Gene structure of prokaryotes and eukaryotes table. Genome structure of prokaryotes and eukaryotes, mobile genetics. The concept of a gene. Structural organization of genes in prokaryotes and eukaryotes. Gene classification

1. prokaryotic genome

The main feature of the molecular organization of prokaryotes is the absence of a nucleus in their cells, which is separated by a nuclear membrane from the cytoplasm. The absence of a core is only outward manifestation special organization of the genome in prokaryotes.

The prokaryotic genome is built very compactly. The number of non-coding nucleotide sequences is minimal. Many mechanisms of gene expression regulation used in eukaryotes never occur in prokaryotes. The simplicity of the structure of the prokaryotic genome is explained by their simplified life cycle.

Gene - a unit of hereditary information that occupies a certain position in the genome or chromosome and controls the performance of a certain function in the body. According to the results of a study of prokaryotes, mainly E. coll, The gene consists of two main elements: the regulatory part and the actual coding part. The regulatory part of the gene provides the first stages of implementation genetic information, contained in the structural part of the gene; the structural part of the gene contains information about the structure of the polypeptide encoded by the given gene. The number of non-coding sequences in the structural part of the gene in prokaryotes is minimal. The 5" end of the prokaryotic gene has a characteristic organization of regulatory elements, especially at a distance of 50 - 70 bp from the transcription initiation point. This region of the gene is called promoter. It is important for gene transcription, but is not transcribed into RNA itself. The opposite 3" end is the terminator region required for transcription termination. It is also not transcribed in RNA. Transcription starts from the starting point (+1).

DNA sequences that signal the termination of transcription are located at the 3' end of the gene and are called transcriptional sequences. terminators. They contain sequences that form the hairpin structure in the transcribed RNA.

In addition to the chromosome, most bacteria have other structures capable of autonomous replication - plasmids. These are double-stranded circular DNAs ranging in size from 0.1 to 5% of the chromosome size, carrying genes that are optional for the host cell, or genes that are required only in a certain environment. It is these extrachromosomal elements that contain genes that confer heritable resistance to one or more antibiotics in cells. They got the name resistance factors, or K-factors. Other plasmids determine the pathogenicity of pathogenic bacteria, such as pathogenic strains E. coli, causative agents of plague and tetanus. Still others determine the ability of soil bacteria to use unusual carbon sources, such as oil hydrocarbons.

^ 2. Eukaryotic genome

Eukaryotic cells are characterized by the presence of a well-formed kernels. The information macromolecule of their genome is DNA, which is unevenly distributed over several chromosomes in the form of complexes with numerous proteins. However, the genetic information in cells contains not only the chromosomes of the nucleus. Vital genetic information is also contained in extrachromosomal DNA molecules. In eukaryotes, this is the DNA of chloroplasts, mitochondria and other plastids. The genome of a eukaryotic organism is currently understood as the total DNA of the haploid set of chromosomes and each of the extrachromosomal genetic elements contained in a single cell of the germline of a multicellular organism.

The eukaryotic genome differs significantly from the prokaryotic genome in a number of ways, among which it should be noted. redundancy. A eukaryotic cell contains many times more genes than a prokaryotic cell. The increased content of DNA in the eukaryotic genome cannot be explained only by an increase in the need of these organisms for additional genetic information due to the complication of organization, since most of their genomic DNA is usually represented by non-coding sequences nucleotides. The phenomenon of a significant redundancy of the eukaryotic genome in relation to non-coding nucleotide sequences is known as the "paradox C".

A eukaryotic gene can be viewed as a collection of DNA segments that together constitute an expressed unit responsible for the formation of a specific functional product, either an RNA molecule or a polypeptide.

The DNA segments that make up a gene include the following elements:

1.
A transcription unit is a section of DNA that codes for
primary transcript. It includes: a) a sequence that is found in mature functional RNA molecules; b) introns (for mRNA); c) intermediate sequences - spacers (for rRNA). Introns and spacers are removed
during processing of primary transcripts; d) 5'- and 3'-untranslated sequences (5'-NTP and 3'-NTP).

2.
Minimum Sequences Needed to Get Started
transcription (promoter) and end of transcription (terminator).

3.
Sequences regulating the frequency of transcription initiation, responsible for the inducibility and repression of transcription, as well as cellular, tissue and temporal specificity of transcription. They are diverse in structure, position and function. These include enhancers
And silencers are DNA sequences located in
thousand base pairs from a eukaryotic gene promoter and
having a remote effect on its transcription.

Unlike prokaryotic genes, which are almost always collinear with their RNA, many eukaryotic genes have mosaic building. Mosaic in this case means the alternation of coding (exons) and non-coding (insertion sequences, or introns) sequences within a transcription unit. Introns are most often found in genes that code for proteins.

A significant part of the eukaryotic genome (10 - 30%) is made up of repetitive sequences that have a certain structural organization and are able to move in the genome both within the same chromosome and between chromosomes. They got the name mobile genetic elements.

There are two main classes of mobile genetic elements: transposons And retrotransposons. This classification is based on the molecular mechanisms by which these elements move.

^ 3. Eukaryotic Organelle Genomes: Mitochondrial and Chloroplast DNA

There are two types of cytoplasmic DNA: one is found in the mitochondria of eukaryotes, the other in the chloroplasts of green plants and algae. Like all cytoplasmic elements, they are inherited through the maternal line, and not according to the laws of Mendel! Most of the proteins of these organelles, encoded in nuclear DNA, are synthesized in the cytoplasm and then pass into the organelle. However, some proteins of mitochondria and chloroplasts and all their RNA are encoded in the DNA of the organelles themselves and are synthesized in them. Thus, organelles are the result of the combined efforts of two genomes and two translational apparatuses. The RNA components of the ribosomes of organelles, as well as the tRNAs used in translation, are encoded by the genomes of mitochondria and chloroplasts.

The size of the chloroplast genome in all studied organisms is similar, while the mitochondrial genomes in plants are much larger than in animals.

All mitochondria and chloroplasts contain several copies of their own genomic DNA. These DNA molecules are usually distributed as separate groups in the mitochondrial matrix and in the chloroplast stroma, where they are attached to the inner membrane. The way DNA is packaged is unknown. In terms of structure, the genome is more similar to the bacterial genome: for example, like bacteria, they do not have histones.

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

These works were carried out François Jacob and Jacques Monod (1961; Nobel Prize 1965). According to the Jacob-Monod concept, the unit of regulation of gene activity in prokaryotes is the operon. Operon is a functional unit of the genome in prokaryotes, which includes cistrons (transcription units) encoding proteins that work together or sequentially and are united under one (or several) promoters, i.e. the size of the operon exceeds the size of the coding DNA sequences. Such a 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 region of DNA that serves to attach RNA polymerase to a DNA molecule.

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

The T-terminator is a regulatory region of DNA that serves to disconnect RNA polymerase after mRNA synthesis is completed.

Transcription of a group of structural genes is regulated by two elements - a regulator gene and an operator. The operator is often located 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 regulator gene is a repressor protein, its attachment to the operator blocks the transcription of structural genes, preventing the attachment of RNA polymerase to a specific site, the promoter, necessary to initiate transcription. On the contrary, if an active apoinducer serves as a regulator protein, its attachment to the operator creates conditions for transcription initiation. The regulation of the work of operons also involves low molecular weight substances - effectors acting as inductors or corepressors of the structural genes that make up the operons.

Operons according to the number of cistrons 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 they usually transfer genetic information in small portions (for example, during transduction or through plasmids). The linkage of functionally related genes is important in itself, which allows bacteria to acquire the necessary function in one stage.

Gene- structural and functional unit of heredity of living organisms. A gene is a DNA sequence that specifies the sequence of a particular polypeptide or functional RNA. Genes (more precisely, alleles of genes) determine the hereditary characteristics of organisms that are transmitted from parents to offspring during reproduction. At the same time, some organelles (mitochondria, plastids) have their own DNA, which is not included in the genome of the organism, which determines their characteristics.

Among some organisms, mostly unicellular, there is horizontal gene transfer that is not associated with reproduction.

The term gene was coined in 1909 by the Danish botanist Wilhelm Johansen.

The science of genetics is engaged in the study of genes, the founder of which is Gregor Mendel, who in 1865 published the results of his research on the transmission of traits by inheritance when crossing peas. The regularities formulated by him were later called Mendel's Laws.

Among scientists there is no consensus from which angle to consider the gene. Some scientists consider it as an information hereditary unit, and the unit of natural selection is a species, group, population or individual. Other scientists, such as Richard Dawkins in his book The Selfish Gene, view the gene as the unit of natural selection and the organism itself as survival car genes.

Currently, in molecular biology it has been established that genes are sections of DNA that carry any integral information - about the structure of one protein molecule or one RNA molecule. These and other functional molecules determine the development, growth and functioning of the organism.

At the same time, each gene is characterized by a number of specific DNA regulatory sequences, such as promoters, which are directly involved in regulating the expression of the gene. Regulatory sequences can be located either in the immediate vicinity of the open reading frame encoding the protein, or the beginning of the RNA sequence, as is the case with promoters (the so-called cis cis-regulatoryelements), and at a distance of many millions of base pairs (nucleotides), as in the case of enhancers, insulators and suppressors (sometimes classified as trans-regulatory elements trans-regulatory elements). Thus, the concept of a gene is not limited to the coding region of DNA, but is a broader concept that includes regulatory sequences.

Originally the term gene appeared as a theoretical unit for the transmission of discrete hereditary information. The history of biology remembers disputes about which molecules can be carriers of hereditary information. Most researchers believed that only proteins can be such carriers, since their structure (20 amino acids) allows you to create more options than the structure of DNA, which is composed of only four types nucleotides. Later it was experimentally proved that it is DNA that includes hereditary information which has been expressed as the central dogma of molecular biology.

Genes can undergo mutations - random or purposeful changes in the sequence of nucleotides in the DNA chain. Mutations can lead to a change in the sequence, and therefore a change biological characteristics protein or RNA, which, in turn, may result in a general or local altered or abnormal functioning of the body. Such mutations in some cases are pathogenic, since their result is a disease, or lethal at the embryonic level. However, not all changes in the nucleotide sequence lead to a change in the protein structure (due to the effect of the degeneracy of the genetic code) or to a significant change in the sequence and are not pathogenic. In particular, the human genome is characterized by single nucleotide polymorphisms and copy number variations. copynumbervariations), such as deletions and duplications, which make up about 1% of the entire human nucleotide sequence. Single nucleotide polymorphisms, in particular, define different alleles of the same gene.

The monomers that make up each strand of DNA are complex organic compounds, including nitrogenous bases: adenine (A) or thymine (T) or cytosine (C) or guanine (G), a five-atom sugar-pentose-deoxyribose, after which DNA itself was named, as well as a phosphoric acid residue. These compounds are called nucleotides.

Genes and memes

By analogy with genes, Richard Dawkins coined the term "meme" - a unit of cultural information. If a gene is distributed in a chemical environment, using for reproduction chemical substances, then the meme is distributed in the information environment: on storage media, in human memory, and also on the network. Just as genes compete for resources: chemicals and memes compete for information space. For a variety of reasons, fairly strong correlations can be observed between the spatial distribution of genes and memes.

Gene Properties

2. discreteness - immiscibility of genes;

3. stability - the ability to maintain the structure;

4. lability - the ability to mutate many times;

5. multiple allelism - many genes exist in a population in a variety of molecular forms;

6. allelism - in the genotype of diploid organisms there are only two forms of the gene;

7. specificity - each gene encodes its own trait;

8. pleiotropy - multiple gene effect;

9. expressivity - the degree of expression of a gene in a trait;

10. penetrance - the frequency of manifestation of a gene in the phenotype;

11. amplification - an increase in the number of copies of a gene.

Classification

Depending on the functions they perform, genes are divided into

1. Structural genes - genes that control the synthesis of structural proteins or enzymes

2. Regulatory genes - genes that control the synthesis of various proteins that affect the activity of structural genes. Regulatory genes, in turn, are divided into:

Genes - modifiers - enhancing and reducing the activity of structural genes.

Genes - suppressors - inhibitory activity of structural genes

According to their influence on the viability of organisms, genes are divided into:

1 Lethal genes - genes that lead to the death of their carriers

Question 71. The structure of the gene in prokaryotes. Operon.

Sublital genes - genes that lead to a violation of the reproductive function (sterility, reduced viability or non-viability of the offspring) of their carriers

3. Neutral genes - not affecting the viability of the organism.

The structure of the structural genes of prokaryotes and eukaryotes is specific. In prokaryotes, in most cases, the coding region is continuous; in eukaryotic genes, along with regions encoding a product specific for this gene (polypeptide, ribosomal RNA, transfer RNA), there are non-coding regions. The coding regions of the gene were, as already mentioned, called exons, non-coding - introns. In a structural gene, exons alternate with introns. The gene is broken.
The number and intragene localization of introns are characteristic of each gene. The sizes of introns are different (from several tens to several thousand nucleotide pairs). Introns in a gene often have more nucleotides than exons. The role of introns is little studied. If they did not perform certain functions, they were not needed by the body, they would be eliminated natural selection.
The study of the gene continues. Modern information allow us to speak of a gene as a section of a genomic molecule nucleic acid, representing a unit of function and capable of changing and acquiring different states through mutation and recombination. This is a complex, but functionally integral unit of heredity.

Eukaryotic genes, unlike bacterial ones, have a discontinuous mosaic structure. Coding sequences (exons) are interspersed with non-coding sequences (introns).

The genetic apparatus of eukaryotic cells.

The genetic apparatus of the cell.

• genome- the genetic material of the nucleus in the haploid set of chromosomes;

The functional unit is the gene.

• Plasmon- the genetic material of the cytoplasm;

The functional unit is the plasmogen.

1962 - D. Gurdon - frogs, experience, stood at the origins of animal cloning.

The role of chromosomes in heredity:

• 1882 - Flemin described the behavior of chromosomes during mitosis;
• 1902 - Theodore Boveri, Walter Setton suggested that genes are located on chromosomes;
• 1909 - Thomas Morgan, Karl Bridges, Alfred Sturtevant experimentally proved the relationship of hereditary material with chromosomes.

Chromosomal theory of heredity.

1. Each chromosome represents a unique linkage group of genes. The number of linkage groups is equal to the haploid set of chromosomes.

2. Genes in the chromosome are arranged in a linear order and occupy a certain place - locus.

3. M-du homologous chromosomes exchange is possible allelic genes –crossing over, which breaks the linkage of genes and provides recombination of genes

4. Crossover frequency is a function of the distance between genes:

the greater the distance between genes, the greater the likelihood of crossing over.

5. The frequency of crossing over depends on the strength of linkage between genes:

The more closely linked genes are, the less likely they are to cross over.(full and incomplete clutch).

Carl Erich Correns (1908) - experiments with " night beauty”, which describes the phenomenon of variegation. The uneven color of the leaves is due to the uneven distribution of chloroplasts during division.

Boris Ephrussi discovered methochondrial inheritance in mammals in 1949.

In 1981, human mitachondrial DNA was sequenced (the exact nucleotide sequence was determined)

Mitochondrial DNA.

• Ring double-helix;
• Contains 37 genes:

encode 13 proteins, 22 t-RNA molecules, 2 r-RNA molecules;

• Genes do not contain introns;
• The traits are maternally inherited and are not Mendelian.

Volume of the mitochondrial genome 200 thousand times less nuclear.

• Replicates independently of nuclear DNA.
• The constancy of presence in the cell,
• ability to self-doubling,
• uniform distribution of genetic information between daughter cells during division.

Chemical composition of chromosomes.

• Histone proteins - H1, H2a, H2b, H3, H4 have basic properties
• Non-histone proteins are acidic
• Lipids (phospholipids, free fatty acids, xc and tg)
• Polysaccharides
• metal ions

The despiralized form of the existence of chromosomes in a non-dividing nucleus is called chromatin.

Deoxyribonucleoprotein complex (DNP).

The degree of chromatin compaction changes during the metotic cell cycle and determines the genetic activity or inactivity of chromosomes.

The higher the degree of compaction, the lower the genetic activity.

Compaction levels:

1. Nucleosomal:

Can only be obtained artificially.

The chromatin fibril looks like a string of beads.

Histone proteins 4 classes (H2a, H2b, H3 and H4) form histone actamers.

A DNA molecule wraps around histone actamers, making 1.75 turns.

There is a free linker site.

In this state, the DNA molecule is shortened to 6-7 times.

fibril diameter 10 nm.

Characteristic for G1 interphase period.

2. Nucleomeric.

The chromatin fibril acquires the structure of a solenoid due to the connection of neighboring nuclei due to the incorporation of the H1 protein into the linker region.

The fibril diameter is 30 nm.

The compactization coefficient is 40 times.

Characteristic for the G2 period of interphase.

3. Chromomeric.

Compaction occurs with the participation of non-histone proteins with the formation of loops.

2. The concept of a gene. Structural organization of genes in prokaryotes and eukaryotes. Classification of genes.

Characteristic for the beginning of the prophase of mitosis.

The fibril diameter is 300 nm.

The compactization factor is 200-400 times.

4. chromonemic.

The loops are stacked.

The compactization factor is 1000 times.

The fibril diameter is 700 nm.

characteristic of the end of the prophase of mitosis.

5. Chromosomal.

The maximum degree of chromatin spiralization is achieved.

fibril diameter 1400 nm.

Compactization coefficient 104-105.

characteristic of the metaphase of mitosis.

The structure of the metaphase chromosome.

Consists of 2 chromatids connected by a primary constriction or centromere. In the region of the centromere is kinetochore- the site to which the fission spindle threads are attached. The primary constriction determines the shape of the chromosome, dividing it into 2 arms: p is a short arm, q is a long one.

By form:

• Metacentric p=q
• Submetacentric p
• Acrocentric p<

To accurately identify chromosomes, the centromeric index is used: the ratio of the length of the short arm to the length of the entire chromosome.

There are also satellites (in acrocentric in humans), connected by a secondary constriction.

The secondary constriction contains genes responsible for the synthesis of ribosomal RNA.

Telomeres- terminal regions of chromosomes.

The role of telomeres:

• Mechanical function (attached to the shell of the nucleus), prevent chromosomes from sticking together, which can lead to the formation of dicentrics.
• Stabilization - protects chromosomes from degradation by cellular nucleases (enzymes that destroy)
• Influence gene expression - the activity of genes located near telomeres is reduced
• Regulate the number of cell divisions in the absence of telomerase.

Varieties of chromatin:

1. Euchromatin - weakly compacted, genetically active, replicated at the beginning of interphase, adenine, thymine predominate, contains all structural genes

1 and 2 compaction level

2. Heterochromatin - highly compacted, genetically inactive, replicated later than chromatin, guanine cytosine predominate, other classes of genes are included. The loss of plots is not affected.

3 and 4 levels of compaction.

• Permanent: located in the telomeric regions of chromosomes and located in the centromere area, functions: regulates the work of structural genes, participates in the formation of a synaptic complex by many homologous chromosomes during meiosis.
• Optional: temporarily inactive euchromatin, an example of which is sex chromatin.

Sex chromatin.

1949 M. Barr and L. Bertram discovered sex chromatin in the interphase nuclei of cat neurons.

Normally, the number of clumps of sex chromatin is 1 less than the number of X-sex chromosomes.

Mammalian female zygote has two functionally active X chromosomes

On the 16th day of embryogenesis, one X chromosome is inactivated in all somatic cells of the embryo. The inactivation process is random.

The meaning of sex chromatin.

Test, Only in somatic cells! To determine the sex of the fetus! For the diagnosis of chromosomal diseases associated with a change in the number of sex chromosomes.

In big sport as sex control.

Karyotype - the chromosome complex of somatic cells of a certain type of plants and animals

Indicators:

• number,
• forms and
• sizes from 0.1 to 10 microns of chromosomes.

1956 - Y. Tio and A. Levan studied the human karyotype.

Chromosome rules.

• Species constancy of the number of chromosomes;
• Chromosome pairings;
• Individuality of chromosomes;
• Chromosome continuity - capable of self-duplication (chromosome comes from chromosome);

In the karyotype, autosomes (the same for both sexes) and sex chromosomes are distinguished. Or 22 pairs.

The method of studying the karyotype - karyological analysis - underlies the cytogenetic method.

The essence is the study of preparations of metaphase chromosomes.

№5

Gene is a fragment of a DNA molecule containing regulatory elements and a structural region, and corresponding to one transcription unit, which determines the possibility of synthesizing a polypeptide chain or an RNA molecule.

The prokaryotic gene is called an operon, it consists of two main sections:

• regulatory (non-informative),
• structural (informative).

In prokaryotes, regulatory elements account for about 10%, and structural elements for 90%.

The structural region of prokaryotic genes (transcription unit) can be represented by one coding region, which is called a cistron, or by several coding regions (polycistronic transcription unit).

The structural zone encodes information about the sequence of amino acids in the form of a genetic code. mRNA is read from the structural region. If prokaryotes have a polycistronic transcription unit, several types of mRNA can be simultaneously synthesized in one structural region.

Regulatory elements of prokaryotic genes include areas that control the operation of the gene:

• promoter,
• operator,
• Terminator.

The promoter determines the start of transcription (site of initiation). The RNA polymerase enzyme, which synthesizes mRNA, is connected to the promoter.

GENE STRUCTURE OF PRO- AND EUKARYOTES

Another element that controls the transcription process is an operator located near or within the promoter. This site can be free, then the RNA polymerase connects to the promoter and transcription begins. If the operator is bound to a repressor protein, the RNA polymerase cannot bind to the promoter normally and transcription is not possible. The next regulatory element, the terminator, is located behind the structural region and contains the transcription stop signaling site.

The mechanism of functioning of the protein synthesis regulation system was discovered in 1962 by Jacob and Monod while studying the cultivation of Escherichia coli in a lactose medium and called the lac operon.

Simplified, this mechanism can be described as follows. Based on the information of the regulator gene, a repressor protein is synthesized; if it is active, it binds to the operator gene, blocking the path for RNA polymerase - the process of translation and subsequent protein synthesis is turned off (prohibited). If an inducer appears (for example, lactose in the lac operon), it attaches to the repressor protein, rendering it inactive. The operator becomes active and turns on the process of reading information from structural genes - it allows translation. Information is read from DNA, the synthesis of the necessary protein, an enzyme, begins (for example, β-galactosidase in the lac operon).

This is just one of the possible mechanisms, which is called forbidding induction. There are other mechanisms of regulation of protein synthesis: permissive induction, permissive and inhibitory repression, in which apoinductors and corepressors take part.

The structure of genes in eukaryotes is much more complex. The genetic system of eukaryotes is called a transcripton. The transcripton also consists of two parts:

• regulatory (non-informative),
• structural (informative),

the relative proportion of which is opposite to the genes of prokaryotes: the share of the regulatory site is 90%, structural - 10%.

The regulatory region is a series of successively located promoters and operators and several terminators. The structural region consists of one transcription unit and has a “discontinuous” structure: coding regions (exons) alternate with non-coding regions (introns). At the same time, only one mRNA molecule can be synthesized in eukaryotes in the structural region, however, due to the presence of alternative splicing at different times (depending on the needs of the cell), different types of mRNA (from one to several tens) can be synthesized on the same structural region.

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The eukaryotic genome is more complex than that of prokaryotes and includes the nucleotide sequences of chromosomes, mitochondrial and plastid DNA (1-10% of the total genome, up to 20% in yeast), plasmid DNA in yeast, DNA of latent and defective viruses.

eukaryotic nucleus well expressed, there is a nuclear membrane surrounding the chromosomes. There are many chromosomes, they are paired, consist of homologous chromatids, each of which represents a double-stranded DNA molecule ( set of chromosomes is diploid ). The chromosome consists of 50% DNA and 50% proteins, which are represented major histone proteins , which are part of the nucleosomes, and acidic proteins , which fill the nucleosome cavity, loosen it, and play an important role in the disintegration of nucleosomes before the start of transcription and replication.

In a relaxed state, eukaryotic chromosomes can reach several centimeters (in humans, up to 5 cm in length). There are several stages of chromosome condensation, as a result of which the chromosome becomes compact, wraps around nucleosomes and forms more complex folded structures.

Stages of compactization (condensation) of chromosomes. The acts of condensation and decondensation of chromosomes replace each other in the cell cycle: in the interphase, DNA looks like elongated tangled threads and is called - chromatin . In this state, DNA is partially relaxed, which facilitates the process of transcription and replication. For divergence ( segregation ) chromosomes in mitosis, it is very important that the chromosomes are supercoiled - condensed. To do this, at the beginning of the prophase of mitosis, DNA begins to compact with the help of positive and negative supercoiling, as well as by wrapping around nucleosomes. The nucleosomal strand of DNA resembles beads, in which the thread (supercoiled DNA molecule) is wound around beads (nucleosomes).

Rice. 3.1. Stages of chromatin compaction

Nucleosome- an octomer of 8 subunits of histone proteins, including 2 molecules of histones H2A, H2B, H3, H4. Nucleosome diameter - 11 nm, height - 5.7 nm. Along the edges of the nucleosomes there are free DNA sections of 20-90 base pairs - linkers . Histone H1 is not part of the nucleosome, but fixes the linker loops, holding the DNA on the nucleosome. Such nucleosomal structure of chromosomes characteristic only of linear eukaryotic chromosomes.

As a result of spiralization and winding around nucleosomes, chromosomes shorten and turn into metaphase chromosomes (metaphase stage), shortening by 10,000 times in length and about 700 times in diameter. This contributes to the normal divergence (segregation) of chromosomes in the anaphase of mitosis. X-ray diffraction analysis revealed the following stages of DNA compaction.

1st stage - double-stranded DNA helix (diameter - 2 nm), usually in a right-handed B-form.

2nd stage– nucleosomal thread (diameter – 11 nm). DNA is wound around particles of nucleosomes, forming 1.75 turns (146 base pairs) on them.

3rd stage– formation of a chromatin fibril (diameter 30 nm). Nucleosomes approach each other, a zigzag "ribbon" is formed, which twists into solenoid - a spiral with a cavity inside.

4th stage– the formation of loop domains (300 nm in diameter) is formed by forming loops from the solenoid filament.

5th stage- the formation of metaphase chromosomes, which are called "lamp brushes" (diameter 1400 nm).

Redundancy of eukaryotic genomes. Only a small part of DNA in eukaryotes is represented by structural and regulatory genes, the rest of the genome is "selfish" (satellite) DNA, which apparently got into the eukaryotic genome through the integration of viruses and other mobile genetic elements. The human genome has 3.5 x 10 9 base pairs. Mammalian genomes differ, but have close values ​​of the molecular weight of chromosomes, reaching hundreds of billions Da. In accordance with the size of the human genome, a person should have had 150,000 or more genes, but in 2003 American scientists announced the existence of 30,000 genes, in recent years there are supposed to be 75 thousand genes, the rest of the genomic DNA is obviously “genetic garbage”. A significant part of the genome is represented by non-coding sequences. In humans, non-coding sequences make up 80-85% (according to other sources - 92%), and in plants - up to 90% or more, i.e. characteristic genome redundancy .

The eukaryotic genome contains the following DNA sequence types :

1)repetitive sequences which there are more than 10 5 repeats per genome. Most often, these are blocks of 5-8 nucleotides, which are repeated in tandem and form fragments of 150-500 base pairs, for example - (AATAT) 30-100. Their function is not fully known, but it is assumed that they can play a role in the regulation of genes - they are located in the region of centromeres, telomeres, introns, transposons. These are the sequences: Alu, B1 , B2, L1. Restriction sites in palindromes are very common among repetitive sequences (see below - the topic "Repairs"). Restriction sites can be those hotspots where plasmids, transposons, viral DNA, transgenes are inserted.

2) moderately repetitive sequences- occur on the genome from 10 to 10 5 . These include sequences encoding histones, ribosomal proteins, r-RNA and t-RNA, IS elements, insert sequences.

3) multigene families - these are groups of genes similar in structure and functions that are “turned on” at different stages of ontogenesis. For example, the hemoglobin b-chain is encoded by 7 genes, 2 of which are defective (pseudogenes), the remaining 5 are switched on sequentially at different stages of development: in early embryogenesis, in the fetal period (8-9 weeks), in childhood, adolescence and adulthood.

4) unique genes - specific genes that encode the synthesis of structural and enzymatic proteins.

The structure of eukaryotic genes. Eukaryotic genes have regulatory elements similar to prokaryotes - promoter And terminator zones between which the DNA sequence that directly codes for the protein is located. Regulatory elements of genes are very important, because it is thanks to them that genes are “turned on” only when there is a need for the corresponding protein products. The promoter zone provides the start of transcription and translation, and the terminator zone provides the end of these processes.

The following conservative sequences can be distinguished in promoters: GC motif, CAAT, TATA, AGGAG, initiating codon ATG (AUG on RNA). Next comes the structural part of the gene, which consists of exons and introns. The structural part of the gene is followed by the terminator zone, represented by the TTA termination codon (TAG or TGA) and the terminator. On fig. 3.1. the main sections of the eukaryotic gene are presented.

Rice. 3.2. Fine structure of a eukaryotic gene

Designations and explanations for fig. 3.2.

Functions of the main regulatory elements of the gene

· HZ motif – one of the most common regulatory elements of a gene. Represented by palindrome YYTsYYY / TsTsGTsT , is found in genes for general functions, that is, those that are expressed in all cells of the body and play an important role in their life support. This site is obviously a transcription operator. Attachment to the GC-motif of the protein-regulator SP1 increases transcription by 10-20 times.

· CAAT is a region of the gene promoter, which, apparently, is recognized by RNA polymerase before the start of transcription. Obviously, this site performs the same function as in prokaryotes. TTGACA (Gilbert block). CCAAT occurs in tissue-specific genes, that is, those that are expressed only in certain tissues and organs. So, the insulin gene is turned on mainly only in the cells of the islets of Langerhans of the pancreas, the alpha-fetoprotein gene - in an adult, only in the liver cells.

· Hognes block - TATA (TATAAAA or TATAATA) , similar Pribnov block (TATAAT) in prokaryotes, it serves to attach RNA polymerase to DNA in the promoter zone, its position in the gene relative to the zero point of the start of transcription is (-30).

· ribosome binding site contains a reduced Shine-Dalgarno sequence AGGAG (see functions of the Shine-Dalgarno sequence AGGAGG in prokaryotes, topic "Prokaryotic genomes").

· start codon represented by triplet ATG (AUG - on RNA), transcribed as part of messenger RNA, translation begins with it. In the synthesis of a polypeptide on the ribosome, this codon corresponds to the amino acid methionine. The synthesis of most proteins begins with methionine.

· structural part of a gene is the DNA sequence that directly codes for the protein itself. In eukaryotes, unlike prokaryotes, it is not whole, but consists of exons (coding regions) and introns (intercalary non-coding regions).

· termination codon - a site that is transcribed into mRNA and ensures the end of translation on ribosomes. On DNA it is represented by nonsense codons - triplets TAA, TAG, TGA, on RNA they correspond to UAA, UAG and UGA. These triplets do not correspond to any of the amino acids; therefore, the synthesis of the polypeptide is terminated on them in the ribosome.

· terminator region obviously represented in each gene by a specific nucleotide sequence.

In the eukaryotic genome, specific regulatory sequences have also been found that can act as enhancers – transcription enhancers, as well as sequences that act as silencers transcription silencing. They can be located at a considerable distance from the gene that regulates, moreover, the same sequences in one cell can be enhancers, and in another - silencers. They regulate gene expression.

Regulatory proteins have also been found that can bind to the promoter region of the gene and provide either activation or suppression of transcription. Thus, the regulatory protein SP1, binding to the GC motif, can enhance transcription by 10–20 times.

The structure of eukaryotic genes. The genes of eukaryotic organisms have the following characteristics:

Single, i.e. unlike prokaryotes, they are not assembled into operons;

Sometimes oligomeric (represented by cluster genes);

Intermittent, i.e. divided into introns and exons;

Overlapping, i.e. Several reading frames can function within one gene region of DNA.

Genetic analysis in eukaryotes, in particular in their simplest representatives, yeast and neurospores, has shown that genes that control different stages of the same metabolic pathway are, as a rule, randomly scattered throughout the genome and usually do not form clusters like bacterial operons. However, several exceptions were found, namely: a compact region of DNA in fungi controls 3 reactions in histidine biosynthesis. A similar situation was found in the study of the genetic control of the biosynthesis of aromatic amino acids (tryptophan, tyrosine, phenylalanine), as well as fatty acids. The researchers had the impression that they were dealing with an operon-like structure encoding a multi-enzyme complex. In fact, it turned out (using mutational analysis) that in fungi all 5 stages of the biosynthesis of aromatic amino acids are controlled by 1 gene, the product of which is a long polypeptide chain with a mass of 150,000 D. This is not an operon, but cluster-gene . Such cluster genes are quite common in eukaryotes. Examples are the following cluster genes:

· his 4 – cluster gene for histidine biosynthesis in saccharomycetes yeast, encodes a single polypeptide with three enzymatic activities;

· arom 1 – gene cluster for the biosynthesis of aromatic amino acids in neurospores, encodes a single polypeptide with five enzymatic activities;

· fas 1 – the first gene cluster for the biosynthesis of fatty acids in saccharomycetes yeast, encodes a polypeptide with three enzymatic activities

· fas 2 – the second cluster gene for fatty acid biosynthesis in saccharomycetes yeast, encodes a single polypeptide with five enzymatic activities.

The existence of cluster genes is an example molecular oligomerization . Obviously, reading information about several enzymes of the metabolic pathway from the cluster gene at once is “economically” more beneficial for the cell, as in prokaryotic operons. In contrast to the operon of bacteria, transcription and subsequent translation on ribosomes results in the synthesis of one long polypeptide molecule in cluster genes, in which individual domains, after spatial folding into a tertiary structure, begin to perform the functions of individual enzymes. In prokaryotic operons, individual operon genes are usually translated into independent protein products.

Most of the eukaryotic genes are single, that is, during the evolution of eukaryotes, gene autonomy. Apparently, this creates favorable conditions for separate, and hence finer regulation of the functions of individual genes. Recall that in prokaryotes, all operon genes are often subject to regulation at once, with the exception of autogenic control, when the regulator gene is located among the structural genes within the operon and allows the operon to be regulated by separate blocks.

eukaryotic genes are discontinuous, namely, they consist of coding regions – exons , and not coding - introns. This gene structure is called intron-exon or mosaic structure. The length of exons reaches 1000 base pairs, and introns - usually 5000-20000 base pairs. The structural part of a gene may include 2-3 (sometimes more) exons separated by long introns. And although there are usually few introns, their number in different species and in different genes can range from 0 (in the histone genes) to 51 (in the collagen structural gene). There are always more exons than introns, but introns have 5-7 times more base pairs than exons because introns are longer. Depending on the number of exons and introns, as well as their length, the length of the eukaryotic gene depends. In different organisms, it can vary greatly. So, in Drosophila, the average length of a gene is 2 yew. bp, and the length of the silk fibroin gene in the silkworm reaches 16 yew. b.s.

The existence of introns in the structural part of the gene creates certain difficulties for the implementation of genetic information, since the transcribed mRNA contains "extra" DNA sections, which subsequently should not be translated on ribosomes. How is this problem solved in a eukaryotic cell? The solution was found by the American scientist Philip Sharp from the Massachusetts Institute of Technology, who discovered the phenomenon of splicing (from English to shift - to sew without knots).

splicing mechanism. First, in the nucleus, a complete DNA sequence is transcribed from a portion of the chromosome (gene) with the formation of pro-i-RNA - an immature, longer RNA that contains both exons and introns. Further, when pro-i-RNA is sent from the nucleus to the cytoplasm, when passing through the nuclear membrane, splicing - maturation of pro-i-RNA, as a result of which introns are cut out, and exons are sewn together using an enzyme called maturase . For the implementation of splicing, an important role is played by special sRNA (up to 160 nucleotides long), which pull together the ends of the introns, which contributes to their excision and subsequent stitching of exons. Mature mRNA, which has no introns, enters the cytoplasm for translation into ribosomes.

Introns are not always non-coding regions. For example, in the mitochondrial genes of yeast, introns encoding the synthesis of the maturase enzyme, which is involved in the excision of introns, were found. Some yeast genes contain introns encoding cytochrome B, etc.

Splicing is carried out by protein complexes called spliceosomes. In addition to the already mentioned maturases and sRNA, spliceosomes also contain proteins that give pro-i-RNA the desired conformation. In addition, the spliceosome is associated with enzymes that carry out polyadenylation of the 3/-terminus of the mRNA.

splicing types: simple; alternative; transsplicing; autosplicing.

Simple splicing characteristic of simple genes, the sequence of exons of which is intended for the synthesis of only one protein. In such genes, exons always occupy a fixed position on DNA, and the removal of introns is always carried out at clearly marked points.

Alternative splicing characteristic of gene regions on which several proteins are encoded at once. In this case, the same sections act either as exons or as introns. Thus, a pituitary neuropeptide and a parathyroid hormone are encoded on the same stretch of DNA. Depending on the excision of certain sections of DNA, an mRNA is formed that encodes a particular protein. Alternative splicing occurs in the synthesis of immunoglobulins (antibodies) and in the synthesis of tissue compatibility antigens (MHC).

transsplicing p occurs when exons from different genes combine into one mRNA molecule. Characteristic for the synthesis of components of the cell cytoskeleton.

Autosplicing first discovered in the macronucleus of ciliates, and later in bacteria, fruit flies and other eukaryotes. Autosplicing is the self-cutting of pro-mRNA without the participation of maturases and other enzymes. RNA that cuts out its own introns is called ribozyme . Autosplicing indicates that the first molecule carrying genetic information in evolution was RNA. She performed both genetic and catalytic functions, later transferred to DNA and proteins, respectively.

How did non-coding introns form in the structure of genes? There is a hypothesis that even at the dawn of the evolution of eukaryotes, they were infected with viruses and, due to the integration of the viral DNA into the genome, excess DNA appeared in the chromosomes. satellite (selfish) DNA . It is present not only in the intron sequences of genes, but is also scattered along the entire length of chromosomes in the form of huge inserts of non-coding sequences.

Eukaryotes, like viruses, have overlapping genes , namely, on the same DNA region, transcription can begin from different points (and/or on different chains) with the formation of different mRNAs encoding different polypeptides.

Replication in eukaryotes multiple, in each chromosome there are 20-100 replication origins and a corresponding number of replicons. Replication in them may not take place simultaneously, however, cell division does not begin until all chromosomes are replicated along their entire length. Replication is discussed in detail in a separate lecture (see above).

Transcription and broadcast in eukaryotes, they are disconnected due to the presence of a nuclear membrane, namely, transcription is carried out in the nucleus, and the resulting messenger RNA must be transported from the nucleus to the cytoplasm for subsequent protein synthesis (translation) on ribosomes. It has already been mentioned that when crossing the nuclear membrane, splicing occurs, i.e. i-RNA maturation. All these processes take time, therefore, from the moment of initiation of transcription to the appearance of a protein product in the process of translation, 6-24 hours pass. For comparison: in prokaryotes, this time is 2-3 minutes.

The entire nucleotide sequence of the gene takes part in encoding the mRNA sequence, however, special enzymes cut out part of the mRNA sections in the process of its so-called maturation, and those that remain form secondary mRNA, from the matrix of which, in fact, protein synthesis occurs.

The segments of DNA that are excised during mRNA maturation are called introns, and those that are then stitched together and serve as a matrix for translation - exons. As a rule, one gene contains from three to five introns. However, it should be noted that there are no introns and exons in the genes of prokaryotes.

Science does not yet know the reason for the complex structure of prokaryotic genes. However, there are several factors that explain this. Since intron-exon organization is a property of eukaryotic genes only, it is a reasonable assumption that such a complex structure of genes is a progressive evolutionary adaptation of eukaryotic organisms. It is believed that, first of all, this may be a mechanism that limits the mutation process. At the same time, introns perform the function of “traps” for mutations. After all, a change in nucleotide sequences in parts of a structural gene is not encoded, will not lead to mutations and the appearance of amino acid substitutions. In addition, if a nucleotide is inserted or dropped out in one of the exons, then this will lead to a shift in the synthesis frame not of the entire mRNA, but only of some of its parts, that is, the effect will not be so detrimental. Obviously, the complex structure of the gene ensures its higher stability and reliability of functioning.

In addition to structural genes encoding one or another form of RNA, the genome of all organisms also contains regulatory genes that determine the start, speed, and sequence of RNA synthesis processes on the DNA template. They are the site of attachment of enzymes and other proteins involved in replication and transcription, and regulate the activity of genes. Regulatory genes are small, only 20–80 bp each, and therefore, compared to structural genes, they occupy much less space in the genome. However, without these genes, which do not encode specific proteins, but only regulate the processes of DNA replication, the interaction of DNA with certain proteins and enzymes, the passage of chromosome conjugation, the functioning of the genetic apparatus is just as impossible as without hormones (substances synthesized by the body in minimal quantities) - the vital activity of the human body.

• A promoter is a regulatory DNA sequence (not transcribed) located to the left of the transcription start point.
• 5' - untranslated region (leader) - starts from the start of transcription to the start codon (transcribed, but not translated, is part of the mature mRNA)
• Coding region - exons and introns (transcribed, but introns are excised from pre-mRNA, only exons remain in mature RNA)
• 3' - unbroadcast area (trailer) – starts from stop codon to sequence AAAAAA (polyadenylation site).

Exons and introns. Most eukaryotic genes have a discontinuous structure, they contain coding sequences - exons, and non-coding sequences - introns.

Genes start and end with exons, but there can be any set of introns. For example, globin genes have 3 exons and 2 introns. Other genes may have a large number of introns (20 or more). For example, the gene encoding the synthesis of the low-density lipoprotein (LDL) receptor protein has 40 introns, while the dystrophin protein gene has 79 introns. Introns are larger than exons, so exons make up a very small fraction.

In a mature mRNA molecule, only exons are present, and introns are excised from the primary transcript (pre-RNA) in the process splicing. Therefore, the sizes of mature eukaryotic mRNAs are always smaller than the sizes of the gene itself and pre-RNA.

Not all eukaryotic genes contain introns. The genes of histone proteins, interferon genes, mammalian and human mitochondrial genes do not contain introns.

Exons are the DNA coding sequences of eukaryotic genes present in the mature RNA molecule.

Introns are non-coding regions of eukaryotic genes that are transcribed but then excised from the primary transcript during splicing and are not part of mature RNA, i.e. are not broadcast.

New gene concept. Thanks to alternative splicing, several different variants of mature mRNA molecules with a different set of exons can be obtained from one molecule of the primary transcript (pre-mRNA). In this regard, we can talk about a new concept of the gene: one gene - a lot of RNA - a lot of polypeptides and formulate a new definition of a gene. A gene is a section of a DNA molecule that codes for the synthesis of one or more functional RNA molecules.

FSBEI HPE "Penza State University"

Pedagogical Institute. V.G. Belinsky

Department of "Biology, methods of teaching biology and life safety"

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 genes in pro- and eukaryotes

The structure of the prokaryotic operon

Regulatory regions and structural genes

Activator, promoter, operator and terminator

Start codon, terminator

Conclusion

Bibliography

Introduction:

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

Approximately such a 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 M. Singer and P. Berg in Genes and Genomes (1998). It is formulated as follows. "A gene is a set of DNA segments that determine the formation of either an RNA molecule or a protein product." In this definition, first of all, it is unambiguously emphasized that a gene is not one continuous segment 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 genes in pro- and eukaryotes

The genome of modern prokaryotic cells is characterized by a relatively small size. In Escherichia coli, it is represented by a circular DNA molecule about 1 mm long, which contains 4 106 base 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 as a nucleoid - a complex of DNA with non-histone proteins.

In eukaryotes, the amount 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 even larger, reaching 1010 and 1011 bp. Unlike prokaryotes, eukaryotic cells simultaneously actively transcribe from 1 to 10% of DNA. The composition of the transcribed sequences and their number depend on the cell type and ontogeny stage. A significant part of the nucleotide sequences in eukaryotes is not transcribed at all - silent DNA.

A large amount of eukaryotic hereditary material is explained by the existence in it, in addition to unique, also 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 primarily in the heterochromatin surrounding the centromeric regions. They are not transcribed. About 20% of the mouse genome is formed by moderate repeats occurring at 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 code amino acid sequences of proteins.

At present, the functions of silent DNA, which makes up a significant part of the genome, is replicated but not transcribed, have not been finally elucidated. There are suggestions about the certain significance of such DNA in providing the structural organization of chromatin. Some of the non-transcribed nucleotide sequences are obviously involved in the regulation of gene expression.

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. In prokaryotic (bacterial) cells, plasmids were found that carry hereditary material that determines properties such as the ability of bacteria to conjugate, as well as their resistance to certain medicinal substances.

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 2 to 10 copies of mtDNA.

Extrachromosomal DNA makes up only a small part of the hereditary material of a eukaryotic cell. For example, human mtDNA contains 16569 bp. and it accounts for less than 1% of all cellular DNA.

Unlike chromosomal DNA, mtDNA is characterized by a high "gene density". They do not have introns, and intergenic gaps are small. The human circular mtDNA contains 13 protein-coding genes (3 cytochrome C-oxidase subunits, 6 ATPase components, etc.) and 22 tRNA genes. A significant part of mitochondrial and plastid proteins is synthesized in the cytoplasm under the control of genomic DNA.

If most of the nuclear genes are present in the cells of the body in a double dose (allelic genes), then the 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, mtDNA is identical. The set of genes located in the cytoplasmic DNA molecules is called a plasmon. It defines a special type of trait inheritance - 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, testify in favor of the unity of their origin from a common ancestor, in which the problem of self-reproduction and recording of information has already been solved 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 hereditary material, different ways of realizing hereditary information and regulating these processes.

Numerous indications of differences in genome organization, details of gene expression processes and mechanisms of its regulation in pro- and eukaryotes testify in favor of 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-reproducing nucleic acid molecules that did not initially carry the function of encoding amino acids in proteins. Due to the ability to self-reproduce these molecules persisted over time. Thus, the initial selection went to the ability to self-preservation through self-reproduction. In accordance with the above assumption, some parts of DNA later acquired the function of coding, i.e. became structural genes, the totality of which at a certain stage of evolution constituted the primary genotype.

The expression of the resulting DNA coding sequences led to the formation of a primary phenotype, which was evaluated by natural selection for the ability to survive in a particular 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 replication, but not carrying a functional load in relation to the formation of the phenotype. It is assumed that 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 sufficiently large volume. Probably, at the early stages of the evolution of the simplest cellular forms, they did not yet have the main mechanisms of information flow (replication, transcription, translation) worked out perfectly. The redundancy of DNA under these conditions made it possible to expand the volume of coding nucleotide sequences at the expense of non-coding ones, providing the emergence of many options for solving the problem of the formation of a viable phenotype.

The structure of the prokaryotic operon

An operon is a method of organizing the 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 the French scientists Jacob and Monod, for which they received the Nobel Prize in 1965.

Operons are classified according to the number of cistrons 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 steps in the biosynthesis of a substance. There may be one, two or more of these genes. They are closely linked to each other and, most importantly, during transcription they work as one single gene: one common mRNA molecule is synthesized on them, which is only then split into several mRNA corresponding to individual genes. The regulatory elements are:

promoter - the binding site of the enzyme that carries out the transcription of DNA - RNA polymerase. It is the site of the start of transcription. It is a short sequence of several tens of nucleotides of DNA, with 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 in which RNA polymerase is disconnected 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 necessary 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 detaches 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, transcription stops.

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

There are two types of regulatory proteins: a repressor protein or an activator protein. They attach to specific nucleotide sequences of the operator's DNA, which either prevents gene transcription (negative, negative regulation) or promotes it (positive, positive regulation); their mechanisms of action are opposite. In addition, the work of repressor proteins can be influenced by substances - effectors: when combined 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 for the implementation of any biochemical reaction.

Genetic information about the structure of proteins and nucleic acids in all organisms is enclosed 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 inductor. One of the most illustrative 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. Literally a few minutes after adding lactose to the nutrient medium for E. coli, bacteria begin to produce three enzymes: galactoside permease, beta-galactosidase and galactoside transacetylase. As soon as the resources of lactose in the medium are exhausted, the synthesis of enzymes immediately stops.

The above example will become more understandable when considering the scheme of the lactose operon (Fig. 81), the study of which allowed the French scientists F. Jacob and J. Monod to develop the concept of the operon itself and clarify the basic principles of transcription regulation in prokaryotes. The operon starts from the A site, which is intended for attaching some activator protein, which, in turn, is necessary for attaching to the RNA polymerase promoter (P) following the A site. The promoter, whose nucleotide sequence is recognized by RNA polymerase, is followed by an operator (O), which plays an important role in the transcription of operon genes, since a regulatory repressor protein binds to it.

The operator is followed by the structural genes for the three previously mentioned enzymes. The operon ends with a terminator that stops the progress of RNA polymerase and transcription of the operon.

The regulatory repressor protein is constantly synthesized in the cell in a small amount, 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 lactose is added 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 translation in progress. Synthesized enzymes transport into the cell and break down lactose. After all the lactose has been used up, there will be nothing to bind the repressor protein and it will again contact the operator, stopping the transcription of the operon. Thus, operon induction is caused by the fact that the regulatory protein does not attach to the operator. This type of induction is called negative.

Another well-known type of induction is positive induction. It is characteristic of another Escherichia coli feather, 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, freeing 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 arabinose reserves 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 (inductor) prevents the attachment of the repressor protein 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 case it prohibited it. The well-studied tryptophan operon of Escherichia coli can serve as an example of negative repression.

It consists of five structural genes that provide the synthesis of the amino acid tryptophan, an operator, and two promoters. The regulatory protein is synthesized outside the tryptophan operon. As long as the cell manages to use up all the synthesized tryptophan, the operon works, and tryptophan synthesis continues. If an excess of tryptophan appears in the cell, it combines with 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 prevents the transcription of structural genes, as a result of which the synthesis of tryptophan stops. With positive repression, the effector deprives the regulatory protein of the ability to bind to the operator, thus causing 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 should be coordinated. Such a 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, transcription of “early early”, then “early” and finally “late” genes occurs first, depending on which proteins are required at different stages of a viral (phage) infection.

Of course, the principle of cascade regulation in phages is one of the simplest. In more complexly organized organisms, for the implementation of 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 differ not only in the more complex organization of the genome, but also in 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 the performance of specific functions by specialized cells. 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 in 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 enter the ribosomes almost in the process of transcription, are translated, and then are quickly destroyed.

At the same time, there are many data indicating that transcription in eukaryotes is carried out from regions similar to prokaryotic operons 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 distinguishing 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 messenger RNA precursor (pro-mRNA) formed during this process then undergoes maturation (processing), which results in the maturation of intros and the formation of mRNA itself (splicing),

In eukaryotes, other types of regulation of gene activity are also known, such as the effect of position or dose 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 any gene (primarily this applies to genes localized in the sex chromosomes of the 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 . On the whole, 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 successive structural genes, the products of which are usually involved in the water and the same metabolic pathway. As a rule, an operon has one set of regulatory elements (regulatory gene, promoter, operator), which ensures the coordination of the processes of gene transcription and the synthesis of the corresponding proteins.

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

The sequence designated "-35" (TTGACA) is responsible for the 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 sequence of bases is most often found in promoters of prokaryotes, it 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 the case of base pair substitutions in the indicated promoter sequences, the efficiency and correct determination of the transcription start point, from which the RNA polymerase enzyme begins RNA synthesis, is impaired. In prokaryotes, along with the promoter, there are other regulatory regions: this is an activator and an operator.

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

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

There are various options for the interaction of regulatory sites with enzymes and regulatory proteins, and the latter with molecules called inductors (effectors).

The genetic information encoded in DNA with the help of 4 nucleotides (four-letter alphabet) in the process of protein biosynthesis is translated into the amino acid sequence of proteins (twenty-letter alphabet) with the help of tRNA adapter molecules ("translators"). Each of the 20 amino acids that make up proteins must attach to its own tRNA. These reactions take place in the cytosol and are catalyzed by twenty APCases (aminoacyl-tRNA synthetases). Each enzyme has a dual affinity: for its “own” amino acid and for its corresponding tRNA (one or more). ATP energy is used for activation.

The process consists of two stages occurring in the active center of the enzyme. At the first stage, as a result of the interaction of the amino acid and ATP, aminoacyl adenylate is formed, at the second stage, the aminoacyl residue is transferred to the corresponding tRNA.
Course of reactions: .Amino acid (R) + ATP + enzyme (ER E?) R (aminoacyl-adenylate) + FFN

ER (aminoacyladenylate) + tRNAR Aminoacyl-tRNA + AMP + E?R
APCaseR Summary Equation:

Amino acid (R) + tRNACR + ATP aminoacyl-tRNACR + AMP + FFN

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

So are all the activated amino acids necessary for protein biosynthesis combined with their corresponding adapters formed in the cytoplasm of the cell? various aminoacyl-tRNA (aa-tRNA).

Terminator (DNA)<#"justify">Conclusion

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

The body of prokaryotes, as a rule, consists of one cell. However, with incomplete divergence of dividing cells, filamentous, colonial and polynucleoid forms (bacteroids) arise. In prokaryotic cells, there are no permanent double-membrane and single-membrane organelles: plastids and mitochondria, the endoplasmic reticulum, the Golgi apparatus and their derivatives. Their functions are performed by mesosomes - folds of the plasma membrane. In the cytoplasm of photoautotrophic prokaryotes, there are various membrane structures on which photosynthesis reactions take place. They are sometimes called bacterial chromatophores.

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

The sizes of prokaryotic cells vary from 0.1-0.15 microns (mycoplasmas) 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 when analyzing the ratio of the number of genes to the amount of DNA in the genome of bacteria and mammals. If the average size of a bacterial gene is 1500 base pairs (bp), and the length of the circular DNA molecule of the chromosomes of E. coli and B. subtilis is over 1 mm, then in such a chromo a catfish can accommodate about 3,000 genes.

Approximately this number of genes was experimentally determined in bacteria by 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, a person has approximately 50,000 genes (meaning only 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) b.p. This means that the coding part of its genome is only 15–20% of the total DNA.

There are a significant number of species whose genome is ten times larger than the human genome, for example, some fish, tailed amphibians, lilies. Excess DNA is characteristic of all eukaryotes. In this regard, it must be emphasized not the unambiguity of the terms genotype and genome. The genotype should be understood as the totality of genes that have a phenotypic manifestation, while the concept of the genome refers to the amount of DNA that is in the haploid set of chromosomes. mosom of this type.

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.: Publishing house of Leningrad State University. - 1985. - 190 p.

Pekhov A.P. Bacterial genetics. M.: Medicine.-1977.-407 p.

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

Rhys E., Sternberg M. An Introduction to Molecular Biology: From Cells to Atoms. M.: Mir.- 2002.-142 p.

Sergeeva G.M., Pashkova E.I. Guide for independent work of students in molecular biology. Petropavlovsk: NKGU im. M. Kozybayeva.-2008.-234 p.

Khesin R.B. Impermanence 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.: Master-Press. - 2000.-343 p.

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