§ 2. Mutation theory
Discovery of intermittent, sudden, hereditary undirected changes - mutations(from lat. mutation- change) *, the distribution of which is purely random, served as an impetus for an even more rapid development of classical genetics at the beginning of the 20th century and for elucidating the role of hereditary changes in evolution.
* (Sudden hereditary changes have long been called mutations (in the 17th and 18th centuries). This term was resurrected by G. De Vries.)
In 1898, a Russian botanist S. I. Korzhinsky, and two years later, the Dutch botanist De Vries (one of those who rediscovered Mendel’s law - see Chapter IV, § 3) independently made another extremely important genetic generalization, called mutation theory.
The essence of this theory is that mutations arise suddenly and undirectedly, but once they occur, the mutation becomes stable. The same mutation can occur repeatedly.
One day, passing by potato field(near the Dutch village of Gilversum), overgrown with the weed brought from America, aspenberry, night candle or evening primrose ( Oenothera Lamarckiana) from the fireweed family (which includes the well-known fireweed, or fireweed), De Vries noticed among ordinary plants specimens that differed sharply from them. The scientist collected the seeds of these exceptional plants and sowed them in his experimental garden. For 17 years, De Vries observed evening primrose (thousands of plants). At first, he discovered three mutants: one of them was dwarf, the other was giant - its leaves, flowers, fruits, seeds turned out to be large, its stems were long (Fig. 29), the third had red veins on the leaves and fruits. Over the course of 10 years, De Vries obtained many new forms from normal plants, differing in a number of characteristics. The scientist closely followed mutants(as mutation carriers are called) and their descendants for several years. Based on observations, complementing the teachings of Darwin, he came to the conclusion about the paramount importance of sharp hereditary deviations - mutations for the emergence of new species. Mutations appear in the most different directions in representatives of any of the species. Since not all mutations allow a mutant to survive (in a certain environment), the further existence of the corresponding form is decided by Darwinian the struggle for existence through natural selection.
Soon, many descriptions of various mutations in animals and plants appeared in the scientific literature.
Without knowing the mechanism by which mutations occur, De Vries believed that all such changes arise spontaneously, spontaneously. This situation is true only for some mutations.
The inevitability of spontaneous mutations follows from the inevitability of the movement of atoms, in which sooner or later, but statistically inevitably, transitions of electrons from one orbit to another occur. As a result, individual atoms and entire molecules change even under the most constant environmental conditions. This inevitable change in any physical and chemical structure is reflected in the appearance of spontaneous mutations (DNA molecules, the custodians of hereditary information, are such a structure).
Spontaneous mutations are constantly found in nature with a certain frequency, which is relatively similar in the most diverse species of living organisms. The frequency of occurrence of spontaneous mutations varies for individual characteristics from one mutation per 10 thousand gametes to one mutation per 10 million gametes. However, due to the large number of genes in each individual in all organisms, 10-25% of all gametes carry certain mutations. Approximately every tenth individual is a carrier of a new spontaneous mutation.
It should be noted that the majority of newly emerging mutations are usually in a recessive state, increasing only the latent, potential variability characteristic of organisms of any species. When conditions change external environment, for example, when the action of natural selection changes, this hidden hereditary variability can manifest itself, since individuals carrying recessive mutations in a heterozygous state will not be destroyed in the new conditions in the process of struggle for existence, but will remain and give offspring. Spontaneous, spontaneous mutations appear without any outside intervention. However, there are many so-called induced mutations. Factors causing (inducing) mutations can be a wide variety of environmental influences: temperature, ultraviolet radiation, radiation (both natural and artificial), the effects of various chemical compounds - mutagens. Mutagens are agents of the external environment that cause certain changes in the genotype - mutation, and the process of mutation formation itself - mutagenesis.
Radioactive mutagenesis began to be studied in the 20s of our century. In 1925, Soviet scientists G. S. Filippov And G. A. Nadson For the first time in history, geneticists used X-rays to produce mutations in yeast. A year later, an American researcher G. Meller(later twice Nobel Prize laureate), who worked for a long time in Moscow, at the institute led by N. K. Koltsov, used the same mutagen on Drosophila.
Numerous mutations have been discovered in Drosophila, two of them, vestigial and curled, are shown in Fig. thirty.
Currently, work in this area has grown into one of the sciences - radiation biology, a science that has a large practical use. For example, some mutations of fungi that produce antibiotics give hundreds and even thousands of times greater yield of medicinal substances. IN agriculture Thanks to mutations, high-yielding plants were obtained. Radiation genetics is important in the study and exploration of outer space.
Chemical mutagenesis was first purposefully studied by N.K. Koltsov’s collaborator V.V. Sakharov in 1931 on Drosophila when its eggs were exposed to iodine, and later M. E. Lobashov.
Chemical mutagens include a wide variety of substances (alkylating compounds, hydrogen peroxide, aldehydes and ketones, nitrous acid and its analogs, various antimetabolites, salts of heavy metals, dyes with basic properties, aromatic substances), insecticides (from the Latin insecta - insects , cida - killer), herbicides (from the Latin herba - grass), drugs, alcohol, nicotine, some medicinal substances and many others.
Behind last years in our country work has begun on the use chemical mutagens to create new breeds of animals. Interesting results have been achieved in changing the coat color of rabbits and increasing the length of wool in sheep. It is important that these achievements were obtained at dosages of mutagens that do not cause death in experimental animals. The strongest chemical mutagens (nitrosoalkylureas, 1,4-bisdiazoacetylbutane) are widely used.
One of the main tasks of selection agricultural plants is to create varieties resistant to fungal and viral diseases. Chemical mutagens are effective means to obtain plant forms resistant to the most various diseases. In cereals (spring and winter wheat, barley, oats), forms resistant to powdery mildew, with increased resistance to various types of rust. It is important that in individual mutants an increase in the amount of protein does not correlate with a deterioration in its quality and it is possible to obtain forms with an increased content of protein and essential amino acids in it (lysine, methionine, threonine).
Among mutants induced by chemical mutagens, forms with a complex of positive characteristics are of great interest. There are frequent cases of obtaining such forms from wheat, peas, tomatoes, potatoes and other crops. Mutations are the material for both natural, and for artificial selection(selection).
In 1920, still young at that time, but one of the greatest geneticists of the 20th century, Nikolai Ivanovich Vavilov, established that there is parallelism of variability among the most diverse systematic units Living creatures. This provision is called the rule homological(from lat. homologis- agreement, common origin) of series, which to a certain extent makes it possible to predict what mutations may occur in related (and sometimes distant) forms. This rule is that between different systematic groups (species, genera, classes and even types) there are repeating series of forms that are similar in their morphological and physiological properties. This similarity is due to the presence of common genes and their similar mutations.
Thus, among the varieties of wheat and rye there are similar forms, winter and spring, with an awned, short-awned or awnless ear; Both have drooping, smooth-spiked, red-, white- and black-spiked races, races with brittle and non-brittle spikes and other characteristics. Similar parallelism between organisms belonging to different types, genera, families, and even different classes, is observed in animals. An example would be gigantism, dwarfism or lack of pigmentation- albinism in mammals, birds, as well as in other animals and plants.
Having found one biological species a series of forms A, B, C, D, D, E and having established forms A 1, B 1, D 1, E 1 in another related species, we can assume that there are not yet open forms B 1 and G 1.
In humans, the mutation rate under natural conditions is 1:1,000,000, but if we take into account the huge number of genes, then at least 10% of gametes, both male and female, carry some kind of newly emerging mutation.
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 source 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 (a large number of different 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, 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 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 are as follows:
1. Mutations arise 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 early period, G. De Vries mistakenly believed that mutations can immediately give rise to new species, i.e. bypassing natural selection.
G. De Vries created his mutation theory based on experiments with various types 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 features necessarily vary, being distributed around a certain average size. 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 among the scientific achievements that led to modern ideas about the universality of many biological structures and functions.
Law N.I. Vavilova has great importance for breeding practice, since it predicts the search for certain forms 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 significance. Each approach in these classification methods reflects some significant aspect of 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, the last two methods of classifying 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.
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, the Russian botanist Sergei Ivanovich Korzhinsky in 1899 developed evolutionary theory of heterogenesis, based on ideas about the leading evolutionary role of discrete (discontinuous) changes.
However, the most famous was mutation theory of the Dutch botanist Hugo (Hugo) De Vries(1901), who introduced the modern, genetic concept of mutation to refer to rare variants of a trait in the offspring of parents who did not have that 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 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 due to the peculiarities of the organization of the chromosomal apparatus of this plant. Besides, rare variants traits may be due to rare combinations of alleles (for example, white color plumage in budgerigars is 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, the following definition of mutations is accepted:
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 the insertion of a nucleotide pair there is a loss of a nucleotide pair (or vice versa), then the amino acid composition of the 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 of the 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, the body 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 the morphology of 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 cultivated and wild plants 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 with D.I. Mendeleev’s periodic law). 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.
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Mutation theory, or more correctly, the theory of mutations, is 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. Korzhinsky (1899) came to the idea of abrupt changes in hereditary properties in his work “Heterogenesis and Evolution.” So it is fair to talk about the Korzhinsky-De Vries mutation theory. The mutation theory is set out in much more detail in the works of G. 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 occurrence. In accordance with the definition of G. De Vries, a mutation is the phenomenon of spasmodic, intermittent changes in a hereditary trait. As will be shown later, the very definition of the concept “mutation” causes difficulties. 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.
Mutations (from the Latin mutatio - change, change) are sudden natural (spontaneous) or artificially caused (induced) persistent changes in the hereditary structures of living matter responsible for the storage and transmission of genetic information. The ability to give M. - to mutate - is a universal property of all forms of life from viruses and microorganisms to higher plants, animals and humans; it underlies hereditary variability (See variability) in living nature. M. that arise in germ cells or spores (generative M.) are inherited; M., arising in cells that do not participate in sexual reproduction (Somatic mutations), lead to genetic mosaicism: part of the body consists of mutant cells, the other - of non-mutant ones. In these cases, M. can be inherited only during vegetative propagation with the participation of mutant somatic parts of the body (buds, cuttings, tubers, etc.).
The sudden occurrence of hereditary changes was noted by many scientists of the 18th and 19th centuries, it was well known to Charles Darwin, but in-depth study M. began only with its inception on the threshold of the 20th century. experimental genetics. The term "M." introduced into genetics in 1901 by H. De Vries.
Types of mutations . Based on the nature of changes in the genetic apparatus, M. are divided into genomic, chromosomal, and gene, or point. Genomic microorganisms involve changing the number of chromosomes in the cells of the body. These include: Polyploidy - an increase in the number of sets of chromosomes, when instead of the usual 2 sets of chromosomes for diploid organisms there can be 3, 4, etc.; Haploidy - instead of 2 sets of chromosomes, there is only one; Aneuploidy - one or more pairs of homologous chromosomes are absent (nullisomy) or are represented not by a pair, but by only one chromosome (monosomy) or, conversely, by 3 or more homologous partners (trisomy, tetrasomy, etc.). Chromosome chromosomes, or chromosomal rearrangements (See Chromosomal rearrangements), include: inversions - a section of a chromosome is turned 180°, so that the genes it contains are located in reverse order compared to normal; translocations - exchange of sections of two or more non-homologous chromosomes; deletions - loss of a significant portion of a chromosome; deficiencies (small deletions) - loss small area chromosomes; duplication - doubling of a chromosome section; fragmentation - breaking a chromosome into 2 or more parts. Gene mutations represent persistent changes in the chemical structure of individual genes and, as a rule, are not reflected in the morphology of chromosomes observed under a microscope. M. genes are also known that are localized not only in chromosomes, but also in some self-reproducing organelles of the cytoplasm (for example, in mitochondria, plastids; see Cytoplasmic heredity).