Who are eukaryotes and prokaryotes: comparative characteristics of cells from different kingdoms. Structural features of cells Shell of a eukaryotic cell

The cells that form the tissues of animals and plants vary considerably in shape, size and internal structure. However, all of them show similarities in the main features of the processes of vital activity, metabolism, in irritability, growth, development, and the ability to change.

Cells of all types contain two main components that are closely related to each other - the cytoplasm and the nucleus. The nucleus is separated from the cytoplasm by a porous membrane and contains nuclear sap, chromatin, and the nucleolus. Semi-liquid cytoplasm fills the entire cell and is penetrated by numerous tubules. Outside, it is covered with a cytoplasmic membrane. It has specialized organelle structures, permanently present in the cell, and temporary formations - inclusions.Membrane organelles : outer cytoplasmic membrane (OCM), endoplasmic reticulum (ER), Golgi apparatus, lysosomes, mitochondria and plastids. The basis of the structure of all membrane organelles is the biological membrane. All membranes have a fundamentally unified structural plan and consist of a double layer of phospholipids, in which protein molecules are immersed from different sides and at different depths. The membranes of organelles differ from each other only in the sets of proteins included in them.

Diagram of the structure of a eukaryotic cell. A - a cell of animal origin; B - plant cell: 1 - nucleus with chromatin and nucleolus, 2 - cytoplasmic membrane, 3 - cell wall, 4 - pores in the cell wall through which the cytoplasm of neighboring cells communicates, 5 - rough endoplasmic reticulum, b - smooth endoplasmic reticulum, 7 - pinocytic vacuole, 8 - Golgi apparatus (complex), 9 - lysosome, 10 - fatty inclusions in the channels of the smooth endoplasmic reticulum, 11 - cell center, 12 - mitochondria, 13 - free ribosomes and polyribosomes, 14 - vacuole, 15 - chloroplast

cytoplasmic membrane. In all plant cells, multicellular animals, protozoa and bacteria, the cell membrane is three-layered: the outer and inner layers consist of protein molecules, the middle layer consists of lipid molecules. It limits the cytoplasm from the external environment, surrounds all organelles of the cell and is a universal biological structure. In some cells, the outer shell is formed by several membranes that are tightly adjacent to each other. In such cases, the cell membrane becomes dense and elastic and allows you to keep the shape of the cell, as, for example, in euglena and shoe ciliates. Most plant cells, in addition to the membrane, also have a thick cellulose membrane on the outside - cell wall. It is clearly visible in a conventional light microscope and performs a supporting function due to a rigid outer layer that gives the cells a clear shape.

On the cell surface, the membrane forms elongated outgrowths - microvilli, folds, protrusions and protrusions, which greatly increases the suction or excretory surface. With the help of membrane outgrowths, cells are connected to each other in the tissues and organs of multicellular organisms; various enzymes involved in metabolism are located on the folds of the membranes. Delimiting the cell from the environment, the membrane regulates the direction of diffusion of substances and simultaneously carries out their active transfer into the cell (accumulation) or out (release). Due to these properties of the membrane, the concentration of potassium, calcium, magnesium, and phosphorus ions in the cytoplasm is higher, while the concentration of sodium and chlorine is lower than in the environment. Through the pores of the outer membrane from the external environment, ions, water and small molecules of other substances penetrate into the cell. Penetration into the cell of relatively large solid particles is carried out by phagocytosis(from the Greek "fago" - I devour, "drink" - a cell). In this case, the outer membrane at the point of contact with the particle bends inside the cell, dragging the particle deep into the cytoplasm, where it undergoes enzymatic cleavage. Drops of liquid substances enter the cell in a similar way; their absorption is called pinocytosis(from the Greek "pino" - I drink, "cytos" - a cell). The outer cell membrane also performs other important biological functions.

Cytoplasm 85% consists of water, 10% of proteins, the rest is lipids, carbohydrates, nucleic acids and mineral compounds; all these substances form a colloidal solution similar in consistency to glycerin. The colloidal substance of a cell, depending on its physiological state and the nature of the influence of the external environment, has the properties of both a liquid and an elastic, denser body. The cytoplasm is permeated with channels of various shapes and sizes, which are called endoplasmic reticulum. Their walls are membranes that are in close contact with all the organelles of the cell and together with them form a single functional and structural system for the exchange of substances and energy and the movement of substances inside the cell.

In the walls of the tubules are the smallest grains-granules, called ribosomes. Such a network of tubules is called granular. Ribosomes can be located on the surface of the tubules separately or form complexes of five to seven or more ribosomes, called polysomes. Other tubules do not contain granules, they make up a smooth endoplasmic reticulum. Enzymes involved in the synthesis of fats and carbohydrates are located on the walls.

The inner cavity of the tubules is filled with waste products of the cell. Intracellular tubules, forming a complex branching system, regulate the movement and concentration of substances, separate various molecules of organic substances and their stages of synthesis. On the inner and outer surfaces of membranes rich in enzymes, proteins, fats and carbohydrates are synthesized, which are either used in metabolism, or accumulate in the cytoplasm as inclusions, or are excreted.

Ribosomes found in all types of cells - from bacteria to cells of multicellular organisms. These are round bodies, consisting of ribonucleic acid (RNA) and proteins in almost equal proportions. Their composition certainly includes magnesium, the presence of which maintains the structure of ribosomes. Ribosomes can be associated with the membranes of the endoplasmic reticulum, with the outer cell membrane, or lie freely in the cytoplasm. They carry out protein synthesis. Ribosomes, in addition to the cytoplasm, are found in the nucleus of the cell. They are produced in the nucleolus and then enter the cytoplasm.

Golgi complex in plant cells it looks like individual bodies surrounded by membranes. In animal cells, this organoid is represented by cisterns, tubules and vesicles. The membrane tubes of the Golgi complex from the tubules of the endoplasmic reticulum receive the secretion products of the cell, where they are chemically rearranged, compacted, and then transferred to the cytoplasm and either used by the cell itself or removed from it. In the tanks of the Golgi complex, polysaccharides are synthesized and combined with proteins, resulting in the formation of glycoproteins.

Mitochondria- small rod-shaped bodies, limited by two membranes. Numerous folds - cristae - extend from the inner membrane of the mitochondria; various enzymes are located on their walls, with the help of which the synthesis of a high-energy substance - adenosine triphosphoric acid (ATP) is carried out. Depending on the activity of the cell and external influences, mitochondria can move, change their size and shape. Ribosomes, phospholipids, RNA and DNA are found in mitochondria. The presence of DNA in mitochondria is associated with the ability of these organelles to reproduce by constriction formation or budding during cell division, as well as the synthesis of some mitochondrial proteins.

Lysosomes- small oval formations limited by the membrane and scattered throughout the cytoplasm. Found in all cells of animals and plants. They arise in the extensions of the endoplasmic reticulum and in the Golgi complex, are filled with hydrolytic enzymes, and then separate and enter the cytoplasm. Under normal conditions, lysosomes digest particles that enter the cell by phagocytosis and organelles of dying cells. Lysosome products are excreted through the lysosome membrane into the cytoplasm, where they are incorporated into new molecules. When the lysosome membrane is ruptured, enzymes enter the cytoplasm and digest its contents, causing cell death.

plastids is found only in plant cells and is found in most green plants. Organic substances are synthesized and accumulated in plastids. There are three types of plastids: chloroplasts, chromoplasts and leukoplasts.

Chloroplasts - green plastids containing the green pigment chlorophyll. They are found in leaves, young stems, unripe fruits. Chloroplasts are surrounded by a double membrane. In higher plants, the inner part of the chloroplasts is filled with a semi-liquid substance, in which the plates are laid parallel to each other. The paired membranes of the plates, merging, form stacks containing chlorophyll (Fig. 6). In each stack of chloroplasts of higher plants, layers of protein molecules and lipid molecules alternate, and chlorophyll molecules are located between them. This layered structure provides maximum free surfaces and facilitates the capture and transfer of energy during photosynthesis.

Chromoplasts - plastids, which contain plant pigments (red or brown, yellow, orange). They are concentrated in the cytoplasm of the cells of flowers, stems, fruits, leaves of plants and give them the appropriate color. Chromoplasts are formed from leukoplasts or chloroplasts as a result of the accumulation of pigments. carotenoids.

Leucoplasts - colorless plastids located in the unpainted parts of plants: in stems, roots, bulbs, etc. Starch grains accumulate in the leukoplasts of some cells, oils and proteins accumulate in the leukoplasts of other cells.

All plastids arise from their predecessors - proplastids. They revealed DNA that controls the reproduction of these organelles.

cell center, or centrosome, plays an important role in cell division and consists of two centrioles . It is found in all cells of animals and plants, except for flowering, lower fungi and some protozoa. Centrioles in dividing cells take part in the formation of the division spindle and are located at its poles. In a dividing cell, the cell center divides first, at the same time an achromatin spindle is formed, orienting the chromosomes when they diverge towards the poles. One centriole leaves each daughter cell.

Many plant and animal cells have special purpose organelles: cilia, performing the function of movement (ciliates, cells respiratory tract), flagella(the simplest unicellular, male germ cells in animals and plants, etc.). Inclusions - temporary elements that arise in a cell at a certain stage of its life as a result of a synthetic function. They are either used or removed from the cell. Inclusions are also reserve nutrients: in plant cells - starch, fat droplets, blocks, essential oils, many organic acids, salts of organic and inorganic acids; in animal cells - glycogen (in liver cells and muscles), fat drops (in subcutaneous tissue); Some inclusions accumulate in cells as waste - in the form of crystals, pigments, etc.

Vacuoles - these are cavities bounded by a membrane; are well expressed in plant cells and are present in protozoa. Arise in different parts of the extensions of the endoplasmic reticulum. And gradually separate from it. Vacuoles maintain turgor pressure, they contain cell or vacuolar juice, the molecules of which determine its osmotic concentration. It is believed that the initial products of synthesis - soluble carbohydrates, proteins, pectins, etc. - accumulate in the cisterns of the endoplasmic reticulum. These accumulations represent the beginnings of future vacuoles.

cytoskeleton . One of distinctive features eukaryotic cell is the development in its cytoplasm of skeletal formations in the form of microtubules and bundles of protein fibers. The elements of the cytoskeleton are closely connected with the outer cytoplasmic membrane and the nuclear membrane, forming complex interlacings in the cytoplasm. The supporting elements of the cytoplasm determine the shape of the cell, ensure the movement of intracellular structures and the movement of the entire cell.

Core cell plays a major role in its life, with its removal, the cell ceases its functions and dies. Most animal cells have one nucleus, but there are also multinucleated cells (human liver and muscles, fungi, ciliates, green algae). Mammalian erythrocytes develop from progenitor cells containing a nucleus, but mature erythrocytes lose it and do not live long.

The nucleus is surrounded by a double membrane penetrated by pores, through which it is closely connected with the channels of the endoplasmic reticulum and the cytoplasm. Inside the nucleus is chromatin- spiralized sections of chromosomes. During cell division, they turn into rod-shaped structures that are clearly visible under a light microscope. Chromosomes are a complex set of proteins and DNA called nucleoprotein.

The functions of the nucleus consist in the regulation of all vital functions of the cell, which it carries out with the help of DNA and RNA-material carriers of hereditary information. In preparation for cell division, DNA is doubled, during mitosis, chromosomes separate and are transferred to daughter cells, ensuring the continuity of hereditary information in each type of organism.

Karyoplasm - the liquid phase of the nucleus, in which the waste products of nuclear structures are in dissolved form

nucleolus- isolated, most dense part of the nucleus. The nucleolus consists of complex proteins and RNA, free or bound phosphates of potassium, magnesium, calcium, iron, zinc, and ribosomes. The nucleolus disappears before the start of cell division and re-forms in the last phase of division.

Thus, the cell has a fine and very complex organization. An extensive network of cytoplasmic membranes and the membrane principle of the structure of organelles make it possible to distinguish between many chemical reactions simultaneously occurring in the cell. Each of the intracellular formations has its own structure and specific function, but only with their interaction is the harmonious life of the cell possible. Based on this interaction, substances from the environment enter the cell, and waste products are removed from it into the external environment - this is how metabolism takes place. The perfection of the structural organization of the cell could arise only as a result of a long biological evolution, during which the functions performed by it gradually became more complicated.

The simplest unicellular forms are both a cell and an organism with all its vital manifestations. In multicellular organisms, cells form homogeneous groups - tissues. In turn, tissues form organs, systems, and their functions are determined by the overall vital activity of the whole organism.

The cell is the elementary unit of the structure and life of all alive organisms(except viruses, which are often referred to as non-cellular life forms), which has its own metabolism, is capable of independent existence, self-reproduction and development. All living organisms either as multicellular animals, plants And mushrooms, consist of many cells, or, as many protozoa And bacteria, are unicellular organisms. The branch of biology that deals with the structure and function of cells is called cytology. Recently, it has also become customary to talk about cell biology, or cell biology.

Distinctive features of plant and animal cell

General features 1. Unity of structural systems - cytoplasm and nucleus. 2. The similarity of the processes of metabolism and energy. 3. Unity of the principle of the hereditary code. 4. Universal membrane structure. 5. The unity of the chemical composition. 6. The similarity of the process of cell division.

cell structure

All cellular life forms on Earth can be divided into two kingdoms based on the structure of their constituent cells:

prokaryotes (pre-nuclear) - simpler in structure and arose earlier in the process of evolution;

eukaryotes (nuclear) - more complex, arose later. The cells that make up the human body are eukaryotic.

Despite the variety of forms, the organization of the cells of all living organisms is subject to uniform structural principles.

The contents of the cell are separated from the environment by a plasma membrane, or plasmalemma. Inside the cell is filled with cytoplasm, which contains various organelles and cellular inclusions, as well as genetic material in the form of a DNA molecule. Each of the organelles of the cell performs its own special function, and together they all determine the vital activity of the cell as a whole.

prokaryotic cell

The structure of a typical prokaryotic cell: capsule, cell wall, plasmalemma, cytoplasm,ribosomes, plasmid, drank, flagellum,nucleoid.

prokaryotes (from lat. pro- before, before and Greek κάρῠον - core, walnut) - organisms that, unlike eukaryotes, do not have a formed cell nucleus and other internal membrane organelles (with the exception of flat tanks in photosynthetic species, for example, in cyanobacteria). The only large circular (in some species - linear) double-stranded molecule DNA, which contains the bulk of the genetic material of the cell (the so-called nucleoid) does not form a complex with proteins- histones(the so-called chromatin). The prokaryotes are bacteria, including cyanobacteria(blue-green algae), and archaea. The descendants of prokaryotic cells are organelles eukaryotic cells - mitochondria And plastids. The main content of the cell, which fills its entire volume, is a viscous granular cytoplasm.

eukaryotic cell

Eukaryotes are organisms that, unlike prokaryotes, have a cellular structure. core separated from the cytoplasm by the nuclear envelope. The genetic material is enclosed in several linear double-stranded DNA molecules (depending on the type of organisms, their number per nucleus can vary from two to several hundred), attached from the inside to the membrane of the cell nucleus and forming in the vast majority (except dinoflagellates) complex with proteins- histones, called chromatin. Eukaryotic cells have a system of internal membranes that form, in addition to the nucleus, a number of other organelles (endoplasmic reticulum, golgi apparatus and etc.). In addition, the vast majority have permanent intracellular symbionts- prokaryotes - mitochondria, and in algae and plants - also plastids.

The structure of a eukaryotic cell

Schematic representation of an animal cell. (By clicking on any of the titles constituent parts cells, you will be taken to the corresponding article.)

Animal cell surface complex

Consists of glycocalyx, plasmalemma and underlying cortical layer cytoplasm. The plasma membrane is also called the plasmalemma, the outer cell membrane. This is a biological membrane, about 10 nanometers thick. Provides primarily a delimiting function in relation to the environment external to the cell. In addition, she performs transport function. The cell does not waste energy on maintaining the integrity of its membrane: the molecules are held according to the same principle by which fat molecules are held together - hydrophobic It is thermodynamically more advantageous for parts of molecules to be located in close proximity to each other. The glycocalyx is a plasmalemma-anchored oligosaccharide, polysaccharide, glycoprotein, and glycolipid molecule. The glycocalyx performs receptor and marker functions. plasma membrane animals cells mainly consists of phospholipids and lipoproteins interspersed with protein molecules, in particular, surface antigens and receptors. In the cortical (adjacent to the plasma membrane) layer of the cytoplasm there are specific elements of the cytoskeleton - actin microfilaments ordered in a certain way. The main and most important function of the cortical layer (cortex) is pseudopodial reactions: ejection, attachment and contraction of pseudopodia. In this case, the microfilaments are rearranged, lengthened or shortened. The shape of the cell also depends on the structure of the cytoskeleton of the cortical layer (for example, the presence of microvilli).

All living organisms can be divided into two main groups: prokaryotes and eukaryotes. These terms are derived from the Greek word karion meaning core. Prokaryotes are pre-nuclear organisms that do not have a formed nucleus. Eukaryotes contain a well-formed nucleus. Prokaryotes include bacteria, cyanobacteria, myxomycetes, rickettsia, and other organisms; eukaryotes are fungi, plants and animals. The cells of all eukaryotes have a similar structure. They consist of cytoplasm And nuclei, which together represent the living contents of the cell - the protoplast. The cytoplasm is a semi-fluid ground substance or hyaloplasm, together with intracellular structures immersed in it - organelles that perform various functions (more details in the table below). From the outside, the cytoplasm is surrounded by a plasma membrane. Plant and fungal cells also have a rigid cell wall. In the cytoplasm of plant and fungal cells there are vacuoles - bubbles filled with water and various substances dissolved in it. In addition, the cell may contain inclusions - reserve nutrients or end products of metabolism.

Cell structure

cell structure

prokaryotic cell

prokaryotes(from lat. pro

The structure of chromosomes

Diagram of the structure of the chromosome in the late prophase - metaphase of mitosis. 1-chromatid; 2-centromere; 3-short shoulder; 4-long shoulder.

Chromosomes(ancient Greek χρῶμα - color and σῶμα - body) - nucleoprotein structures in the nucleus of a eukaryotic cell (a cell containing a nucleus), which become easily visible in certain phases cell cycle(during mitosis or meiosis). Chromosomes are a high degree condensation of chromatin, constantly present in the cell nucleus. The term was originally proposed to refer to structures found in eukaryotic cells, but in recent decades, bacterial chromosomes have been increasingly spoken of. Chromosomes contain most of the genetic information.

Chromosome morphology is best seen in a cell at the metaphase stage. The chromosome consists of two rod-shaped bodies - chromatids. Both chromatids of each chromosome are identical to each other in terms of gene composition.

Chromosomes are differentiated in length. Chromosomes have a centromere or primary constriction, two telomeres, and two arms. On some chromosomes, secondary constrictions and satellites are isolated. The movement of the chromosome determines the Centromere, which has a complex structure.

Centromere DNA is distinguished by a characteristic nucleotide sequence and specific proteins. Depending on the location of the centromere, acrocentric, submetacentric and metacentric chromosomes are distinguished.

As mentioned above, some chromosomes have secondary constrictions. They, unlike the primary constriction (centromeres), do not serve as a place of attachment for the spindle threads and do not play any role in the movement of chromosomes. Some secondary constrictions are associated with the formation of nucleoli, in which case they are called nucleolar organizers. The nucleolar organizers contain the genes responsible for RNA synthesis. The function of other secondary constrictions is not yet clear.

Some acrocentric chromosomes have satellites - regions connected to the rest of the chromosome by a thin filament of chromatin. The shape and dimensions of the satellite are constant for a given chromosome. Humans have satellites on five pairs of chromosomes.

The ends of chromosomes rich in structural heterochromatin are called telomeres. Telomeres prevent the ends of chromosomes from sticking together after reduplication and thus contribute to the preservation of their integrity. Therefore, telomeres are responsible for the existence of chromosomes as individual formations.

Chromosomes that have the same order of genes are called homologous. They have the same structure (length, location of the centromere, etc.). Non-homologous chromosomes have a different gene set and a different structure.

The study of the fine structure of chromosomes showed that they are composed of DNA, protein and a small amount of RNA. The DNA molecule carries negative charges distributed along its entire length, and the proteins attached to it - histones - are positively charged. This complex of DNA and protein is called chromatin. Chromatin can have different degrees of condensation. Condensed chromatin is called heterochromatin, decondensed chromatin is called euchromatin. The degree of chromatin decondensation reflects its functional state. Heterochromatic regions are functionally less active than euchromatic regions, in which most of the genes are localized. Structural heterochromatin is distinguished, the amount of which varies in different chromosomes, but it is constantly located in the pericentromeric regions. In addition to structural heterochromatin, there is facultative heterochromatin, which appears in the chromosome during supercoiling of euchromatic regions. The existence of this phenomenon in human chromosomes is confirmed by the fact of genetic inactivation of one X chromosome in the somatic cells of a woman. Its essence lies in the fact that there is an evolutionarily formed mechanism of inactivation of the second dose of genes localized in the X chromosome, as a result of which, despite the different number of X chromosomes in the male and female organisms, the number of genes functioning in them is equalized. Chromatin is maximally condensed during mitotic cell division, then it can be detected in the form of dense chromosomes

The dimensions of the DNA molecules of chromosomes are huge. Each chromosome is represented by one DNA molecule. They can reach hundreds of micrometers and even centimeters. Of the human chromosomes, the largest is the first; its DNA has a total length of up to 7 cm. The total length of the DNA molecules of all chromosomes of one human cell is 170 cm.

Despite the gigantic size of DNA molecules, it is quite densely packed in chromosomes. Histone proteins provide such specific packing of chromosomal DNA. Histones are arranged along the length of the DNA molecule in the form of blocks. One block includes 8 histone molecules, forming a nucleosome (a formation consisting of a DNA strand wound around a histone octamer). The size of the nucleosome is about 10 nm. Nucleosomes look like beads strung on a string. Nucleosomes and the DNA segments connecting them are densely packed in the form of a helix, with six nucleosomes for each turn of such a helix. This is how the structure of the chromosome is formed.

The hereditary information of an organism is strictly ordered according to individual chromosomes. Each organism is characterized by a certain set of chromosomes (number, size and structure), which is called a karyotype. The human karyotype is represented by twenty-four different chromosomes (22 pairs of autosomes, X and Y chromosomes). A karyotype is a species passport. Karyotype analysis allows to identify disorders that can lead to developmental abnormalities, hereditary diseases or death of fetuses and embryos early stages development.

For a long time it was believed that the human karyotype consists of 48 chromosomes. However, at the beginning of 1956, a message was published, according to which the number of chromosomes in the human karyotype is 46.

Human chromosomes differ in size, location of the centromere and secondary constrictions. The first division of the karyotype into groups was carried out in 1960 at a conference in Denver (USA). The description of the human karyotype was originally based on the following two principles: the arrangement of chromosomes along their length; grouping of chromosomes according to the location of the centromere (metacentric, submetacentric, acrocentric).

The exact constancy of the number of chromosomes, their individuality and the complexity of the structure indicate the importance of the function they perform. Chromosomes perform the function of the main genetic apparatus of the cell. They contain genes in a linear order, each of which occupies a strictly defined place (locus) in the chromosome. There are many genes in each chromosome, but for the normal development of an organism, a set of genes of a complete chromosome set is necessary.

Structure and functions of DNA

DNA- a polymer whose monomers are deoxyribonucleotides. The model of the spatial structure of the DNA molecule in the form of a double helix was proposed in 1953 by J. Watson and F. Crick (to build this model, they used the work of M. Wilkins, R. Franklin, E. Chargaff).

DNA molecule formed by two polynucleotide chains, spirally twisted around each other and together around an imaginary axis, i.e. is a double helix (exception - some DNA-containing viruses have single-stranded DNA). The diameter of the DNA double helix is ​​2 nm, the distance between adjacent nucleotides is 0.34 nm, and there are 10 pairs of nucleotides per turn of the helix. The length of the molecule can reach several centimeters. Molecular weight - tens and hundreds of millions. The total length of DNA in the human cell nucleus is about 2 m. In eukaryotic cells, DNA forms complexes with proteins and has a specific spatial conformation.

DNA monomer - nucleotide (deoxyribonucleotide)- consists of residues of three substances: 1) a nitrogenous base, 2) a five-carbon monosaccharide (pentose) and 3) phosphoric acid. The nitrogenous bases of nucleic acids belong to the classes of pyrimidines and purines. Pyrimidine bases of DNA(have one ring in their molecule) - thymine, cytosine. Purine bases(have two rings) - adenine and guanine.

The monosaccharide of the DNA nucleotide is represented by deoxyribose.

The name of the nucleotide is derived from the name of the corresponding base. Nucleotides and nitrogenous bases are indicated by capital letters.

A polynucleotide chain is formed as a result of nucleotide condensation reactions. In this case, between the 3 "-carbon of the deoxyribose residue of one nucleotide and the phosphoric acid residue of the other, phosphoether bond(belongs to the category of strong covalent bonds). One end of the polynucleotide chain ends with a 5 "carbon (it is called the 5" end), the other ends with a 3 "carbon (3" end).

Against one chain of nucleotides is a second chain. The arrangement of nucleotides in these two chains is not random, but strictly defined: thymine is always located against adenine of one chain in the other chain, and cytosine is always located against guanine, two hydrogen bonds arise between adenine and thymine, three hydrogen bonds between guanine and cytosine. The pattern according to which the nucleotides of different DNA strands are strictly ordered (adenine - thymine, guanine - cytosine) and selectively connect to each other is called the principle of complementarity. It should be noted that J. Watson and F. Crick came to understand the principle of complementarity after reading the works of E. Chargaff. E. Chargaff, having studied a huge number of samples of tissues and organs of various organisms, found that in any DNA fragment the content of guanine residues always exactly corresponds to the content of cytosine, and adenine to thymine ( "Chargaff's rule"), but he could not explain this fact.

From the principle of complementarity, it follows that the nucleotide sequence of one chain determines the nucleotide sequence of another.

DNA strands are antiparallel (opposite), i.e. nucleotides of different chains are located in opposite directions, and, therefore, opposite the 3 "end of one chain is the 5" end of the other. The DNA molecule is sometimes compared to a spiral staircase. The "railing" of this ladder is the sugar-phosphate backbone (alternating residues of deoxyribose and phosphoric acid); "steps" are complementary nitrogenous bases.

Function of DNA- storage and transmission of hereditary information.

Reparation ("repair")

reparations is the process of repairing damage to the nucleotide sequence of DNA. It is carried out by special enzyme systems of the cell ( repair enzymes). The following steps can be distinguished in the process of DNA structure repair: 1) DNA-repairing nucleases recognize and remove the damaged area, resulting in a gap in the DNA chain; 2) DNA polymerase fills this gap by copying information from the second (“good”) strand; 3) DNA ligase “crosslinks” the nucleotides, completing the repair.

Three repair mechanisms have been studied the most: 1) photoreparation, 2) excise or pre-replicative repair, 3) post-replicative repair.

Changes in the structure of DNA occur constantly in the cell under the influence of reactive metabolites, ultraviolet radiation, heavy metals and their salts, etc. Therefore, defects in repair systems increase the rate of mutation processes and cause hereditary diseases (xeroderma pigmentosa, progeria, etc.).

Structure and functions of RNA

RNA- a polymer whose monomers are ribonucleotides. Unlike DNA, RNA is formed not by two, but by one polynucleotide chain (exception - some RNA-containing viruses have double-stranded RNA). RNA nucleotides are capable of forming hydrogen bonds with each other. RNA chains are much shorter than DNA chains.

RNA monomer - nucleotide (ribonucleotide)- consists of residues of three substances: 1) a nitrogenous base, 2) a five-carbon monosaccharide (pentose) and 3) phosphoric acid. The nitrogenous bases of RNA also belong to the classes of pyrimidines and purines.

Pyrimidine bases of RNA - uracil, cytosine, purine bases - adenine and guanine. The RNA nucleotide monosaccharide is represented by ribose.

Allocate three types of RNA: 1) informational(matrix) RNA - mRNA (mRNA), 2) transport RNA - tRNA, 3) ribosomal RNA - rRNA.

All types of RNA are unbranched polynucleotides, have a specific spatial conformation and take part in the processes of protein synthesis. Information about the structure of all types of RNA is stored in DNA. The process of RNA synthesis on a DNA template is called transcription.

Transfer RNAs usually contain 76 (from 75 to 95) nucleotides; molecular weight - 25,000–30,000. tRNA accounts for about 10% of the total RNA content in the cell. tRNA functions: 1) transport of amino acids to the site of protein synthesis, to ribosomes, 2) translational mediator. About 40 types of tRNA are found in the cell, each of them has a nucleotide sequence characteristic only for it. However, all tRNAs have several intramolecular complementary regions, due to which tRNAs acquire a conformation that resembles a clover leaf in shape. Any tRNA has a loop for contact with the ribosome (1), an anticodon loop (2), a loop for contact with the enzyme (3), an acceptor stem (4), and an anticodon (5). The amino acid is attached to the 3' end of the acceptor stem. Anticodon- three nucleotides that "recognize" the mRNA codon. It should be emphasized that a particular tRNA can transport a strictly defined amino acid corresponding to its anticodon. The specificity of the connection of amino acids and tRNA is achieved due to the properties of the enzyme aminoacyl-tRNA synthetase.

Ribosomal RNA contain 3000–5000 nucleotides; molecular weight - 1,000,000–1,500,000. rRNA accounts for 80–85% of the total RNA content in the cell. In complex with ribosomal proteins, rRNA forms ribosomes - organelles that carry out protein synthesis. In eukaryotic cells, rRNA synthesis occurs in the nucleolus. rRNA functions A: 1) Required structural component ribosomes and thus ensuring the functioning of ribosomes; 2) ensuring the interaction of the ribosome and tRNA; 3) initial binding of the ribosome and the mRNA initiator codon and determination of the reading frame, 4) formation of the active center of the ribosome.

Information RNA varied in nucleotide content and molecular weight (from 50,000 to 4,000,000). The share of mRNA accounts for up to 5% of the total RNA content in the cell. Functions of mRNA: 1) transfer of genetic information from DNA to ribosomes, 2) a matrix for the synthesis of a protein molecule, 3) determination of the amino acid sequence of the primary structure of a protein molecule.

The structure and functions of ATP

Adenosine triphosphoric acid (ATP)- universal source and main accumulator of energy in living cells. ATP is found in all plant and animal cells. The amount of ATP averages 0.04% (of the raw mass of the cell), the largest amount of ATP (0.2–0.5%) is found in skeletal muscles.

ATP consists of residues: 1) a nitrogenous base (adenine), 2) a monosaccharide (ribose), 3) three phosphoric acids. Since ATP contains not one, but three residues of phosphoric acid, it belongs to ribonucleoside triphosphates.

For most types of work occurring in cells, the energy of ATP hydrolysis is used. At the same time, when the terminal residue of phosphoric acid is cleaved off, ATP passes into ADP (adenosine diphosphoric acid), when the second phosphoric acid residue is cleaved off, into AMP (adenosine monophosphoric acid). The yield of free energy during the elimination of both the terminal and the second residues of phosphoric acid is 30.6 kJ each. Cleavage of the third phosphate group is accompanied by the release of only 13.8 kJ. The bonds between the terminal and the second, second and first residues of phosphoric acid are called macroergic (high-energy).

ATP reserves are constantly replenished. In the cells of all organisms, ATP synthesis occurs in the process of phosphorylation, i.e. addition of phosphoric acid to ADP. Phosphorylation occurs with different intensity during respiration (mitochondria), glycolysis (cytoplasm), photosynthesis (chloroplasts).

ATP is the main link between processes accompanied by the release and accumulation of energy, and processes that require energy. In addition, ATP, along with other ribonucleoside triphosphates (GTP, CTP, UTP), is a substrate for RNA synthesis.

Gene Properties

1. discreteness - immiscibility of genes;
2. stability - the ability to maintain a structure;
3. lability - the ability to repeatedly mutate;
4. multiple allelism - many genes exist in a population in a variety of molecular forms;
5. allelism - in the genotype of diploid organisms, only two forms of the gene;
6. specificity - each gene encodes its own trait;
7. pleiotropy - multiple effect of a gene;
8. expressivity - the degree of expression of a gene in a trait;
9. penetrance - the frequency of manifestation of a gene in the phenotype;
10. amplification - an increase in the number of copies of a gene.

Classification

1. Structural genes are unique components of the genome, representing a single sequence encoding a specific protein or some types of RNA. (See also the article housekeeping genes).
2. Functional genes - regulate the work of structural genes.

Genetic code- a method inherent in all living organisms to encode the amino acid sequence of proteins using a sequence of nucleotides.

Four nucleotides are used in DNA - adenine (A), guanine (G), cytosine (C), thymine (T), which in Russian-language literature are denoted by the letters A, G, C and T. These letters make up the alphabet of the genetic code. In RNA, the same nucleotides are used, with the exception of thymine, which is replaced by a similar nucleotide - uracil, which is denoted by the letter U (U in Russian-language literature). In DNA and RNA molecules, nucleotides line up in chains and, thus, sequences of genetic letters are obtained.

Genetic code

There are 20 different amino acids used in nature to build proteins. Each protein is a chain or several chains of amino acids in a strictly defined sequence. This sequence determines the structure of the protein, and therefore all its biological properties. The set of amino acids is also universal for almost all living organisms.

The implementation of genetic information in living cells (that is, the synthesis of a protein encoded by a gene) is carried out using two matrix processes: transcription (that is, the synthesis of mRNA on a DNA template) and translation of the genetic code into an amino acid sequence (synthesis of a polypeptide chain on mRNA). Three consecutive nucleotides are enough to encode 20 amino acids, as well as the stop signal, which means the end of the protein sequence. A set of three nucleotides is called a triplet. Accepted abbreviations corresponding to amino acids and codons are shown in the figure.

Properties

1. Tripletity- a significant unit of the code is a combination of three nucleotides (triplet, or codon).
2. Continuity- there are no punctuation marks between the triplets, that is, the information is read continuously.
3. non-overlapping- the same nucleotide cannot be part of two or more triplets at the same time (not observed for some overlapping genes of viruses, mitochondria and bacteria that encode several frameshift proteins).
4. Unambiguity (specificity)- a certain codon corresponds to only one amino acid (however, the UGA codon in Euplotes crassus codes for two amino acids - cysteine ​​and selenocysteine)
5. Degeneracy (redundancy) Several codons can correspond to the same amino acid.
6. Versatility- the genetic code works the same way in organisms different levels complexity - from viruses to humans (genetic engineering methods are based on this; there are a number of exceptions, shown in the table in the section "Variations of the standard genetic code" below).
7. Noise immunity- mutations of nucleotide substitutions that do not lead to a change in the class of the encoded amino acid are called conservative; nucleotide substitution mutations that lead to a change in the class of the encoded amino acid are called radical.

Protein biosynthesis and its steps

Protein biosynthesis- a complex multi-stage process of synthesis of a polypeptide chain from amino acid residues, occurring on the ribosomes of cells of living organisms with the participation of mRNA and tRNA molecules.

Protein biosynthesis can be divided into stages of transcription, processing and translation. During transcription, the genetic information encoded in DNA molecules is read and this information is written into mRNA molecules. During a series of successive stages of processing, some fragments that are unnecessary in subsequent stages are removed from mRNA, and the nucleotide sequences are edited. After the code is transported from the nucleus to the ribosomes, the actual synthesis of protein molecules occurs by attaching individual amino acid residues to the growing polypeptide chain.

Between transcription and translation, the mRNA molecule undergoes a series of successive changes that ensure the maturation of a functioning template for the synthesis of the polypeptide chain. A cap is attached to the 5' end, and a poly-A tail is attached to the 3' end, which increases the lifespan of the mRNA. With the advent of processing in a eukaryotic cell, it became possible to combine gene exons to obtain a greater variety of proteins encoded by a single DNA nucleotide sequence - alternative splicing.

Translation consists in the synthesis of a polypeptide chain in accordance with the information encoded in messenger RNA. The amino acid sequence is arranged using transport RNA (tRNA), which form complexes with amino acids - aminoacyl-tRNA. Each amino acid has its own tRNA, which has a corresponding anticodon that “matches” the mRNA codon. During translation, the ribosome moves along the mRNA, as the polypeptide chain builds up. Energy for protein synthesis is provided by ATP.

The finished protein molecule is then cleaved from the ribosome and transported to the right place in the cell. Some proteins require additional post-translational modification to reach their active state.

Causes of Mutations

Mutations are divided into spontaneous And induced. Spontaneous mutations occur spontaneously throughout the life of an organism under normal environmental conditions with a frequency of about 10 - 9 - 10 - 12 per nucleotide per cell generation.

Induced mutations are called heritable changes in the genome resulting from certain mutagenic effects in artificial (experimental) conditions or under adverse environmental influences.

Mutations appear constantly in the course of processes occurring in a living cell. The main processes leading to the occurrence of mutations are DNA replication, impaired DNA repair, and genetic recombination.

The role of mutations in evolution

With a significant change in the conditions of existence, those mutations that were previously harmful may turn out to be beneficial. Thus, mutations are the material for natural selection. Thus, melanistic mutants (dark-colored individuals) in populations of the birch moth in England were first discovered by scientists among typical light individuals in the middle of the 19th century. Dark coloration occurs as a result of a mutation in one gene. Butterflies spend the day on the trunks and branches of trees, usually covered with lichens, against which the light color is masking. As a result of the industrial revolution, accompanied by atmospheric pollution, lichens died, and the light trunks of birches were covered with soot. As a result, by the middle of the 20th century (for 50-100 generations) in industrial areas, the dark morph almost completely replaced the light one. It has been shown that the main reason for the predominant survival of the black form is the predation of birds, which selectively ate the light-colored butterflies in polluted areas.

If a mutation affects “silent” DNA sections, or leads to the replacement of one element of the genetic code with a synonymous one, then it usually does not manifest itself in the phenotype in any way (the manifestation of such a synonymous replacement may be associated with different frequencies of codon use). However, such mutations can be detected by gene analysis methods. Since mutations most often occur as a result of natural causes, then, assuming that the basic properties of the external environment did not change, it turns out that the frequency of mutations should be approximately constant. This fact can be used to study phylogeny - the study of the origin and relationships of various taxa, including humans. Thus, mutations in silent genes serve as a kind of “molecular clock” for researchers. The "molecular clock" theory also proceeds from the fact that most mutations are neutral, and the rate of their accumulation in a given gene does not depend or weakly depends on the action of natural selection and therefore remains constant for a long time. For different genes, this rate, however, will vary.

The study of mutations in mitochondrial DNA (inherited through the maternal line) and in Y-chromosomes (inherited through the paternal line) is widely used in evolutionary biology to study the origin of races and nationalities, to reconstruct the biological development of mankind.

Cell structure

cell structure

All cellular life forms on earth can be divided into two kingdoms based on the structure of their constituent cells - prokaryotes (pre-nuclear) and eukaryotes (nuclear). Prokaryotic cells are simpler in structure, apparently, they arose earlier in the process of evolution. Eukaryotic cells - more complex, arose later. The cells that make up the human body are eukaryotic.

Despite the variety of forms, the organization of the cells of all living organisms is subject to uniform structural principles.

The living contents of the cell - the protoplast - is separated from the environment by the plasma membrane, or plasmalemma. Inside the cell is filled with cytoplasm, which contains various organelles and cellular inclusions, as well as genetic material in the form of a DNA molecule. Each of the organelles of the cell performs its own special function, and together they all determine the vital activity of the cell as a whole.

prokaryotic cell

The structure of a typical prokaryotic cell: capsule, cell wall, plasmalemma, cytoplasm, ribosomes, plasmid, pili, flagellum, nucleoid.

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

eukaryotic cell(eukaryotes) (from the Greek ευ - good, completely and κάρῠον - core, nut) - organisms that, unlike prokaryotes, have a well-shaped cell nucleus, delimited from the cytoplasm by the nuclear membrane. The genetic material is enclosed in several linear double-stranded DNA molecules (depending on the type of organisms, their number per nucleus can vary from two to several hundred), attached from the inside to the membrane of the cell nucleus and forming in the vast majority (except dinoflagellates) a complex with histone proteins, called chromatin. Eukaryotic cells have a system of internal membranes that form, in addition to the nucleus, a number of other organelles (endoplasmic reticulum, Golgi apparatus, etc.). In addition, the vast majority have permanent intracellular symbionts-prokaryotes - mitochondria, and algae and plants also have plastids.

The structure of a eukaryotic cell

Schematic representation of an animal cell. (When you click on any of the names of the components of the cell, you will be taken to the corresponding article.)

In most cases, eukaryotic cells are part of multicellular organisms. However, in nature there is a considerable number of unicellular eukaryotes, which are structurally a cell, and physiologically - a whole organism. In turn, eukaryotic cells, which are part of a multicellular organism, are not capable of independent existence. They are usually divided into cells of plants, animals and fungi. Each of them has its own characteristics and has its own subtypes of cells that form different tissues.

Despite the diversity, all eukaryotes have a common ancestor, presumably appeared in the process.

In the cells of unicellular eukaryotes (protozoa) there are structural formations that perform the functions of organs at the cellular level. So ciliates have a cellular mouth and pharynx, powder, digestive and contractile vacuoles.

All eukaryotic cells are isolated, delimited from the external environment. In the cytoplasm there are various cell organelles already delimited from it by their membranes. The nucleus contains the nucleolus, chromatin, and nuclear juice. Numerous (larger than in prokaryotes) various inclusions are present in the cytoplasm.

Eukaryotic cells are characterized by a high orderliness of the internal contents. Such compartmentation achieved by dividing the cell into parts by membranes. Thus, the separation of biochemical processes is achieved in the cell. The molecular composition of membranes, the set of substances and ions on their surface is different, which determines their functional specialization.

In the cytoplasm there are proteins-enzymes of glycolysis, sugar metabolism, nitrogenous bases, amino acids and lipids. Microtubules are assembled from certain proteins. The cytoplasm performs unifying and framework functions.

Inclusions are relatively unstable components of the cytoplasm, which are nutrient reserves, secretion granules (products for removal from the cell), ballast (a number of pigments).

Organelles are permanent and perform vital functions. They include organelles general meaning(, ribosomes, polysomes, microfibrils and, centrioles, and others) and special in specialized cells (microvilli, cilia, synaptic vesicles, etc.).

The structure of an animal eukaryotic cell

Eukaryotic cells are capable of endocytosis (uptake of nutrients by the cytoplasmic membrane).

Eukaryotes (if any) are of a different chemical nature than prokaryotes. In the latter, it is based on murein. In plants, it is mainly cellulose, and in fungi, it is chitin.

The genetic material of eukaryotes is contained in the nucleus and is packaged in chromosomes, which are a complex of DNA and proteins (mainly histones).