detecting a mutation depends on the number of individuals examined.
6. Similar mutations can occur repeatedly.
Like many geneticists of the early period, G. De Vries mistakenly believed that mutations could immediately give rise to new species, i.e. bypassing natural selection.
G. De Vries created his mutation theory based on experiments with various species of Oenothera. In fact, he did not receive mutations, but observed the result of combinative variability, since the forms with which he worked turned out to be complex heterozygotes for translocation.
The honor of rigorous proof of the occurrence of mutations belongs to V. Johansen, who studied inheritance in pure (self-pollinating) lines of beans and barley. The result he obtained concerned a quantitative characteristic—seed mass. The dimensional values of such characteristics necessarily vary, being distributed around a certain average value. Mutational changes in such characteristics were discovered by V. Johannsen (1908-1913). This fact itself already poses one of the provisions of G. De Vries (point 3, mutation theory of G. De Vries).
One way or another, the hypothesis about the possibility of abrupt hereditary changes - mutations, which was discussed by many geneticists at the turn of the century (including W. Bateson), received experimental confirmation.
The largest generalization of work on the study of variability at the beginning of the 20th century. became the law of homological series in hereditary variability N.I. Vavilov, which he formulated in 1920 in a report at the III All-Russian Selection Congress in Saratov. According to this law, similar species and genus of organisms are characterized by similar series of hereditary variability. The closer the taxonomically considered organisms are, the greater the similarity is observed in the series (spectrum) of their variability. The fairness of this law N.I. Vavilov illustrated it using a huge amount of botanical material.
Law N.I. Vavilova finds confirmation in the study of the variability of animals and microorganisms, not only at the level of whole organisms, but also of individual structures. It is obvious that the law of N.I. Vavilova stands in a row scientific achievements, which led to modern ideas about the universality of many biological structures and functions.
Law N.I. Vavilova is of great importance for breeding practice, since it predicts the search for certain forms of cultivated plants and animals. Knowing the nature of variability of one or several closely related species, one can purposefully search for forms that are not yet known in a given organism, but have already been discovered in its taxonomic relatives.
Classification of mutations
The difficulties of defining the concept of “mutation” are best illustrated by the classification of its types.
There are several principles for this classification.
A. By the nature of the genome change:
1. Genomic mutations – changes in the number of chromosomes.
2. Chromosomal mutations, or chromosomal rearrangements, are changes in the structure of chromosomes.
3. Gene mutations - changes in genes.
B. By manifestation in a heterozygote:
1. Dominant mutations.
2. Recessive mutations.
B. By deviation from the norm or the so-called wild type:
1. Direct mutations.
2. Reversions. Sometimes they talk about reverse mutations, but it is obvious that they represent only a part of the reversions, since in reality so-called suppressor mutations are widespread.
D. Depending on the reasons, causing mutations:
1. Spontaneous, occurring for no apparent reason, i.e. without any inducing influences from the experimenter.
2. Induced mutations.
Only these four methods of classifying changes in genetic material are quite strict and have universal meaning. Each approach in these classification methods reflects some essential aspect the occurrence or manifestation of mutations in any organisms: eukaryotes, prokaryotes and their viruses.
There are also more specific approaches to classifying mutations:
D. By localization in the cell:
1. Nuclear.
2. Cytoplasmic. In this case, mutations of non-nuclear genes are usually implied.
E. In relation to the possibility of inheritance:
1. Generative, occurring in germ cells.
2. Somatic, occurring in somatic cells.
Obviously two latest methods classifications of mutations are applicable to eukaryotes, and consideration of mutations from the point of view of their occurrence in somatic or germ cells is relevant only to multicellular eukaryotes.
Very often, mutations are classified according to their phenotypic manifestation, i.e. depending on the changing characteristic. Then lethal, morphological, biochemical, behavioral, resistance or sensitivity to damaging agents mutations, etc. are considered.
In general terms, we can say that mutations are inherited changes in genetic material. Their appearance is judged by changes in signs. This primarily applies to gene mutations. Chromosomal and genomic mutations are also expressed in changes in the nature of inheritance of traits.