Analysis of the results of violations of linked inheritance of genes allows you to determine the sequence of genes in the chromosome and make genetic maps. How are the concepts of "crossover frequency" and "distance between genes" related? What is the importance of studying the genetic maps of various objects for evolutionary research?

Explanation.

1. The frequency (percentage) of crossover between two genes located on the same chromosome is proportional to the distance between them. Crossing over between two genes occurs less frequently the closer they are to each other. As the distance between genes increases, the likelihood that crossing over will separate them on two different homologous chromosomes increases more and more.

Based on the linear arrangement of genes on a chromosome and the frequency of crossing over as an indicator of the distance between genes, maps of chromosomes can be built.

2. In studies of the evolutionary process, genetic maps of different types of living organisms are compared.

Just as DNA analysis allows you to determine the degree of relationship between two people, the same DNA analysis (comparison of individual genes or entire genomes) allows you to find out the degree of relationship between species, and knowing the number of accumulated differences, researchers determine the time of divergence of two species, that is, the time when their last common ancestor lived.

Note.

With the development of molecular genetics, it was shown that the processes of evolution leave traces in genomes in the form of mutations. For example, the genomes of chimpanzees and humans are 96% the same, and the few regions that differ allow us to determine the time of existence of their common ancestor.

Just as DNA analysis allows you to determine the degree of relationship between two people, the same DNA analysis (comparison of individual genes or entire genomes) allows you to find out the degree of relationship between species, and knowing the number of accumulated differences, researchers determine the time of divergence of two species, that is, the time when their last common ancestor lived. For example, according to paleontological data, the common ancestor of humans and chimpanzees lived about 6 million years ago (for example, fossil finds of Orrorin and Sahelanthropus, forms morphologically close to the common ancestor of humans and chimpanzees, have this age). In order to get the observed number of differences between genomes, for every billion nucleotides there should have been an average of 20 changes per generation.

Human DNA is homologous to macaque DNA by 78%, bull by 28%, rat by 17%, salmon by 8%, E. coli by 2%.

In order to build a phylogenetic tree, it is enough to consider a few genes that are present in all organisms that we want to include in this tree (usually, the more genes, the more statistically reliable the elements of the tree are obtained - the branching order and branch lengths).

It is possible, using genetic techniques (studying the structure of chromosomes, comparing genetic maps, establishing the alleles of genes), to determine with sufficient accuracy the phylogeny of several related species over a period of time during which they diverged from the general order. But this approach is applicable only to very close forms, well genetically studied and, preferably, crossed with each other, i.e. to very few and very narrow systematic groups that have arisen relatively recently.

nuclear envelope

This structure is characteristic of all eukaryotic cells. The nuclear envelope consists of outer and inner membranes separated by a perinuclear space 20 to 60 nm wide. The nuclear envelope contains nuclear pores.

The membranes of the nuclear membrane do not differ morphologically from other intracellular membranes: they are about 7 nm thick and consist of two osmiophilic layers.

In general, the nuclear membrane can be represented as a hollow two-layer bag that separates the contents of the nucleus from the cytoplasm. Of all the intracellular membrane components, only the nucleus, mitochondria, and plastids have this type of membrane arrangement. However, the nuclear membrane has a characteristic feature that distinguishes it from other membrane structures of the cell. This is the presence of special pores in the nuclear membrane, which are formed due to numerous fusion zones of two nuclear membranes and are, as it were, rounded perforations of the entire nuclear membrane.

The structure of the nuclear envelope

The outer membrane of the nuclear envelope, which is in direct contact with the cytoplasm of the cell, has a number of structural features that allow it to be attributed to the proper membrane system of the endoplasmic reticulum. Thus, a large number of ribosomes are usually located on the outer nuclear membrane. In most animal and plant cells, the outer membrane of the nuclear membrane does not represent a perfectly flat surface - it can form protrusions or outgrowths of various sizes towards the cytoplasm.

The inner membrane is in contact with the chromosomal material of the nucleus (see below).

The most characteristic and conspicuous structure in the nuclear envelope is the nuclear pore. The pores in the shell are formed by the fusion of two nuclear membranes in the form of rounded through holes or perforations with a diameter of 80-90 nm. The rounded through hole in the nuclear envelope is filled with intricately organized globular and fibrillar structures. The combination of membrane perforations and these structures is called the core pore complex. Thus, it is emphasized that the nuclear pore is not just a through hole in the nuclear membrane through which the substances of the nucleus and cytoplasm can communicate directly.

The complex complex of pores has octagonal symmetry. Along the border of the rounded hole in the nuclear membrane there are three rows of granules, 8 pieces each: one row lies on the side of the nucleus, the other on the side of the cytoplasm, the third is located in the central part of the pores. The granule size is about 25 nm. Fibrillar processes extend from these granules. Such fibrils extending from the peripheral granules can converge in the center and create, as it were, a partition, a diaphragm, across the pore. In the center of the hole, one can often see the so-called central granule.

The number of nuclear pores depends on the metabolic activity of the cells: the higher the synthetic processes in the cells, the more pores per unit surface of the cell nucleus.

Number of nuclear pores in various objects

Nuclear envelope chemistry

In the composition of the nuclear membranes, small amounts of DNA (0-8%), RNA (3-9%) are found, but the main chemical components are lipids (13-35%) and proteins (50-75%), which is for all cell membranes.

The composition of lipids is similar to that in the membranes of microsomes or membranes of the endoplasmic reticulum. The nuclear membranes are characterized by a relatively low content of cholesterol and a high content of phospholipids enriched in saturated fatty acids.

The protein composition of membrane fractions is very complex. Among proteins, a number of enzymes common with ER were found (for example, glucose-6-phosphatase, Mg-dependent ATPase, glutamate dehydrogenase, etc.), RNA polymerase was not found. Here, the activities of many oxidative enzymes (cytochrome oxidase, NADH-cytochrome-c-reductase) and various cytochromes were revealed.

Among the protein fractions of nuclear membranes, there are basic histone-type proteins, which is explained by the connection of chromatin regions with the nuclear envelope.

Nuclear envelope and nuclear-cytoplasmic exchange

The nuclear membrane is a system that delimits the two main cell compartments: the cytoplasm and the nucleus. The nuclear membranes are completely permeable to ions, to substances of small molecular weight, such as sugars, amino acids, nucleotides. It is believed that proteins with a molecular weight of up to 70 thousand and a size of no more than 4.5 nm can freely diffuse through the shell.

The reverse process is also known - the transfer of substances from the nucleus to the cytoplasm. This primarily concerns the transport of RNA synthesized exclusively in the nucleus.

Another way of transporting substances from the nucleus to the cytoplasm is associated with the formation of outgrowths of the nuclear membrane, which can be separated from the nucleus in the form of vacuoles, their contents are then poured out or thrown into the cytoplasm.

Thus, from the numerous properties and functional loads of the nuclear membrane, its role should be emphasized as a barrier that separates the contents of the nucleus from the cytoplasm, restricting free access to the nucleus of large aggregates of biopolymers, a barrier that actively regulates the transport of macromolecules between the nucleus and the cytoplasm.

One of the main functions of the nuclear envelope should also be considered its participation in the creation of intranuclear order, in the fixation of chromosomal material in the three-dimensional space of the nucleus.

The nucleus of the cell is the central organelle, one of the most important. Its presence in the cell is a sign of the high organization of the body. A cell that has a well-formed nucleus is called a eukaryotic cell. Prokaryotes are organisms consisting of a cell that does not have a formed nucleus. If we consider in detail all its components, we can understand what function the cell nucleus performs.

Core structure

1. Nuclear shell.
2. Chromatin.
3. Nucleoli.
4. Nuclear matrix and nuclear juice.

The structure and functions of the cell nucleus depend on the type of cells and their purpose.

nuclear envelope

The nuclear envelope has two membranes - outer and inner. They are separated from each other by the perinuclear space. The shell has pores. Nuclear pores are necessary so that various large particles and molecules can move from the cytoplasm to the nucleus and vice versa.

Nuclear pores are formed by the fusion of the inner and outer membranes. The pores are rounded openings having complexes, which include:

1. A thin diaphragm covering the opening. It is pierced by cylindrical channels.
2. Protein granules. They are located on both sides of the diaphragm.
3. Central protein granule. It is associated with peripheral granules fibrils.

The number of pores in the nuclear envelope depends on how intensively synthetic processes take place in the cell.

The nuclear envelope consists of outer and inner membranes. The outer one passes into the rough EPR (endoplasmic reticulum).

Chromatin

Chromatin is the most important substance in the cell nucleus. Its functions are the storage of genetic information. It is represented by euchromatin and heterochromatin. All chromatin is a collection of chromosomes.

Euchromatin are parts of chromosomes that are actively involved in transcription. Such chromosomes are in a diffuse state.

Inactive sections and whole chromosomes are condensed clumps. This is heterochromatin. When the state of the cell changes, heterochromatin can turn into euchromatin, and vice versa. The more heterochromatin in the nucleus, the lower the rate of synthesis of ribonucleic acid (RNA) and the lower the functional activity of the nucleus.

Chromosomes

Chromosomes are special formations that appear in the nucleus only during division. The chromosome consists of two arms and a centromere. According to their form they are divided into:

• Rod-shaped. Such chromosomes have one large arm and the other small.
• Equal-shouldered. They have relatively equal shoulders.
• Diverse. The arms of the chromosome are visually different from each other.
• With secondary straps. Such a chromosome has a non-centromeric constriction that separates the satellite element from the main part.

In each species, the number of chromosomes is always the same, but it is worth noting that the level of organization of the organism does not depend on their number. So, a person has 46 chromosomes, a chicken has 78, a hedgehog has 96, and a birch has 84. The fern Ophioglossum reticulatum has the largest number of chromosomes. It has 1260 chromosomes per cell. The male ant of the species Myrmecia pilosula has the smallest number of chromosomes. It has only 1 chromosome.

It was by studying the chromosomes that scientists understood what the functions of the cell nucleus are.

Chromosomes are made up of genes.

Gene

Genes are sections of deoxyribonucleic acid (DNA) molecules that encode certain compositions of protein molecules. As a result, the body manifests one or another sign. The gene is inherited. Thus, the nucleus in the cell performs the function of transferring genetic material to the next generations of cells.

Nucleoli

The nucleolus is the densest part that enters the nucleus of the cell. The functions that it performs are very important for the entire cell. Usually has a rounded shape. The number of nucleoli varies in different cells - there may be two, three, or none at all. So, in the cells of crushing eggs there are no nucleoli.

The structure of the nucleolus:

1. granular component. These are granules that are located on the periphery of the nucleolus. Their size varies from 15 nm to 20 nm. In some cells, HA may be evenly distributed throughout the nucleolus.
2. Fibrillar component (FC). These are thin fibrils, ranging in size from 3 nm to 5 nm. FC is the diffuse part of the nucleolus.

Fibrillar centers (FCs) are low-density fibril regions, which, in turn, are surrounded by high-density fibrils. The chemical composition and structure of PCs are almost the same as those of the nucleolar organizers of mitotic chromosomes. They include fibrils up to 10 nm thick, which contain RNA polymerase I. This is confirmed by the fact that the fibrils are stained with silver salts.

Structural types of nucleoli

1. Nucleolonemic or reticular type. It is characterized by a large number of granules and dense fibrillar material. This type of nucleolus structure is characteristic of most cells. It can be observed both in animal cells and in plant cells.
2. Compact type. It is characterized by a small severity of nucleonoma, a large number of fibrillar centers. It is found in plant and animal cells, in which the process of protein and RNA synthesis is actively taking place. This type of nucleoli is characteristic of actively proliferating cells (tissue culture cells, plant meristem cells, etc.).
3. Ring type. In a light microscope, this type is visible as a ring with a bright center - a fibrillar center. The average size of such nucleoli is 1 µm. This type is typical only for animal cells (endotheliocytes, lymphocytes, etc.). In cells with this type of nucleoli, the level of transcription is rather low.
4. Residual type. In cells of this type of nucleoli, RNA synthesis does not occur. Under certain conditions, this type can turn into reticular or compact, i.e., be activated. Such nucleoli are characteristic of the cells of the prickly layer of the skin epithelium, normoblast, etc.
5. segregated type. In cells with this type of nucleoli, rRNA (ribosomal ribonucleic acid) synthesis does not occur. This happens if the cell is treated with some kind of antibiotic or chemical. The word "segregation" in this case means "separation" or "isolation", since all components of the nucleoli are separated, which leads to its reduction.

Almost 60% of the dry weight of the nucleoli is protein. Their number is very large and can reach several hundred.

The main function of the nucleoli is the synthesis of rRNA. The embryos of ribosomes enter the karyoplasm, then through the pores of the nucleus they seep into the cytoplasm and onto the endoplasmic reticulum.

Nuclear matrix and nuclear juice

The nuclear matrix occupies almost the entire nucleus of the cell. Its functions are specific. It dissolves and evenly distributes all nucleic acids in the interphase state.

The nuclear matrix, or karyoplasm, is a solution that includes carbohydrates, salts, proteins and other inorganic and organic substances. It contains nucleic acids: DNA, tRNA, rRNA, mRNA.

In the state of cell division, the nuclear envelope dissolves, chromosomes form, and the karyoplasm mixes with the cytoplasm.

The main functions of the nucleus in the cell

1. informative function. It is in the nucleus that all the information about the heredity of the organism is located.
2. Inheritance function. Thanks to the genes that are located on the chromosomes, the body can pass on its traits from generation to generation.
3. Union function. All organelles of the cell are united into one whole precisely in the nucleus.
4. regulation function. All biochemical reactions in the cell, physiological processes are regulated and coordinated by the nucleus.

One of the most important organelles is the cell nucleus. Its functions are important for the normal functioning of the whole organism.

The structure and functions of the kernel

Nucleus(Latin nucleus, Greek karion-nucleus) is an essential component of eukaryotic cells. It is well distinguishable in non-dividing cells and performs a number of important functions:

1) storage and transmission of hereditary information in the cell;

2) creation of an apparatus for protein synthesis - the synthesis of all types of RNA and the formation of ribosomes.

Loss or violation of any of these functions leads the cell to death.

Fig.24. Diagram of the ultramicroscopic structure of the nucleus.

The cell contains, as a rule, one nucleus, but there are binuclear and multinuclear cells.

Interphase nuclei consist of: nuclear membrane, nuclear juice (karyoplasm, karyolymph or nucleoplasm), nuclear protein backbone, chromatin and nucleoli.

nuclear envelope(karyolemma) consists of two membranes, between which there is a perinuclear space 10-40 nm wide, filled with an electron-microscopically loose substance. The outer membrane of the nuclear envelope from the side of the cytoplasm in a number of areas passes into the membranes of the endoplasmic reticulum, and polyribosomes are located on its surface. The inner membrane of the nuclear membrane is involved in ensuring the internal order in the nucleus - in the fixation of chromosomes in three-dimensional space. This connection is mediated by a layer of fibrillar proteins similar to the intermediate filaments of the cytoplasm.

The nuclear envelope contains pores about 90 nm in diameter. In these areas, along the edges of the opening, the membranes of the nuclear envelope merge. The holes themselves are filled with complexly organized globular and fibrillar structures. The totality of membrane perforations and structures filling them is called pore complex.

Along the edge of the pore opening, granules are arranged in three rows (8 granules in each row). In this case, one row lies on the side of the cytoplasm, the other - on the side of the internal contents of the nucleus, and the third - between them. Fibrillar processes extend radially from the granules of these layers, forming in the pore, as it were, a membrane - a diaphragm. Fibrillar processes are sent to the centrally located granule.

Fig.25. Structure of nuclear pores (pore complex).

Pore ​​complexes are involved in the reception of macromolecules transported through the pores (proteins and nucleoproteins), as well as in the active transfer of these substances through the nuclear envelope using ATP.

The number of nuclear pores depends on the metabolic activity of the cells. The more intense the synthesis processes in the cell, the more pores. On average, there are several thousand pore complexes per core.

Main functions nuclear envelope are as follows:

Barrier (separation of the contents of the nucleus from the cytoplasm and restriction of free access to the nucleus of large biopolymers);

Regulation of the transport of macromolecules between the nucleus and the cytoplasm;

Participation in the creation of intranuclear order (fixation of the chromosomal apparatus).

Karyoplasm(nuclear juice, or nucleoplasm, or karyolymph) is the contents of the nucleus, which has the form of a gel-like matrix. It contains various chemicals: proteins (including enzymes), amino acids and nucleotides in the form of a true or colloidal solution.

Nuclear or protein backbone (matrix). In interphase nuclei, non-histone proteins form a network - a "protein matrix". It consists of a peripheral fibrillar layer lining the nuclear envelope (lamin) and an internal network to which chromatin fibrils are attached. The matrix is ​​involved in maintaining the shape of the nucleus, organizing the spatial position of chromosomes. In addition, it contains enzymes necessary for the synthesis of RNA and DNA, as well as proteins involved in DNA compaction in interphase and mitotic chromosomes.

Chromatin- a complex of DNA and proteins (histone and non-histone). Chromatin is an interphase form of the existence of chromosomes.

1.Euchromatin; 2. Heterochromatin

Fig.26. Chromatin of interphase chromosomes.

During this period, different parts of the chromosomes have an unequal degree of compaction. Genetically inert regions of chromosomes have the highest degree of compaction. They stain well with nuclear dyes and are called heterochromatin. Distinguish constitutive and optional heterochromatin.

Constitutive heterochromatin made up of non-transcribed DNA. It is believed that it is involved in maintaining the structure of the nucleus, attaching chromosomes to the nuclear envelope, recognizing homologous chromosomes during meiosis, separating neighboring structural genes, and in the processes of regulating their activity.

Optional heterochromatin, unlike constitutive, can become transcribed at certain stages of cell differentiation or ontogenesis. An example of facultative heterochromatin is the Barr body, which is formed in organisms of the homogametic sex due to the inactivation of one of the X chromosomes.

Decompacted regions of chromosomes that stain poorly with nuclear dyes are called euchromatin.This is a functionally active, transcribed chromatin.

Nucleoli- compacted bodies, usually rounded, with a diameter of less than 1 micron. They are present only in interphase nuclei. Their number varies in diploid cells from 1 to 7, but in some types of cells, for example, ciliate micronuclei, nucleoli are absent.

The nucleus is found in every eukaryotic cell. There may be one nucleus, or there may be several nuclei in a cell (depending on its activity and function).

The cell nucleus consists of a membrane, nuclear juice, nucleolus and chromatin. The nuclear envelope consists of two membranes separated by a perinuclear (perinuclear) space, between which there is a liquid. The main functions of the nuclear membrane are the separation of genetic material (chromosomes) from the cytoplasm, as well as the regulation of bilateral relationships between the nucleus and the cytoplasm.

The nuclear envelope is permeated with pores that have a diameter of about 90 nm. The pore area (pore complex) has a complex structure (this indicates the complexity of the mechanism for regulating the relationship between the nucleus and the cytoplasm). The number of pores depends on the functional activity of the cell: the higher it is, the more pores (there are more pores in immature cells).

The basis of nuclear juice (matrix, nucleoplasm) is proteins. Juice forms the internal environment of the nucleus, plays an important role in the work of the genetic material of cells. Proteins: filamentous or fibrillar (support function), heteronuclear RNA (products of primary transcription of genetic information) and mRNA (processing result).

The nucleolus is the structure where the formation and maturation of ribosomal RNA (rRNA) takes place. The rRNA genes occupy certain regions of several chromosomes (in humans, these are 13–15 and 21–22 pairs), where nucleolar organizers are formed, in the region of which the nucleoli themselves are formed. In metaphase chromosomes, these areas are called secondary constrictions and look like constrictions. Electron microscopy revealed filamentous and granular components of the nucleoli. Filamentous (fibrillar) is a complex of proteins and giant rRNA precursor molecules, which subsequently give rise to smaller molecules of mature rRNA. During maturation, the fibrils are transformed into ribonucleoprotein granules (granular component).

Chromatin got its name for its ability to stain well with basic dyes; in the form of clumps, it is scattered in the nucleoplasm of the nucleus and is an interphase form of the existence of chromosomes.

Chromatin consists mainly of DNA strands (40% of the mass of the chromosome) and proteins (about 60%), which together form the nucleoprotein complex. There are histone (five classes) and non-histone proteins.

Histones (40%) have regulatory (strongly connected to DNA and prevent reading information from it) and structural functions (organization of the spatial structure of the DNA molecule). Non-histone proteins (more than 100 fractions, 20% of the chromosome mass): enzymes of RNA synthesis and processing, DNA replication repair, structural and regulatory functions. In addition, RNA, fats, polysaccharides, and metal molecules were found in the composition of chromosomes.

Depending on the state of chromatin, euchromatic and heterochromatic regions of chromosomes are distinguished. Euchromatin is less dense and genetic information can be read from it. Heterochromatin is more compact, and information cannot be read within it. There are constitutive (structural) and facultative heterochromatin.

5. Structure and functions of semi-autonomous cell structures: mitochondria and plastids

Mitochondria (from Gr. mitos - “thread”, chondrion - “grain, grain”) are permanent membrane organelles of a round or rod-shaped (often branching) shape. Thickness - 0.5 microns, length - 5-7 microns. The number of mitochondria in most animal cells is 150-1500; in female eggs - up to several hundred thousand, in spermatozoa - one helical mitochondria twisted around the axial part of the flagellum.

The main functions of mitochondria:

1) play the role of energy stations of cells. The processes of oxidative phosphorylation (enzymatic oxidation of various substances with subsequent accumulation of energy in the form of molecules of adenosine triphosphate - ATP) proceed in them;

2) store hereditary material in the form of mitochondrial DNA. Mitochondria require the proteins encoded in the nuclear DNA genes to function, since their own mitochondrial DNA can provide the mitochondria with only a few proteins.

Side functions - participation in the synthesis of steroid hormones, some amino acids (for example, glutamine). The structure of mitochondria

Mitochondria have two membranes: outer (smooth) and inner (forming outgrowths - leaf-shaped (cristae) and tubular (tubules)). Membranes differ in chemical composition, set of enzymes and functions.

In mitochondria, the internal content is a matrix - a colloidal substance in which grains with a diameter of 20–30 nm were found using an electron microscope (they accumulate calcium and magnesium ions, reserves of nutrients, for example, glycogen).

The matrix houses the organelle protein biosynthesis apparatus: 2–6 copies of circular DNA devoid of histone proteins (like in prokaryotes), ribosomes, a set of t-RNA, enzymes of reduplication, transcription, translation of hereditary information. This apparatus as a whole is very similar to that of prokaryotes (in terms of the number, structure and size of ribosomes, the organization of its own hereditary apparatus, etc.), which confirms the symbiotic concept of the origin of the eukaryotic cell.

Both the matrix and the surface of the inner membrane are actively involved in the implementation of the energy function of mitochondria, on which the electron transport chain (cytochromes) and ATP synthase are located, which catalyzes the phosphorylation of ADP coupled with oxidation, which converts it into ATP.

Mitochondria multiply by ligation, so during cell division they are more or less evenly distributed between daughter cells. Thus, succession is carried out between the mitochondria of cells of successive generations.

Thus, mitochondria are characterized by relative autonomy within the cell (unlike other organelles). They arise during the division of maternal mitochondria, have their own DNA, which differs from the nuclear system of protein synthesis and energy storage.

plastids

These are semi-autonomous structures (they can exist relatively autonomously from the cell's nuclear DNA) that are present in plant cells. They are formed from proplastids, which are present in the embryo of the plant. Delimited by two membranes.

There are three groups of plastids:

1) leukoplasts. They are round, not colored and contain nutrients (starch);

2) chromoplasts. They contain molecules of coloring substances and are present in the cells of colored plant organs (fruits of cherries, apricots, tomatoes);

3) chloroplasts. These are the plastids of the green parts of the plant (leaves, stems). In structure, they are in many ways similar to the mitochondria of animal cells. The outer membrane is smooth, the inner one has outgrowths - lamelosomes, which end in thickenings - thylakoids containing chlorophyll. The stroma (liquid part of the chloroplast) contains a circular DNA molecule, ribosomes, reserve nutrients (starch grains, fat drops).