The modern understanding of the structure of the genome is brief. Modern idea of the gene
The chromosome of any organism, be it a bacterium or a human, contains a long, continuous strand of DNA along which many genes are located. Establishing the number of genes, their exact location on the chromosome and their detailed internal structure, including knowledge of the complete nucleotide sequence, is a task of exceptional complexity and importance.
Genome organization.
Different organisms differ dramatically in the amount of DNA that makes up their genomes. In viruses, depending on their size and complexity, the genome size ranges from several thousand to hundreds of nucleotide pairs. Genes in such simply arranged genomes are located one after another and occupy up to 100% of the length of the corresponding nucleic acid (RNA and DNA). For many viruses, the complete DNA nucleotide sequence has been determined. Bacteria have a much larger genome size. E. coli has a single strand of DNA - the bacterial chromosome consists of 4.2x106 (degree 6) nucleotide pairs. More than half of this amount consists of structural genes, i.e. genes encoding certain proteins. The rest of the bacterial chromosome consists of nucleotide sequences that cannot be transcribed, the function of which is not entirely clear. The vast majority of bacterial genes are unique, i.e. presented once in the genome. The exception is the genes for transport and ribosomal RNAs, which can be repeated dozens of times.
The genome of eukaryotes, especially higher ones, sharply exceeds the size of the genome of prokaryotes and, as noted, reaches hundreds of millions and billions of nucleotide pairs. The number of structural genes does not increase very much. The amount of DNA in the human genome is sufficient to form approximately 2 million structural genes. The actual number is estimated at 50-100 thousand genes, i.e. 20-40 times less than what could be encoded by a genome of this size. Consequently, we have to admit the redundancy of the eukaryotic genome. The reasons for redundancy have now become largely clear: firstly, some genes and nucleotide sequences are repeated many times, secondly, there are many genetic elements in the genome that have a regulatory function, thirdly, some DNA does not contain genes at all
Regulation of genes
How does a cell know which protein to produce and how much?
At the beginning of each gene there is a segment of DNA that contains the controlling elements of that particular gene. This segment is called a promoter. It functions as a watchtower, raising a flag, that is, giving a signal to the gene it controls. Take, for example, the production of insulin (which we produce to ensure that blood sugar is burned). When a message molecule with a message of more insulin appears in the cell, a messenger molecule is produced that binds to the insulin watchtower. After this, the watchtower lever moves and opens the way for reading the insulin gene.
How does the information contained in DNA turn into proteins at the right time?
Each gene consists of three main components: a watchtower (promoter), an information block, and a poly-A signaling element.
If there is not enough of a protein in the cell, a message is sent to the nucleus to find the corresponding gene. If the watchtower recognizes the received message, a signal will be sent to open the gate to the information block. The information is immediately copied - or read (transcribed) - into a thread-like molecule called RNA. RNA is very similar to DNA, only it is one strand rather than two. After the information has been copied, a tail of 200 type A nuclides is attached to the end of the molecule. This process is called polyadenylation, and it begins with a poly-A signal located at the end of the gene. The poly-A tail helps store messenger RNAs in the nucleus for a limited time. After this, copies of the gene (RNA) leave the nucleus into the cytoplasm and bind to mini-organelles - ribosomes, which perform the function of synthesizing proteins from amino acids. Ribosomes read the code from RNA and link amino acids into the polypeptide chain of a protein molecule.
No cell will ever be able to use all the information contained in DNA. Cells share the work among themselves - they specialize. Brain cells will not produce insulin, liver cells will not produce saliva, just as skin cells will not build bone tissue.
The gene problem is the central problem of genetics. Ideas about the gene have always reflected and reflect the level of development of this science, its achievements of problems. The concept of a gene as a discrete unit, identified by the method of hybridological analysis developed by G. Mendel, was introduced by Johannsen in 1909. He did not connect this concept with any hypotheses regarding its essence and material nature.
In the subsequent period, not only “materialization” occurred, but also itself! genes - sections of DNA molecules - became objects and “workers!” tools" of genetic engineering and biotechnology. The primary structure of many thousands of genes has been deciphered, the main features of their structure in various objects have been elucidated. These| The information is stored in computer information banks, updated and used by scientists all over the world.
Currently, a gene is defined as a section of a DNA molecule (in some RNA viruses), encoding the primary structure of a polypeptide, a transport or ribosome RNA molecule, or interacting with a regulatory protein.
In G. Mendel’s view, the unit of heredity was a factor that controls the manifestation of one trait in a dominant or recessive state. Subsequently, the concept of a gene was developed in the works of T. Morgan, who showed that a gene is a locus (section) of a chromosome that occupies a strictly defined position in it.
In the modern understanding, a gene is a functional unit of a DNA molecule that controls the sequence of amino acids in the encoded polypeptide chain. The specificity of a gene is determined by the number of nucleotides and their unique sequence. A gene has a certain size, expressed by the number of nucleotides and molecular weight. The gene encoding the synthesis of polypeptide value is called structural. It is an integral part of the operon and has a complex system of regulation carried out by acceptor genes. Each Structural gene is characterized by a unique nucleotide sequence that allows it to be identified.
A structural gene is a discrete integral unit encoding the synthesis of one polypeptide chain. Any change in the order of alternation of nucleotides - deletion, addition or replacement of at least one nucleotide - inactivates the structural gene or changes its function.
It was previously noted that the structural genes of eukaryotes are characterized by a mosaic structure of the sections of the DNA molecule encoding the amino acids of the polypeptide chain - EXONS (cDNAs) alternate with sections that do not have this ability - nitrones.
The acceptor genes of each operon are highly specific; only certain protein molecules can attach to them, including the repressor protein that suppresses the activity of structural genes, the Cap protein, as well as enzymatic proteins that ensure replication and transcription. The proportion of structural and acceptor genes in the total DNA in the genome of different organisms ranges from 98 to 15%. The rest of the genome DNA is called excess DNA. There is especially a lot of excess DNA in plant genomes; excess DNA is characterized by the presence of repeats of identical nucleotide sequences. R. Britten and D. Cohn in 1968 found that in mice, 7% of the DNA consists of unique nucleotide sequences, and 30% are repeats; in humans, 66% are unique sequences, and 34% are repeats,
DNA repeats in eukaryotes can be of different natures. Some structural genes that have a unique nucleotide sequence can be represented in several copies. Genes; Coding histones, the main proteins that make up chromosomes, are represented in a DNA molecule by different numbers of copies; for example, the haploid mouse genome contains 30 structural genes encoding H4 histones. Animals have repeats of structural genes encoding globin, immunoglobulin, interferon and other vital protein molecules. Among the gene repeats there are non-functioning genes that, due to the loss or addition of a nucleotide, have lost the ability to synthesize mRNA. They are called pseudogenes.
Repeats of structural genes that control the synthesis of ribosomal and transfer RNA are especially numerous in the DNA molecule. Thus, in the haploid genome of a frog there are about 8000 tRNA genes, in the chicken genome there are about 100 rRNA genes, in the Drosophila genome there are about 130 of them.
The DNA of genomes also contains other types of repeats. They are short sequences of nucleotides, each containing about 300 nucleotide pairs, as well as 40,000-80,000 B1 repeats containing approximately 140 nucleotide pairs each |
Excess DNA in eukaryotes contains quite large quantities of nucleotide sequences, the genetic role of which remains unclear. They are called satellite DNA, which is a sequence of several nucleotide pairs. In the mouse, they consist of 6 nucleotide pairs, including 5 AT pairs and CT pairs; in the guinea pig, satellite DNA consists of 6 pairs of nucleotides, including 3 pairs of CG and 3 pairs of TA, AG and AT. Blocks (clusters) of satellite DNA are predominantly concentrated in heterochromatic regions of chromosomes located near the centromere.
Transposons. For a long time it was believed that the position of genes in the chromosome and, consequently, in the DNA molecule is strictly fixed, although B. McClintock back in 1953. proved that the corn genome contains so-called mobile! genetic elements. In 1975 -1977, the Soviet scientist G.P. Georgiev discovered in the Drosophila genome genes represented by dozens of copies and scattered across different chromosomes. He found that these genes are mobile or “jumping”, since they can be localized in different lines and even in individual individuals on different chromosomes and in different loci of the same chromosome.
The movement of a DNA fragment containing a gene or genes from one chromosome to another, unusual for them, is called transposition. DNA fragments that can be mixed from one chromosome to another or from one locus to another are called transposons. Transposition involves two processes: excision and insertion. Excision is the release of a transposon from the DNA molecule into which it was embedded, and insertion is the process of inserting a transposon into a new DNA locus.
Transposons can be divided into several classes. One of them, the most studied and widely represented, was discovered by G. IJL Georgiev in Drosophila and called them “mobile dispersed genes (MDGs). Drosophila has ABOUT 20 families of such MDHs, each of which contains from 10 to 150 copies, the localization of which in the genome varies greatly. A characteristic feature of MDHs are identically oriented, long terminal repeats (LTRs). DNI MDH contains 5 thousand nucleotide pairs, including 250-1500 nucleotide pairs - this is DIP. The formation of a large number of MDH copies occurs as follows: RNA is synthesized on the DNA matrix at the MDH element locus, on which, with the participation of the enzyme reverse transcriptase, many copies of DNI fragments corresponding to MDH are formed, which are introduced into new DNI loci of the genome. The DCT of MDH elements contains signal sequences for the beginning and end of transcription, as well as amplifiers (enhancers) that sharply increase the intensity of transcription. They also contain an operon encoding reverse transcriptase.
Another class of active transposons (MDTs) includes DNI sequences encoding the transpphase enzyme, which is responsible for MDT transposition—excision and insertion of transposons. These include the well-studied P-element of Drosophila and the Ac-element of maize.
A special class includes passive transposons - DNA fragments that do not encode anything, but numerous copies of which can serve as a substrate for transposase. These may also include long inversions: 1st repetitions; and even some MDG elements,
Transposons also include other parts of the genome if they actively synthesize RNA and then, with the participation of the enzyme reversese, form numerous copies of DNA that are inserted into various parts of the genome. These include two classes of short mouse transposons, called B and B2; B1 contains 130, B2-190 nucleotide pairs. They are scattered throughout the genome, and almost every DNA fragment contains B1 or B2, or both. Being in the DNA of the genome, they actively transcribe RNA, and then, with the participation of reverse transcriptase, form a huge number of copies (a cell can contain up to 100,000 copies of each transposon).
IN human genome also found in! and 132, as well as transposons, containing 300 base pairs and represented by 300,000 copies.
Transposition plays a significant role in the implementation of hereditary information and can be the cause of a hereditary change in a trait (mutation). Many transposons serve as templates for the transcription of mRNA encoding various enzymes, including reverse transcriptase. By introducing themselves into new loci of the cell’s genetic apparatus, transposons affect the functioning of surrounding genes. Sometimes an introduced transposon changes the structure of a gene up to the creation of a new one, unusual for a given locus. Transposons can cause profound genome rearrangements, including fission; inversions, translocations, for different genetic loci, from 10 to 90% of all spontaneous mutations are the result of MHD transposition.
Under normal conditions, transposition occurs very rarely, but under the influence of certain factors, so-called transposition explosions are observed, when a large number of transposons belonging to different classes move simultaneously in the cell.
In recent years, it has been established that transposition and the formation of a large number of MDH repeats are similar to retroviruses of birds and mammals. Retroviruses are viruses whose genetic information is written on RNA (RNA-containing viruses). When such an RNA virus enters a cell, DNA copies of the virus RNA are synthesized with the participation of the enzyme reverse transcriptase. DNA is inserted into various loci of the cell genome and becomes an integral part of the DNA molecule. This DNA is called a provirus. The mouse genome may contain several families of proviruses localized at different DNA loci. RNA can be synthesized from these DNAs and even virus-like particles can be formed, but an infectious virus does not arise. Viruses, information about which is contained in the DNA of higher organisms, are called endogenous viruses (EV), and the genetic elements encoding them are called endogenous proviruses (EP). Thus, 29 EP loci were identified in chickens, which occur in various combinations and with different frequencies. None of the EP loci is an obligatory element of the chicken genome.
The vast majority of EPs are defective and cannot encode virions, therefore, they are not infectious for (parental cells. However, some EPs should be considered as genetic risk factors that increase the likelihood of the onset of a carcinogenic process or the emergence of a new oncogenic virus,
Modern idea of the gene
A gene is a section of a DNA molecule (in some viruses RNA) that encodes the primary structure of a polypeptide, tRNA, rRNA molecule, or interacts with a regulatory protein. The gene has a discrete structure. The structural unit of a gene, at the level of which mutations and recombinations occur, is one pair of nucleotides - a site. The number of nucleotide pairs of a gene can range from 150 to several thousand. The shortest genes are RNA proimase (10 bp) and tRNA (70-80 bp). Eukaryotic genes encoding the order of amino acids in a polypeptide molecule have a discontinuous structure, introns (silent sections) alternate with exons (sense). The total length of the intron is many times greater than the length of the exons. The initial, initiating, and terminal, terminating parts of the gene have a special structure. A gene is a complex unique structure characterized by specific features depending on its functions.
Currently, in connection with the establishment of the structure of DNA molecules and their role in the transmission of hereditary information, the concept of a gene has undergone further changes. A genome is a localized section of a DNA molecule that has a specific biochemical function and has a specific effect on the property of an individual. Each gene has the function of programming the synthesis of a specific protein in the cell (the “gene-protein” concept). In other words, a gene is a specific biological unit in a system of genetic information associated with a specific sequence of nucleotides in DNA and RNA
The size of a gene is determined by the number of paired bases that provide coding for the structure of the molecule of the protein that is associated with it in its genesis. If we assume that each amino acid is encoded by a triplet, the number of nucleotides that determine the length of the gene will be 3 and, where right -BUT the number of consecutive molecules of amino acids in the molecule of a given protein. Therefore, the length of individual (genes can correspond to several thousand base pairs.
Within a chromosome, certain genes are strictly localized. The place that a gene occupies on a chromosome is called locus. Allelic genes occupy identical loci. The question of whether neighboring genes are separated from each other has not yet been answered. There are two opposing points of view. According to one of them, genes are located continuously one after another! to others. Another view believes that genes are separated by segments of protein that have no genetic significance.
Cytoplasmic inheritance. Although the leading role in heredity belongs to chromosomes with genes contained in them, the latter are not the only carriers of hereditary information. In the cytoplasm of the cell there are structures that, like the nucleus, also reproduce by division and, apparently, can, in turn, provide specific hereditary information. These include, for example, plastids in plant cells.
A characteristic feature of cytoplasmic heredity is the inheritance of certain properties through the maternal line. This is explained by the fact that the egg contains a large amount of protoplasm, while the sperm is almost devoid of it.
54) Structure and structure of DNA and RNA. Nucleic acids, their main functions. Depending on which monosaccharide is contained in the structural unit of the polynucleotide - ribose or 2-deoxyribose, they distinguish between (RNA) and (DNA). The main chain of RNA contains ribose residues, and DNA contains 2-deoxyribose residues.
The nucleotide units of DNA macromolecules may contain adenine, guanine, cytosine and thymine. The composition of RNA differs in that uracil is present instead of thymine. DNA is contained mainly in the nuclei of cells, RNA - in ribosomes and protoplasm of cells. When describing the structure of nucleic acids, different levels of organization of macromolecules are taken into account: primary. and secondary structure. The primary structure of nucleic acids is the nucleotide composition and a certain sequence of nucleotide units in the polymer chain. In the abbreviated one-letter designation, this structure is written as ...– A – G – C –...
Secondary structure of DNA represents two parallel unbranched polynucleotide chains twisted around a common axis into a double helix.
Such a spatial structure is held together by many hydrogen bonds formed by nitrogenous bases directed into the helix. Hydrogen bonds occur between the purine base of one chain and the pyrimidine base of another chain. These bases make up complementary pairs. The pattern of hydrogen bonds between complementary base pairs is determined by their spatial correspondence. A pyrimidine base is complementary to a purine base. Thus, thymine complex. adenine, cytosine comp. guanine. The complement of bases determines the complementarity of chains in DNA molecules.
A set of polynucleotide chains serves as the chemical basis for the main function of DNA - storage and transmission of hereditary characteristics.
Properties of DNA:
DNA molecules are capable of replication (doubling), i.e. can make it possible to synthesize other DNA molecules identical to the original ones, since the sequence of bases in one of the strands of the double helix controls their location in the other strand
DNA molecules can direct in a very precise and specific way the synthesis of proteins specific to organisms of a given species.
Secondary structure of RNA. Unlike DNA, RNA molecules consist of a single polynucleotide chain and do not have a strictly defined spatial shape (the secondary structure of RNA depends on their biological functions).
The main role of RNA is direct participation in protein biosynthesis. Three types of cellular RNA are known, which differ in location in the cell, composition, size and properties that determine their specific role in the formation of protein macromolecules:
Messenger RNAs transmit information about the structure of the protein encoded in DNA from the cell nucleus to the ribosomes, where protein synthesis occurs;
Transfer RNAs collect amino acids in the cell cytoplasm and transfer them to the ribosome; RNA molecules of this type “learn” from the corresponding sections of the messenger RNA chain which amino acids should participate in protein synthesis;
Ribosomal RNAs ensure the synthesis of a protein of a certain structure by reading information from messenger RNA
N.c. molecules are long polymer chains with a molecular weight of 2.5 x 104-4 x 109, built from monomeric molecules - nucleotides so that the hydroxyl groups at the 31st and 51st carbon atoms of the carbohydrate of neighboring nucleotides are linked by a phosphoric acid residue. RNA contains ribose as a carbohydrate, and the nitrogenous components are represented by A, G (purine bases), U and C (pyrimide bases). In DNA, the carbohydrate component is deoxyribose, and uracil is replaced by thymine. Phosphate and sugar make up the nonspecific part in the nucleotide molecule, and the purine or pyrimidine base makes up the specific part. As part of the majority of N., some other (mainly methylated) derivatives of purines and pyrimidines were also found in small quantities. n. minor bases. N. chains contain from several tens to many thousands of nucleotide residues arranged linearly in a specific sequence that is unique to a given N. c. Thus, both RNA and DNA are represented by a huge variety of individual compounds. The linear sequence of nucleotides determines the primary structure of the NK. The secondary structure of the NK arises as a result of the convergence of certain base pairs, namely: G with C and A with U (or thymine) according to the principle of complementation due to hydrogen bonds, as well as hydrophobic interactions between them.
The biological role of N. to. lies in the storage, implementation and transmission of hereditary information, “recorded” in the molecules of N. to. in the form of a sequence of nucleotides - i.e. n. genetic code. During cell division - mitosis - self-copying of DNA occurs - its replication, as a result of which each daughter cell receives an equal amount of DNA, which contains the development program for all the characteristics of the mother cell. The implementation of this genetic Inflation into certain characteristics is carried out through the biosynthesis of RNA molecules on a DNA molecule (transcription) and subsequent biosynthesis of proteins involving different types of RNA (translation).
58) Quantitative and qualitative traits, features of their inheritance.
Phenotype is the sum of all external and internal characteristics (properties) of a given organism. All organisms have qualitative and quantitative characteristics. Qualitative features are those that can be photographed or described by looking at them, and the degree of reliability in the description depends on the skill of the describer. Thus, K. Linnaeus so vividly described the qualitative characteristics of a domestic dog that these descriptions have been passed from one textbook to another without changes for two centuries. Such characteristics of organisms are sexual differences, body shape, structure, color of the animal, color of flowers and fruits, shape of seeds, fruits, etc. Qualitative characteristics in humans are especially diverse. They are specific to each individual.
Quantitative characteristics are those that can be determined by measurements. For example, quantitative traits in plants are the mass of seeds, fruits, number, shape and size of leaves, height of stems, productivity, etc. In domestic animals, quantitative traits are milk and meat productivity, protein content of meat, amount of fat in cows’ milk, egg production chickens, egg weight, payment for feed, etc. In crop production and livestock farming, taking into account quantitative traits is very important not only in economic terms, but also in the fact that they are used in the selection of high-yielding plant varieties and highly productive animal breeds, selecting for economically useful traits. As a rule, quantitative traits in both plants and animals are controlled not by one, but by a large number of genes acting in the same direction. In humans, quantitative traits are body weight, brain weight, weight and size of internal organs, height, number of blood cells, degree of skin pigmentation, general intelligence, etc. As in the case of plants and animals, human quantitative traits are also subject to genetic control, i.e. they are polygenes.
In genetics, there are two classes of traits - qualitative and quantitative. They differ in the nature of variability and features of inheritance. Qualitative characteristics are characterized by intermittent, and quantitative - continuous variability. The first of them provide clear boundaries when splitting into dominant or recessive traits. This is because each of them is usually controlled by a single allelic gene. Quantitative traits do not provide clear boundaries for splitting under different crossing options, although they differ from qualitative traits in a higher degree of variability. A feature of quantitative traits is the complex nature of inheritance. Each of them is determined not by one, but by many loci in the chromosomes. This type of inheritance, when one trait is determined by many genes, is called polygenic. The level of development of a quantitative trait depends on the ratio of dominant and recessive genes, other genetic factors and the degree of the modifying effect of environmental factors. Variability in a quantitative trait in a population consists of genetic and paralogical (external environmental) variability.
Pleiotropy is the effect of one gene on several phenotypic traits. The product of virtually every gene is usually involved in several, and sometimes in very many processes that form the metabolic network of the body. Pleiotropy is especially characteristic of genes encoding signaling proteins.
· The gene that causes red hair causes lighter skin color and the appearance of freckles.
· Phenylketonuria (PKU), a disease that causes mental retardation, hair loss and skin pigmentation, can be caused by a mutation in the gene encoding the enzyme phenylalanine 4-hydroxylase, which normally catalyzes the conversion of the amino acid phenylalanine to tyrosine.
· A recessive mutation in the gene encoding the synthesis of the globin part in hemoglobin (replacement of one amino acid), causing sickle-shaped red blood cells, changes in the cardiovascular, nervous, digestive and excretory systems.
· Arachnodactyly, caused by a dominant mutation, manifests itself simultaneously in changes in the fingers and toes, dislocation of the lens of the eye and congenital heart defects.
· Galactosemia, caused by a recessive mutation in the gene encoding the enzyme galactose-1-phosphate uridyl transferase, leads to dementia, cirrhosis and blindness.
The history of views on the units of heredity (genes) discovered by Mendel can be divided into several periods. In accordance with the “classical” point of view, which prevailed in the 30s. XX century, the gene was considered as an indivisible unit of genetic transmission, function, mutation and recombination. Since the 1940s, in connection with the establishment of the genetic role of DNA, a “neoclassical” concept has been formed, according to which a gene (cistron) is a section of a DNA molecule with a specific nucleotide sequence that determines the primary structure of the synthesized mRNA molecule and the corresponding polypeptide or single molecule tRNA or rRNA. In this case, the gene is divided into its component parts in the form of elementary units of mutation (mutons) and recombination (recons), which can be identified as certain sections of the polynucleotide. Genes that determine the structure of polypeptides and RNA molecules are called structural genes. The modern period of understanding the gene, which began in the 1970s, is associated with the emergence of new knowledge about the discontinuous (“mosaic”) structure of eukaryotic genes and a number of other features of the genetic organization of various organisms (overlapping genes, repeating genes, pseudogenes, mobile genes, etc. ).
Within the framework of classical (formal) genetics, it is customary to consider a gene as a structural unit that determines an elementary trait (phene) of an organism. The totality of all the genes of a separate organism (individual) is called it genotype, and the set of signs - phenotype. The term genome It is customary to denote the totality of all genetic elements (DNA of chromosomes, mitochondria, plasmids, etc.) that are constant for organisms of a given species. It should be noted that the sizes of genomes (the amount of genomic DNA or RNA in the corresponding viruses) have significant differences in organisms belonging to different levels of organization of living matter (viruses, bacteria, eukaryotes).
In accordance with modern concepts, most of the structural genes of prokaryotes (bacteria) are represented by continuous sections of the DNA molecule, all of the information of which is used in the synthesis of encoded polypeptide chains. Consequently, the genetic information of the prokaryotic gene is fully realized. In some small viruses, an unusual structural and functional organization of genetic material was discovered in the form of overlapping genes (according to the “gene within a gene” principle), which allows for even more economical use of the existing very limited information capabilities of the genome. Thus, some sections of the DNA of one of the smallest bacteriophages fX174 contain information from not one, but simultaneously two different genes, which allows a genome of such a small size to encode at least nine different protein molecules. Reading information from overlapping genes begins from different starting points of the same nucleotide sequence, i.e. There are different reading frames for this sequence.
Unlike prokaryotes, eukaryotes are characterized by the discontinuous nature of the structural and functional organization of genes. Information from such a gene about the structure of the synthesized polypeptide does not exist in the form of a continuous nucleotide sequence of a certain section of the DNA molecule, but in the form of coding fragments ( exons), which are interrupted (separated) by “uninformative” nucleotide sequences ( introns), not directly involved in the coding of this polypeptide. Consequently, the genes of various eukaryotic organisms are a mosaic of several exons and nitrons alternating in a certain order. The sizes of nitrons in the composition of such genes range from ten to more than 1000 nucleotide pairs. It has been suggested that introns may play a role in regulating RNA processing, which will be discussed below. There is evidence to suggest that they are likely to significantly influence recombination processes between homologous genes. There is also a known hypothesis that genes of different proteins or genes that determine proteins of the same family, but have accumulated different mutations, can recombine relatively easily and often along intronic regions. It can be assumed that such properties of nitrons should accelerate the evolution of protein molecules, facilitating the processes of evolution of eukaryotes as a whole, which gives them significant advantages over prokaryotes. The elements found in their genomes can probably be considered as an “evolutionary reserve” of eukaryotes. pseudogenes, which are DNA nucleotide sequences that are homologous to the sequences of known (functioning) genes, but for one reason or another do not exhibit information activity, i.e. not giving the final mature product.
One of the features of the genetic organization of eukaryotes is also the presence in their genomes of a significant number of repeating genes encoding the primary structure of tRNA, rRNA, histone proteins, etc., as well as other (less extensive and not always identified in terms of functional significance) repeating DNA sequences, the number of copies of which can vary from a few to several thousand or more. For example, in the haploid human genome, containing about 3 x 10 9 base pairs, repeated DNA sequences make up approximately 30%, while the remaining 70% of the genome is represented by “unique” sequences that exist in single copies.
Mobile (transposable) genes have also been found in the genomes of various organisms (prokaryotes and eukaryotes), the role of which will be described below.
TASKS FOR INDEPENDENT WORK
- 1. Calculate the linear dimensions (in nucleotide pairs and in length units) of a bacterial gene encoding a polypeptide consisting of 100 amino acid residues.
- 2. Explain the reason for the situation in which a eukaryotic cell gene, occupying a DNA section of 2400 nucleotide pairs in size, encodes a polypeptide consisting of 180 amino acid residues.
- 3. Draw a diagram of the discontinuous structure of a hypothetical gene, consisting of five exons and four nitrones and encoding a polypeptide of 300 amino acid residues (the relative sizes of individual exons and nitrones can be chosen arbitrarily).
Genome organization
The study of evolution and the construction of hypotheses regarding the mechanisms of evolutionary changes are currently difficult to imagine without the involvement of genetics. At the beginning of the twentieth century, it was the use of genetic methods that provided a breakthrough in evolutionary research and raised it to a qualitatively new level.
The evolutionary concepts of Lamarck and Darwin were based on the study of external characteristics and properties of living organisms; assumptions about the mechanism of their inheritance were speculative. The idea of the material carriers of heredity was reduced to the principle of continuity, an analogue of a liquid capable of infinite dilution, despite the well-known rather eloquent facts that clearly indicate the discrete nature of the inheritance of certain traits according to the well-studied pedigrees of noble families of Europe, the degree of expression of which did not change over generations.
These views on heredity were closely related to ideas about the limitless and undirected variability of the characteristics and properties of living things, which arose as an adaptive reaction to the influence of external factors. Similar ideas about the nature of heredity and variability were, in general, shared by Charles Darwin, which was reflected in his concept pangenesis.
The foundations of genetics, the science of heredity, were laid by the research of the canon of the monastery of Brno, Czech Republic, Gregor Mendel. He was the first to experimentally prove the discrete inheritance of traits and their independent distribution among descendants during backcrossing in subsequent generations. His works were ahead of their time and were recognized only at the beginning of the 20th century, 25 years after publication.
During this period, ideas about mutations as discrete heritable changes in the properties of organisms were formulated (G. de Vries), ideas about genes as units of heredity (V. Johansen) without indicating their localization in the cell. The concept " genotype " to denote the hereditary constitution of gametes and zygotes as opposed to phenotype – a set of external, directly observable signs and properties of living organisms.
Subsequently, the work of American geneticists of the Morgan school established the linear arrangement of genes as loci chromosomes in the chromosomes of the nucleus, the phenomenon of gene recombination between homologous chromosomes as a result genetic crossing over during the process of meiosis. These and other works served as the basis for the formation of the chromosomal theory of heredity, according to which discrete elements (loci) of chromosomes serve as material carriers of the properties and characteristics of organisms.
Until the 50s of the twentieth century, the molecular nature of genes remained unknown. It was only in 1953 that the structure of DNA was deciphered and a hypothesis was put forward about its role as a custodian of hereditary information, the principles of its encoding and implementation in protein synthesis. Was formulated "central dogma" genetics, which embodied in molecular form Weisman’s position on the impossibility of transmitting information from germ plasm to somatic cells. According to it, hereditary information can only be transmitted in the direction DNA→RNA→protein. Over the course of two decades, the molecular mechanisms of transmission and implementation of genetic information in the cell were deciphered, as a result of which the chromosomal theory of heredity received a molecular justification. The following were considered proven: the unchanged position of genes in the chromosome, the connection between the gene and the product it encodes is unambiguous (one gene - one enzyme), the functional activity of all DNA in the chromosomes of the nucleus, the dependence of the complexity and level of organization of objects on the amount of DNA. The main principles of genetics of this period are reflected in Table 4.1.
Table 4.1. Changing ideas about the structure and function of genetic material (Golubovsky, 2000)