Types of prokaryotic genes. Genome structure of prokaryotes and eukaryotes, mobile genetics

FSBEI HPE "Penza" State University»

Pedagogical Institute them. V.G. Belinsky

Department of “Biology, methods of teaching biology and life skills”

Coursework in the discipline "Biology"

". The structure of the prokaryotic operon. . Activator, promoter, operon and terminator. Start codon, terminator"

Penza-2013

Introduction

Features of the organization of pro- and eukaryotic genes

Structure of the prokaryotic operon

Regulatory regions and structural genes

Activator, promoter, operator and terminator

Start codon, terminator

Conclusion

Bibliography

Introduction:

The study of gene structure and its expression is currently one of the main directions in modern genetics. But, as often happens with the rapid development of any scientific field, the huge flow of obtained facts is not immediately comprehended, the identified contradictions are not immediately resolved, and the introduced terminology is not immediately recognized. The same phenomenon sometimes has so many different names that one can easily determine from them the number of researchers who have studied this phenomenon.

Approximately the same situation is now developing in the direction that elucidates the structure and function of an individual gene and the genome of living beings.

There are many definitions of a gene, but none of them completely satisfies all scientists. We will adhere to the definition given by Singer M. and Berg P. in the book “Genes and Genomes” (1998). It is formulated as follows. “A gene is a collection of DNA segments that determine the formation of either an RNA molecule or a protein product.” In this definition, first of all, it is clearly emphasized that a gene is not one continuous piece of DNA, but a collection of several segments (segments) of DNA. And, secondly, the gene carries information not only about the structure of the polypeptide, but also about the structure of any RNA. In this case, it may not contain information about the structure of the protein.

Features of the organization of pro- and eukaryotic genes

The genome of modern prokaryotic cells is characterized by relatively little large sizes. In Escherichia coli it is represented by a circular DNA molecule about 1 mm long, which contains 4 106 nucleotide pairs, forming about 4000 genes. The bulk of prokaryotic DNA (about 95%) is actively transcribed at any given time. As mentioned above, the genome of a prokaryotic cell is organized in the form of a nucleoid - a complex of DNA with non-histone proteins.

In eukaryotes, the volume of hereditary material is much larger. In yeast it is 2.3 107 bp; in humans, the total length of DNA in the diploid chromosome set of cells is about 174 cm. Its genome contains 3 109 bp. and includes, according to the latest data, 30-40 thousand genes.

In some amphibians and plants, the genome is characterized by even larger sizes, reaching 1010 and 1011 bp. Unlike prokaryotes, in eukaryotic cells, from 1 to 10% of DNA is actively transcribed at a time. The composition of transcribed sequences and their number depend on the cell type and stage of ontogenesis. A significant part of nucleotide sequences in eukaryotes is not transcribed at all - silent DNA.

The large volume of hereditary material of eukaryotes is explained by the existence in it, in addition to unique ones, of moderately and highly repetitive sequences. Thus, about 10% of the mouse genome consists of short nucleotide sequences arranged in tandem (one after another), repeated up to 106 times. These highly repetitive DNA sequences are located mainly in the heterochromatin surrounding the centromeric regions. They are not transcribed. About 20% of the mouse genome is formed by moderate repeats, occurring with a frequency of 103-105 times.

Such repeats are distributed throughout the genome and are transcribed into RNA. These include genes that control the synthesis of histones, tRNA, rRNA and some others. The remaining 70% of the mouse genome is represented by unique nucleotide sequences. In plants and amphibians, moderately and highly repetitive sequences account for up to 60% of the genome.

The redundancy of the eukaryotic genome is also explained by the exon-intron organization of most eukaryotic genes, in which a significant part of the transcribed RNA is removed during processing following synthesis and is not used to encode the amino acid sequences of proteins.

At present, the functions of silent DNA, which constitutes a significant part of the genome and is replicated but not transcribed, have not been fully elucidated. It has been suggested that such DNA plays a certain role in ensuring the structural organization of chromatin. Some of the non-transcribed nucleotide sequences are apparently involved in the regulation of gene expression.

When characterizing the hereditary material of a prokaryotic cell as a whole, it should be noted that it is contained not only in the nucleoid, but is also present in the cytoplasm in the form of small circular DNA fragments - plasmids. Plasmids have been found in prokaryotic (bacterial) cells that carry hereditary material that determines properties such as the ability of bacteria to conjugate, as well as their resistance to certain drugs.

In eukaryotic cells, extrachromosomal DNA is represented by the genetic apparatus of organelles - mitochondria and plastids, as well as nucleotide sequences that are not vital for the cell (virus-like particles). The hereditary material of organelles is located in their matrix in the form of several copies of circular DNA molecules not associated with histones. Mitochondria, for example, contain from 2 to 10 copies of mtDNA.

Extrachromosomal DNA constitutes only a small part of the hereditary material of a eukaryotic cell. For example, human mtDNA contains 16,569 bp. and it accounts for less than 1% of all cellular DNA.

Unlike chromosomal DNA, mtDNA is characterized by high “gene density.” They have no introns, and intergenic spaces are small. Human circular mtDNA contains 13 genes encoding proteins (3 subunits of cytochrome C oxidase, 6 ATPase components, etc.) and 22 tRNA genes. A significant portion of mitochondrial and plastid proteins are synthesized in the cytoplasm under the control of genomic DNA.

If most nuclear genes are represented in the cells of the body in a double dose (allelic genes), then mitochondrial genes are represented by many thousands of copies per cell.

The mitochondrial genome is characterized by interindividual differences, but in the cells of one individual, as a rule, the mtDNA is identical. A set of genes located in cytoplasmic DNA molecules is called a plasmon. It defines a special type of inheritance of traits - cytoplasmic inheritance.

The general principles of organization of hereditary material represented by nucleic acids, as well as the principles of recording genetic information in pro- and eukaryotes, indicate the unity of their origin from a common ancestor, who had already solved the problem of self-reproduction and recording information based on DNA replication and the universality of the genetic code. However, the genome of such an ancestor retained great evolutionary possibilities associated with the development of the supramolecular organization of the hereditary material, different ways of implementation hereditary information and regulation of these processes.

Numerous indications of differences in genome organization, details of gene expression processes and mechanisms of its regulation in pro- and eukaryotes indicate the evolution of these cell types in different directions after their divergence from a common ancestor.

There is an assumption that in the process of the emergence of life on Earth, the first step was the formation of self-replicating nucleic acid molecules that did not initially carry the function of encoding amino acids in proteins. Thanks to their ability to self-replicate, these molecules were preserved over time. Thus, the initial selection was aimed at the ability of self-preservation through self-reproduction. In accordance with the assumption considered, later some sections of DNA acquired a coding function, i.e. became structural genes, the totality of which at a certain stage of evolution constituted the primary genotype.

The expression of the emerging DNA coding sequences led to the formation of a primary phenotype, which was assessed by natural selection for the ability to survive in a specific environment.

An important point in the hypothesis under consideration is the assumption that an essential component of the first cellular genomes was excess DNA, capable of replicating, but not carrying a functional load in relation to the formation of the phenotype. It is assumed that the different directions of evolution of the genomes of pro- and eukaryotes are associated with the different fate of this excess DNA of the ancestral genome, which should have been characterized by a fairly large volume. Probably, at the early stages of the evolution of the simplest cellular forms, they had not yet fully developed the main mechanisms of information flow (replication, transcription, translation). The redundancy of DNA under these conditions created the possibility of expanding the volume of coding nucleotide sequences at the expense of non-coding ones, providing the emergence of many options for solving the problem of forming a viable phenotype.

Structure of the prokaryotic operon

An operon is a way of organizing genetic material in prokaryotes, in which cistrons (genes, transcription units), encoding jointly or sequentially working proteins, are combined under one (or several) promoters. This functional organization makes it possible to more effectively regulate the expression (transcription) of these genes.

The concept of an operon for prokaryotes was proposed in 1961 by French scientists Jacob and Monod, for which they received the Nobel Prize in 1965.

Based on the number of cistrons, operons are classified into mono-, oligo- and polycistronic, containing, respectively, only one, several or many cistrons (genes).

The prokaryotic operon includes structural genes and regulatory elements. Structural genes encode proteins that carry out sequential stages of biosynthesis of a substance. There may be one, two or several of these genes. They are closely linked to each other and, most importantly, during transcription they work as one single gene: they synthesize one common mRNA molecule, which is only then split into several mRNAs corresponding to individual genes. The regulatory elements are the following:

promoter - a binding site for the enzyme that transcribes DNA - RNA polymerase. Is the starting point of transcription. It is a short sequence of several tens of DNA nucleotides to which RNA polymerase specifically binds. In addition, the promoter determines which of the two DNA strands will serve as a template for mRNA synthesis;

operator - a site to which a repressor is attached, which prevents RNA polymerase from moving along DNA.

terminator - the site where RNA polymerase disconnects from DNA.

The lactose operon was discovered by Jacob, Monod and Lvov in 1961. His work:

When there is no lactose in the medium, E. coli does not produce the enzymes needed to break it down because a repressor is attached to the operator, which prevents transcription from occurring.

When lactose appears in the medium, it combines with the repressor protein, it denatures and disconnects from the operator. Now nothing prevents RNA polymerase from making mRNA, on which ribosomes immediately make proteins.

Enzyme proteins break down lactose, including the one that was attached to the repressor, it returns to its place, and transcription stops.

The operation of the operator of this operon is influenced by an independent gene regulator that synthesizes the corresponding regulatory protein. This gene is not necessarily located next to an operon. In addition, one regulator can regulate the transcription of several operons. The gene regulator also has its own promoter and terminator.

Regulatory proteins are of two types: repressor protein or activator protein. They attach to specific operator DNA nucleotide sequences, which either prevents gene transcription (negative, negative regulation) or promotes it (positive, positive regulation); the mechanisms of their operation are opposite. In addition, the work of repressor proteins can be influenced by substances - effectors: by combining with the repressor, they affect its interaction with the operator.

Regulatory regions and structural genes

Structural genes - contain information about the structure of the protein. In prokaryotes, one operon contains the genes for several proteins necessary to carry out any biochemical reaction.

Genetic information about the structure of proteins and nucleic acids in all organisms is contained in DNA or RNA molecules in the form of nucleotide sequences called genes<#"290" src="doc_zip1.jpg" />

This type of regulation of enzyme synthesis is called induction, and the substance that causes this synthesis is called an inducer. One of the most obvious examples of this type of regulation is the lactose operon of Escherichia coli - a group of genes that controls the synthesis of enzymes that catabolize milk sugar - lactose. Just a few minutes after lactose is added to the E. coli culture medium, the bacteria begin to produce three enzymes: galactoside permease, beta-galactosidase and galactoside transacetylase. As soon as lactose resources in the medium are exhausted, enzyme synthesis immediately stops.

The given example will become more understandable when considering the operating diagram of the lactose operon (Fig. 81), the study of which allowed the French scientists F. Jacob and J. Monod to develop the actual concept of the operon and to clarify the basic principles of transcription regulation in prokaryotes. The operon begins with section A, intended for the attachment of a certain activator protein, which in turn is necessary for attachment to the RNA polymerase promoter (P) following section A. The promoter, the nucleotide sequence of which is recognized by RNA polymerase, is followed by an operator (O), which plays an important role in the transcription of the operon genes, since a regulatory repressor protein binds to it.

The operator is followed by the structural genes for the three enzymes mentioned earlier. The operon ends with a terminator, which stops the progression of RNA polymerase and transcription of the operon.

The regulatory repressor protein is constantly synthesized in small quantities in the cell, so that no more than 10 of its molecules are simultaneously present in the cytoplasm. This protein has an affinity for the nucleotide sequence in the operator region, and the same affinity for lactose. In the absence of lactose, the repressor protein binds to the operator site and prevents RNA polymerase from moving along DNA: mRNA is not synthesized, and enzymes are not synthesized. After adding lactose to the medium, the repressor protein binds to it faster than to the operator site: the latter remains free and does not interfere with the advancement of RNA polymerase. Transcription and broadcast in progress. The synthesized enzymes transport lactose into the cell and break down lactose. After all the lactose is consumed, there will be nothing to bind the rep spring protein with and it will again contact the operator, stopping the transcription of the operon. Thus, induction of the operon is caused by the regulatory protein not being attached to the operator. This type of induction is called negative.

Another well-known type of induction is positive induction. It is characteristic of another operen of Escherichia coli, encoding enzymes for the catabolism of another sugar - arabinose. This operon is structurally very similar to the previous one. The difference in regulation is that arabinose added to the medium interacts with the repressor protein and, releasing the operator site, simultaneously converts the repressor protein into an activator protein that promotes the attachment of RNA polymerase to the promoter. Under these conditions, transcription takes place. As soon as the reserves of arabinose in the medium are exhausted, the synthesized repressor protein again binds to the operator, turning off transcription.

In addition to induction, two types (negative and positive) of regulation based on the principle of repression are also known. If during negative induction the effector (inducer) prevents the repressor protein from attaching to the operator, then during negative repression, on the contrary, the effector gives the regulatory protein the ability to attach to the operator. If in the first case the connection of the effector with the regulatory protein allowed transcription, then in the second it prohibited it. An example of negative repression is the well-studied tryptophan operon of Escherichia coli.

It consists of five structural genes that provide the synthesis of the amino acid tryptophan, an operator and two promoters. The regulator protein is synthesized outside the tryptophan operon. While the cell manages to consume all the synthesized tryptophan, the operon works and tryptophan synthesis continues. If an excess of tryptophan appears in the cell, it binds to the regulatory protein and changes it in such a way that this protein acquires an affinity for the operator. The altered regulatory protein interacts with the operator and interferes with the transcription of structural genes, as a result of which tryptophan synthesis stops. In positive repression, the effector deprives the regulatory protein of the ability to bind to the operator, thus determining the transcription of structural genes.

The described types of regulation characterize the mechanisms of regulation of individual operons, practically without touching the regulation of genome expression as a whole, while it is quite obvious that the regulation of different operons must be coordinated. This coordinated nature of the work of different operons and genes is called cascade regulation in viruses and phages. According to the principle of cascade regulation, first the transcription of “early”, then “early” and finally “late” genes occurs, depending on which proteins are required at different stages of viral (phage) infection.

Of course, the principle of cascade regulation in phages is one of the simplest. In more complex organisms, in order to carry out a large number of functions that occur simultaneously or with a certain sequence, the coordinated work of many genes and operons is necessary. This is especially true for eukaryotes, which are distinguished not only by a more complex genome organization, but also by many other features of the mechanisms of regulation of gene activity.

According to the principles of regulation, eukaryotic genes can be divided into three groups: 1) functioning in all cells of the body; 2) functioning only in tissues of one type; 3) ensuring that specialized cells perform specific functions. In addition, in eukaryotes, simultaneous group shutdown of gene activity is known, carried out by histones, the main proteins that make up the chromosomes. Another significant difference between transcription in eukaryotes is that many mRNAs are stored in the cell for a long time in the form of special particles - informosomes, while prokaryotic mRNAs, almost during the process of transcription, enter ribosomes, are translated, and are then quickly destroyed.

At the same time, there is a lot of evidence indicating that transcription in eukaryotes occurs from regions similar to the operons of prokaryotes and consisting of regulatory and structural genes.

A distinctive feature of eukaryotic operons is that they almost always contain only a structural gene, and the genes that control various stages of a certain chain of metabolic transformations are scattered throughout the chromosome and even across different chromosomes. Another distinctive feature of eukaryotic operons is that they consist of significant (exons) and insignificant (introns) regions alternating with each other. During transcription, both exons and introns are read, and the resulting messenger RNA precursor (pro-mRNA) then undergoes maturation (processing), as a result of which intros mature and the mRNA itself is formed (splicing),

In eukaryotes, other types of regulation of gene activity are also known, such as the position effect or dosage compensation. In the first case, we are talking about a change in gene activity depending on the specific environment: the movement of a gene from one place on the chromosome to another can lead to a change in the activity of both this gene and nearby ones. In the second case, the lack of one dose of a gene (primarily this applies to genes localized in the sex chromosomes of heterogametic sex, when one of the homologous sex chromosomes is either genetically inert or completely absent) does not manifest itself phenotypically due to a compensatory increase in the activity of the remaining gene . In general, the regulation of gene activity in eukaryotes has not been sufficiently studied.

Activator, promoter, operator, terminator

The unit of transcription in prokaryotes can be individual genes, but more often they are organized into structures called operons. The operon consists of structural genes located one behind the other, the products of which usually participate in the same metabolic pathway. As a rule, an operon has one set of regulatory elements (regulatory gene, promoter, operator), which ensures coordination of gene transcription processes and the synthesis of corresponding proteins.

A promoter is a section of DNA responsible for binding to RNA polymerase. In the case of prokaryotes, the most important sequences for transcriptional regulation are those designated "-35" and "-10". Nucleotides located before the initiation codon (“upstream”) are written with a “-” sign, and with a “+” sign are all nucleotides starting from the first in the initiation codon (starting point). The direction in which the transcription process moves is called "downstream".

The sequence designated “-35” (TTGACA) is responsible for recognition of the promoter by RNA polymerase, and the sequence “-10” (or Pribnow box) is the site from which the unwinding of the DNA double helix begins. This box most often contains TATAAT bases. This base sequence is most often found in prokaryotic promoters and is called consensus. The TATA box contains adenine and thymine, between which there are only two hydrogen bonds, which facilitates the unwinding of DNA chains in this region of the promoter. In case of base pair substitutions in the specified promoter sequences, the efficiency and correct definition the transcription start point at which the enzyme RNA polymerase begins RNA synthesis. In prokaryotes, along with the promoter, there are other regulatory regions: these are the activator and the operator.

An operator is a section of DNA to which a repressor protein binds, preventing RNA polymerase from starting transcription.

In the lactose operon, the left part of the promoter (activator) binds to the catabolite activator protein (BAK, or CAP in English terminology, catabolite activator protein), and the right part binds to RNA polymerase. The BAC protein, unlike the repressor protein, plays a positive role by helping RNA polymerase begin transcription.

Possible various options interactions of regulatory sites with enzymes and regulatory proteins, and the latter with molecules called inducers (effectors).

Genetic information encoded in DNA using 4 nucleotides (a four-letter alphabet), during the process of protein biosynthesis, is translated into the sequence of amino acids of proteins (a twenty-letter alphabet) using adapter molecules (“translators”) tRNA. Each of the 20 amino acids that make up proteins must attach to its own tRNA. These reactions occur in the cytosol and are catalyzed by twenty APCase enzymes (aminoacyl-tRNA synthetases). Each enzyme has a double affinity: for “its” amino acid and for its corresponding tRNA (one or more). ATP energy is used for activation.

The process consists of two stages that take place in the active site of the enzyme. At the first stage, as a result of the interaction of an amino acid and ATP, an aminoacyl adenylate is formed, at the second, the aminoacyl residue is transferred to the corresponding tRNA.
Reaction progress: Amino acid (R) + ATP + enzyme (ER E?) R (aminoacyl adenylate) + FPN

ER (aminoacyl adenylate) + tRNAR Aminoacyl-tRNA + AMP + E?R
APCaseR Summary equation:

Amino acid (R) + tRNAR + ATP aminoacyl-tRNAR + AMP + FPN

The ester bond between the aminoacyl and tRNA is high-energy, the energy used in the synthesis of the peptide bond.

So, all the activated amino acids necessary for protein biosynthesis are formed in the cytoplasm of the cell, connected to their corresponding adapters? various aminoacyl-tRNAs (aa-tRNAs).

Terminator (DNA)<#"justify">Conclusion

Prokaryotes are organisms whose cells lack a formed nucleus. Its functions are performed by a nucleoid (that is, “like a nucleus”); Unlike the nucleus, the nucleoid does not have its own shell.

The body of prokaryotes usually consists of one cell. However, with incomplete divergence of dividing cells, filamentous, colonial and polynucleoid forms (bacteroids) arise. Prokaryotic cells lack permanent double-membrane and single-membrane organelles: plastids and mitochondria, endoplasmic reticulum, Golgi apparatus and their derivatives. Their functions are performed by mesosomes - folds of the plasma membrane. The cytoplasm of photoautotrophic prokaryotes contains various membrane structures on which photosynthetic reactions occur. They are sometimes called bacterial chromatophores.

A specific substance in the cell wall of prokaryotes is murein, but some prokaryotes lack murein. There is often a mucous capsule on top of the cell wall. The space between the membrane and the cell wall serves as a reservoir of protons for photosynthesis and aerobic respiration.

The sizes of prokaryotic cells vary from 0.1-0.15 microns (mycoplasma) to 30 microns or more. Most bacteria are 0.2-10 microns in size. Motile bacteria have flagella, which are based on flagellin proteins.

The main quantitative feature of the genetic material of eukaryotes is the presence of excess DNA. This fact is easily revealed by analyzing the ratio of the number of genes to the amount of DNA in the genome of bacteria and mammals. If the average bacterial gene size is 1500 nucleotide pairs (bp), and the length of the circular DNA molecule of the E. coli and B. subtilis chromosome is over 1 mm, then in such a chromo the soma can accommodate about 3 thousand genes.

Approximately the same number of genes was experimentally determined in bacteria based on the number of mRNA types.

If this number is multiplied by the average gene size, it turns out that about 95% of the bacterial genome consists of coding (gene) sequences. The remaining 5% appears to be occupied by regulatory elements. A different picture is observed in eukaryotic organisms. For example, humans have approximately 50 thousand genes (this refers only to the total length of the coding sections of DNA - exons). At the same time, the size of the human genome is 3 ×10 9 (three billion) bp This means that the coding part of its genome makes up only 15...20% of the total DNA.

There are a significant number of species whose genome is tens of times larger than the human genome, for example, some fish, tailed amphibians, and liliaceae. Excess DNA is common to all eukaryotes. In this regard, it is necessary to emphasize not unambiguity of the terms genotype and genome. The genotype should be understood as a set of genes that have a phenotypic manifestation, while the concept of genome denotes the amount of DNA found in the haploid set of chromosomes mosom of this species.

Bibliography:

1. Avramenko I.F. Microbiology. M.: Kolos.- 1979.-176 p.

Mishustin E.N., Emtsev V.T. Microbiology. M.: Agropromizdat. - 1987. - 336 p.

Bakulina N.A. Microbiology. M.: Medicine.-1976.-325 p.

Singer M., Berg P. Genes and genomes in 2 volumes. T 2. M.: Mir. - 1988.-391 p.

Konichev A.S. Molecular biology. M.: Publishing Center Academy.-2005-400 p.

Blokhina I.N. Genosystematics of bacteria. M.: Nauka.- 1976.-151 p.

Gramov B.V. The structure of bacteria. L.: Leningrad State University Publishing House. - 1985. - 190 p.

Pekhov A.P. Genetics of bacteria. M.: Medicine.-1977.-407 p.

Stent G.S. Molecular genetics. M.: Mir.-1981.-646 p.

Rees E., Sternberg M. Introduction to molecular biology: From cells to atoms. M.: Mir.- 2002.-142 p.

Sergeeva G.M., Pashkova E.I. Guide for independent work students in molecular biology. Petropavlovsk: NKSU named after. M.Kozybaeva.-2008.-234 p.

Khesin R.B. Variability of the genome. M.: Nauka.-1984.-472 p.

Ed. J. Lengler, G. Drews, G. Schlegel. Modern microbiology. Prokaryotes. M.: Mir.- 2005.-469 p.

Yu.P. Altukhova. Modern natural science. Encyclopedia. M.: Magistr-Press. - 2000.-343 p.

Tutoring

Need help studying a topic?

Our specialists will advise or provide tutoring services on topics that interest you.
Submit your application indicating the topic right now to find out about the possibility of obtaining a consultation.

Determination of the fine structure of a gene, i.e. its organization, as well as operating principles, i.e. regulation of activity (on-off), were originally established for prokaryotic cells.

These works were carried out Francois Jacob and Jacques Monod(1961; Nobel Prize 1965). According to the Jacob–Monot concept, the unit of regulation of gene activity in prokaryotes is the operon. Operon- a functional unit of the genome in prokaryotes, which includes cistrons (transcription units) encoding jointly or sequentially working proteins and united under one (or several) promoters, i.e. the size of the operon exceeds the size of the coding DNA sequences. This functional organization makes it possible to more effectively regulate the expression (manifestation) of these genes.

In general, the structure of the operon includes: promoter, operator, structural genes, terminator (Fig. 1).

P - A promoter is a regulatory section of DNA that serves to attach RNA polymerase to a DNA molecule.

The C-Operator is a regulatory section of DNA that is capable of attaching a repressor protein, which is encoded by the corresponding gene. If a repressor is attached to an operator, then RNA polymerase cannot move along the DNA molecule and synthesize mRNA.

T-Terminator is a regulatory region of DNA that serves to disconnect RNA polymerase after the end of mRNA synthesis.

Transcription of a group of structural genes is regulated by two elements - a regulator gene and an operator. The operator is often localized between the promoter and structural genes; the regulator gene can be localized next to the operon or at some distance from it.

If the product of the gene regulator is a repressor protein, its attachment to the operator blocks the transcription of structural genes, preventing the attachment of RNA polymerase to a specific region - the promoter, necessary for the initiation of transcription. On the contrary, if the regulatory protein is an active apoinducer, its attachment to the operator creates the conditions for the initiation of transcription. Low-molecular substances, effectors, that act as inducers or corepressors of structural genes that are part of operons, also participate in the regulation of operons.

Based on the number of cistrons, operons are divided into mono-, oligo- and polycistronic, containing, respectively, only one, several or many cistrons (genes).

The combination of functionally similar genes into operons apparently gradually developed in the evolution of bacteria for the reason that in them the transfer of genetic information is usually carried out in small portions (for example, during transduction or through plasmids). What is important is the linkage of functionally related genes, which allows bacteria to acquire the necessary function in one step.

Gene- a structural and functional unit of heredity that controls the development of a certain trait or property. Parents pass on a set of genes to their offspring during reproduction. However, gene transfer from parents to offspring is not the only way to transmit genes. In 1959, a case of horizontal gene transfer was described. Unlike vertical transfer, in horizontal transfer an organism transfers genes to an organism that is not its descendant. This mode of transmission is widespread among unicellular organisms and to a lesser extent among multicellular organisms.

Eukaryotic genes

Let us first note that in eukaryotic organisms DNA is present not only in the nuclei, but also in organelles - mitochondria, which are found in all eukaryotes, and chloroplasts, found in green plants. Based on many features, it is assumed that organelles originate from prokaryotes: mitochondria from a-purple bacteria, and chloroplasts from cyanobacteria. They are similar to prokaryotes in many features of the protein-synthesizing apparatus. Considering the direction of interests in genetic engineering, we will limit ourselves here to considering only nuclear genes.

Structure. Eukaryotic genes differ significantly in structure and transcription from prokaryotic genes. Their distinctive feature is discontinuity, i.e., the alternation of nucleotide sequences in them that are represented (exons) or not represented (introns) in the mRNA. It is clear from this that introns belong to non-coding sequences. They can be located not only in the area limited by the initiation and termination codons, but also outside them, at the beginning or end of the gene. Their length can exceed 10 kb. In lower eukaryotes, discontinuous genes make up a minority of all genes (5% in yeast), and in higher eukaryotes they make up the majority (94% in mammals). Note that gene mosaicism has also been found in prokaryotic cells.

Evolutionarily related genes that have a high degree of physical homology form families. Proteins encoded by such genes, acting simultaneously or at different stages of organism development, perform the same functions. For example, the composition of proteins in the a- and p-chains of hemoglobin in the blood of mammals is different in the embryo, fetus, and adult organism, which is caused by differential expression of genes included in the a- and p-families of globin genes. Along with functioning genes, non-functioning ones were found in families. Such genes are called pseudogenes. They are not expressed for various reasons (change in reading frame due to deletion or insertion, lack of an intron, etc.).

A characteristic feature of the genes included in the family is a similar pattern of localization of most introns. This similarity is not limited to a specific genome. Thus, in the case of globin genes, the genes in all studied animals turned out to be similar in the location of introns - in mammals, birds and frogs. However, the lengths and nucleotide sequences of introns can vary significantly, thereby changing the size of the genes themselves.

Transcription. Eukaryotic genes are not grouped into operons, so each of them has its own promoter and transcription terminator. Transcription is carried out by three different RNA polymerases: I, II and III, which synthesize rRNA, mRNA and tRNA, respectively. As in the case of prokaryotes, we will consider only the mechanism of expression of genes encoding proteins. Therefore, in what follows, eukaryotic RNA polymerase refers to RNA polymerase II. It consists of more than a dozen subunits, but still cannot bind directly to the promoter. Its placement on the promoter is facilitated by protein transcription factors. A number of them recognize specific sequences (boxes) in the promoter.

The length of a typical promoter of higher eukaryotes is about 100 bp. It should distinguish between two parts - basic and additional. Genes that have only the basic part of the promoter function in any cells of the body and are not subject to tissue-specific control. This part serves to initiate transcription and precisely orient RNA polymerase II relative to the first nucleotide to be transcribed. The additional part, together with enhancers, is used to increase transcription efficiency and regulate gene activity.

Prokaryotes(lat. Procaryota, from ancient Greek. προ "before" and κάρυον "core"), or pre-nuclear- unicellular living organisms that do not (unlike eukaryotes) have a formed cell nucleus and other internal membrane organelles (with the exception of flat cisterns in photosynthetic species, for example, cyanobacteria). The only large circular (in some species - linear) double-stranded DNA molecule, which contains the bulk of the genetic material of the cell (the so-called nucleoid), does not form a complex with histone proteins (the so-called chromatin). Prokaryotes include bacteria, including cyanobacteria (blue-green algae), and archaea. The descendants of prokaryotic cells are the organelles of eukaryotic cells - mitochondria and plastids.

Prokaryotes are divided into two taxa according to the rank of domain (superkingdom): Bacteria ( Bacteria) and Archaea ( Archaea).

Prokaryotic cells are characterized by the absence of a nuclear membrane; DNA is packaged without the participation of histones. The type of nutrition is osmotrophic.

The genetic material of prokaryotes is represented by one DNA molecule closed in a ring; there is only one replicon. The cells do not have organelles with a membrane structure. Mobile genetic elements may be present in the genome, and some prokaryotes (for example, Wolbachia) contain unusually many of them. The study of bacteria led to the discovery of horizontal gene transfer, which was described in Japan in 1959. This process is widespread among prokaryotes and also in some eukaryotes. The discovery of horizontal gene transfer in prokaryotes has forced us to take a different look at the evolution of life. Previously, evolutionary theory was based on the fact that species cannot exchange hereditary information. Prokaryotes can exchange genes among themselves directly (conjugation, transformation) and also with the help of bacteriophage viruses (transduction).

Unique genes- these are genes that occur in a cell two or more times (up to 10-20). Most researchers believe that in multicellular organisms the total number of genes is on average one hundred thousand and the overwhelming number of them are unique genes. A characteristic feature of eukaryotic genes is a mosaic exon-intron structure. Introns that do not carry genetic information are cut out (splicing). The number and size of introns varies among species. Their presence in the gene leads to a significant increase in the size of the gene. Introns stabilize exons, but it is believed that intron- this is the so-called “selfish” DNA, which does not give the organism any evolutionary advantages. Exons control protein synthesis: 1 exon - 1 domain.

Repeated genes include primarily the genes of large and small rRNAs and histones. Their number varies greatly and can reach more than 2000. Large rRNA genes are organized into blocks in which the 18S rRNA, 58S rRNA and 28S rRNA genes sequentially occur. Between them there are gaps that vary in length in different organisms. Intergenic regions have repeats different types, with an unusual sequence, rich in GC pairs. Low molecular weight nuclear RNA genes do not form blocks. Histone genes are repeated in the genome tens (in mammals), and hundreds (in Drosophila), and thousands (in axolotl) times. Moreover, it is not possible to discern the connection between this indicator and the position of the organism on the evolutionary ladder.

Rearranging, or recombining, genes are genes that encode light and heavy chains of proteins immunoglobulins, performing the functions of antibodies. The genes of these proteins consist of two types of genes for light chains and five types for heavy chains. Light chains are encoded by three separate genetic elements, heavy chains by four. Genome rearrangements lead to the connection of different sections and, ultimately, to the formation of immunoglobulins of different classes.

Jumping genes, or transposons, - mobile genetic elements. Being a normal component of the genome, they make up a significant part of it (in Drosophila, 7% of the genome), can be represented by many copies scattered throughout the genome, and have varying localization. The structure of different classes of migratory elements (MEs) varies, but all of them are characterized by the presence of inverted repeats at the ends. In the middle, MEs may have unique sequences. MEs exhibit high locus specificity, since they can be integrated into a specific sequence on the chromosome.

Principles and stages of DNA replication.

The process of synthesis of a daughter molecule of deoxyribonucleic acid on the matrix of the parent DNA molecule. During the subsequent division of the mother cell, each daughter cell receives one copy of a DNA molecule that is identical to the DNA of the original mother cell. This process ensures that genetic information is accurately passed on from generation to generation.

Replication can be divided into 4 stages: formation of a replication fork (initiation), synthesis of new chains (elongation), elimination of primers, completion of the synthesis of two daughter DNA strands (termination).

Remember what substance is the carrier of hereditary information in living organisms. Repeat what a gene is. What are the types of genes? What is the difference between structural and regulatory genes?

Gene structure diagram

All genes have the same structure. They consist of several sections (Fig. 20.1). The main section of any gene is the one that contains information about the structure of the protein or RNA molecule (gene product). This is the coding part of the gene. The remaining regions of the gene are non-coding. They do not contain information about the structure of the molecules whose synthesis is provided by the gene. But they are responsible for the functioning of the gene.

The non-coding regions of the gene are the promoter and terminator. A promoter is a section of a gene where RNA synthesis begins, a terminator is a section where this synthesis ends. In addition, the gene contains regulatory regions that regulate its operation.

Prokaryotic genes

Prokaryotic genes have a relatively simple structure. Most often, each of these genes contains information about only one structure - a protein or RNA molecule.

Genes of prokaryotic organisms are often organized into operons. An operon is a structure consisting of several structural genes (Fig. 20.2). It allows prokaryotes to synthesize the products of several genes at once. The structural genes in the operon are located one after another and on all of them there is one common promoter, one common terminator and one common operator that regulates its operation.

An example of an operon is the lactose operon of Escherichia coli. It contains genes encoding enzymes necessary for the synthesis of the carbohydrate lactose.

Eukaryotic genes

Unlike the genes of prokaryotes, the genes of eukaryotic organisms do not form operons. Each of them has its own promoter and terminator. In addition, the structure of these genes is more complex. they contain sections of DNA that do not contain the information necessary for the synthesis of a gene product (protein or RNA molecule). Such regions are called introns. Those regions that contain the necessary information are called exons. Typically, a eukaryotic gene contains several introns and exons (Fig. 20.3).

Important components of eukaryotic genes are regulatory regions. Using these sites, the cell can speed up or slow down the synthesis of gene products. This structure allows eukaryotic organisms to very finely regulate the functioning of genes.

For the functioning of genes in living organisms, it is necessary to have special sites for the start (promoter), regulation and completion (terminator) of information reading. Genes of prokaryotic organisms can be combined into special groups - operons, which have a common promoter, operator and terminator. The genes of eukaryotic organisms contain non-coding (introns) and coding (exons) regions of DNA. In addition, these genes have regulatory regions that change the speed at which they operate.

Test your knowledge

1. Why do genes need a promoter? 2. Why do genes need a terminator? 3. What is an operon? 4. What are introns? 5. Compare the genes of prokaryotes and eukaryotes.

This is textbook material

26. Gene structure in prokaryotes and eukaryotes. The concept of the genome.

Gene is the simplest functionally integral unit of hereditary information. Materially, a gene is a section of a DNA molecule containing information about a simple specific structure or function - genetic information. The specificity of genetic information is determined by the nucleotide sequence of the gene. Thus, different genes differ structurally from each other in nucleotide sequence, which determines their functional differences.

The primary functions of genes are realized in the process of transcription (RNA synthesis). Moreover, the implementation of the function of each gene depends on other genes, i.e. manifests itself in a certain gene system - the genome.

Genome- this is a set of genes characteristic of a certain systematic group of organisms. Structurally, the genome is a set of genes located in one copy of all DNA molecules of a cell of a given type of organism, i.e. one copy of all the genetic material of a cell.

Genome is a specific genetic characteristic characteristic of a specific biological species of organisms: the human genome, the dysentery amoeba genome, the HIV genome. Due to the fact that evolution occurs through divergence, similarity in the content and organization of the genome occurs between related systematic groups.