Cellular structure of the body. What role do chromosomes play in a cell: structure and functions
Chromosomes- cell structures that store and transmit hereditary information. A chromosome consists of DNA and protein. A complex of proteins bound to DNA forms chromatin. Proteins play an important role in packaging DNA molecules in the nucleus.
The DNA in chromosomes is packaged in such a way that it fits in the nucleus, the diameter of which usually does not exceed 5 microns (5-10 -4 cm). The DNA packaging takes on the appearance of a loop structure, similar to the lampbrush chromosomes of amphibians or the polytene chromosomes of insects. The loops are maintained by proteins that recognize specific nucleotide sequences and bring them together. The structure of the chromosome is best seen in metaphase of mitosis.
The chromosome is a rod-shaped structure and consists of two sister chromatids, which are held by the centromere in the region of the primary constriction. Each chromatid is made up of chromatin loops. Chromatin does not replicate. Only DNA is replicated.
Rice. 14. Chromosome structure and replication
When DNA replication begins, RNA synthesis stops. Chromosomes can be in two states: condensed (inactive) and decondensed (active).
The diploid set of chromosomes of an organism is called a karyotype. Modern research methods make it possible to identify each chromosome in a karyotype. To do this, take into account the distribution of light and dark bands visible under a microscope (alternating AT and GC pairs) in chromosomes treated with special dyes. The chromosomes of representatives of different species have transverse striations. Related species, such as humans and chimpanzees, have very similar patterns of alternating bands in their chromosomes.
Each type of organism has a constant number, shape and composition of chromosomes. There are 46 chromosomes in the human karyotype - 44 autosomes and 2 sex chromosomes. Males are heterogametic (XY) and females are homogametic (XX).
The Y chromosome differs from the X chromosome in the absence of some alleles (for example, the blood clotting allele). Chromosomes of the same pair are called homologous. Homologous chromosomes at identical loci carry allelic genes.
1.14. Reproduction in the organic world- this is the reproduction of genetically similar individuals of a given species, ensuring continuity and continuity of life.
Asexual reproduction carried out in the following ways:
- simple division into two or many cells at once (bacteria, protozoa);
- vegetatively (plants, coelenterates);
- dividing a multicellular body in half with subsequent regeneration (starfish, hydra);
- budding (bacteria, coelenterates);
- formation of disputes.
Asexual reproduction usually ensures an increase in the number of genetically homogeneous offspring. But when spore nuclei are produced by meiosis, the offspring from asexual reproduction will be genetically different.
Sexual reproduction- a process in which genetic information from two individuals is combined.
Individuals of different sexes form gametes. Females produce eggs, males produce sperm, and hermaphrodites produce both eggs and sperm. And in some algae, two identical sex cells merge.
When haploid gametes fuse, fertilization occurs and a diploid zygote is formed.
The zygote develops into a new individual.
All of the above is true only for eukaryotes. Prokaryotes also have a sexual process, but it occurs differently.
Thus, during sexual reproduction, the genomes of two different individuals of the same species are mixed. Offspring carry new genetic combinations that differentiate them from their parents and from each other.
One type of sexual reproduction is parthenogenesis, or the development of individuals from an unfertilized egg (aphids, drones of bees, etc.).
The structure of germ cells
Ovules- round, relatively large, immobile cells. Dimensions - from 100 microns to several centimeters in diameter. They contain all the organelles characteristic of a eukaryotic cell, as well as the inclusion of reserve nutrients in the form of yolk. The egg cell is covered with an egg membrane, consisting mainly of glycoproteins.
Rice. 15. The structure of a bird's egg: 1 - chalaza; 2 - shell; 3 - air chamber; 4 - outer subshell membrane; 5 - liquid protein; 6 - dense protein; 7 - germinal disc; 8 - light yolk; 9 - dark yolk.
In mosses and ferns, the eggs develop in archegonia; in flowering plants, in ovules located in the ovary of the flower.
Oocytes are divided as follows:
- isolecithal - the yolk is evenly distributed and there is little of it (in worms, mollusks);
- alecithal - almost devoid of yolk (mammals);
- telolecithal - contain a lot of yolk (fish, birds);
- polylecithal - contain a significant amount of yolk.
Oogenesis is the formation of eggs in females.
In the reproduction zone there are oogonia - primary germ cells that reproduce by mitosis.
From the oogonia, after the first meiotic division, first-order oocytes are formed.
After the second meiotic division, second-order oocytes are formed, from which one egg and three guiding bodies are formed, which then die.
Sperm- small, mobile cells. They have a head, neck and tail.
In the anterior part of the head there is an acrosomal apparatus - an analogue of the Golgi apparatus. It contains an enzyme (hyaluronidase) that dissolves the egg membrane during fertilization. The neck contains centrioles and mitochondria. Flagella are formed from microtubules. During fertilization, only the nucleus and centrioles of the sperm enter the egg. Mitochondria and other organelles remain outside. Therefore, cytoplasmic inheritance in humans is transmitted only through the female line.
The sex cells of sexually reproducing animals and plants are formed through a process called gametogenesis.
A cell is a single living system consisting of two inextricably linked parts - the cytoplasm and the nucleus (color table XII).
Cytoplasm- this is the internal semi-liquid environment in which the nucleus and all organelles of the cell are located. It has a fine-grained structure, penetrated by numerous thin threads. It contains water, dissolved salts and organic matter. The main function of the cytoplasm is to unite into one and ensure the interaction of the nucleus and all organelles of the cell.
Outer membrane surrounds the cell with a thin film consisting of two layers of protein, between which is a fatty layer. It is permeated with numerous small pores through which the exchange of ions and molecules occurs between the cell and the environment. The thickness of the membrane is 7.5-10 nm, the pore diameter is 0.8-1 nm. In plants, a membrane of fiber is formed on top of it. The main functions of the outer membrane are to limit the internal environment of the cell, protect it from damage, regulate the flow of ions and molecules, remove metabolic products and synthesized substances (secrets), connect cells and tissues (due to outgrowths and folds). The outer membrane ensures the penetration of large particles into the cell by phagocytosis (see sections in “Zoology” - “Protozoa”, in “Anatomy” - “Blood”). In a similar way, the cell absorbs drops of liquid - pinocytosis (from the Greek “pino” - drink).
Endoplasmic reticulum(EPS) is a complex system of channels and cavities consisting of membranes that penetrate the entire cytoplasm. There are two types of EPS - granular (rough) and smooth. On the membranes of the granular network there are many tiny bodies - ribosomes; there are none in a smooth network. The main function of the EPS is participation in the synthesis, accumulation and transportation of the main organic substances produced by the cell. Protein is synthesized in granular EPS, and carbohydrates and fats are synthesized in smooth EPS.
Ribosomes- small bodies, 15-20 nm in diameter, consisting of two particles. There are hundreds of thousands of them in each cell. Most ribosomes are located on the membranes of granular ER, and some are located in the cytoplasm. They consist of proteins and r-RNA. The main function of ribosomes is protein synthesis.
Mitochondria- these are small bodies, 0.2-0.7 microns in size. Their number in a cell reaches several thousand. They often change shape, size and location in the cytoplasm, moving to their most active part. The outer cover of the mitochondrion consists of two three-layer membranes. The outer membrane is smooth, the inner membrane forms numerous outgrowths on which respiratory enzymes are located. The inner cavity of mitochondria is filled with fluid, which houses ribosomes, DNA and RNA. New mitochondria are formed when old ones divide. The main function of mitochondria is ATP synthesis. They synthesize a small amount of proteins, DNA and RNA.
Plastids characteristic only of plant cells. There are three types of plastids - chloroplasts, chromoplasts and leucoplasts. They are capable of mutual transition into each other. Plastids reproduce by fission.
Chloroplasts(60) are green in color and oval in shape. Their size is 4-6 microns. From the surface, each chloroplast is bounded by two three-layer membranes - outer and inner. Inside it is filled with liquid, in which there are several dozen special, interconnected cylindrical structures - grana, as well as ribosomes, DNA and RNA. Each grana consists of several dozen flat membrane sacs superimposed on each other. In cross section, it has a rounded shape, its diameter is 1 micron. All the chlorophyll is concentrated in the granas; the process of photosynthesis occurs in them. The resulting carbohydrates first accumulate in the chloroplast, then enter the cytoplasm, and from there into other parts of the plant.
Chromoplasts determine the red, orange and yellow colors of flowers, fruits and autumn leaves. They have the form of multifaceted crystals located in the cytoplasm of the cell.
Leukoplasts colorless. They are found in uncolored parts of plants (stems, tubers, roots) and have a round or rod-shaped shape (5-6 microns in size). Spare substances are deposited in them.
Cell center found in the cells of animals and lower plants. It consists of two small cylinders - centrioles (about 1 μm in diameter), located perpendicular to each other. Their walls consist of short tubes, the cavity is filled with a semi-liquid substance. Their main role is the formation of a spindle and uniform distribution of chromosomes among daughter cells.
Golgi complex was named after the Italian scientist who first discovered it in nerve cells. It has a varied shape and consists of cavities bounded by membranes, tubes extending from them and vesicles located at their ends. The main function is the accumulation and excretion of organic substances synthesized in the endoplasmic reticulum, the formation of lysosomes.
Lysosomes- round bodies with a diameter of about 1 micron. On the surface, the lysosome is bounded by a three-layer membrane; inside it there is a complex of enzymes capable of breaking down carbohydrates, fats and proteins. There are several dozen lysosomes in a cell. New lysosomes are formed in the Golgi complex. Their main function is to digest food that has entered the cell by phagocytosis and remove dead organelles.
Organoids of movement- flagella and cilia - are cell outgrowths and have the same structure in animals and plants (their common origin). The movement of multicellular animals is ensured by muscle contractions. The main structural unit of a muscle cell is myofibrils - thin filaments more than 1 cm long, 1 micron in diameter, located in bundles along the muscle fiber.
Cellular inclusions- carbohydrates, fats and proteins - belong to the non-permanent components of the cell. They are synthesized periodically, accumulate in the cytoplasm as reserve substances and are used in the process of vital activity of the body.
Carbohydrates are concentrated in starch grains (in plants) and glycogen (in animals). There are many of them in liver cells, potato tubers and other organs. Fats accumulate in the form of droplets in plant seeds, subcutaneous tissue, connective tissue, etc. Proteins are deposited in the form of grains in animal eggs, plant seeds and other organs.
Core- one of the most important organelles of the cell. It is separated from the cytoplasm by a nuclear envelope consisting of two three-layer membranes, between which there is a narrow strip of semi-liquid substance. Through the pores of the nuclear membrane, substances are exchanged between the nucleus and the cytoplasm. The cavity of the nucleus is filled with nuclear juice. It contains a nucleolus (one or more), chromosomes, DNA, RNA, proteins and carbohydrates. The nucleolus is a round body ranging in size from 1 to 10 microns or more; it synthesizes RNA. Chromosomes are visible only in dividing cells. In the interphase (non-dividing) nucleus they are present in the form of thin long strands of chromatin (DNA-protein connections). They contain hereditary information. The number and shape of chromosomes in each species of animal and plant are strictly defined. Somatic cells, which make up all organs and tissues, contain a diploid (double) set of chromosomes (2n); sex cells (gametes) - haploid (single) set of chromosomes (n). A diploid set of chromosomes in the nucleus of a somatic cell is created from paired (identical) homologous chromosomes. Chromosomes of different pairs(non-homologous) differ from each other in shape, location centromeres And
secondary constrictions. Prokaryotes
- these are organisms with small, primitively arranged cells, without a clearly defined nucleus. These include blue-green algae, bacteria, phages and viruses. Viruses are DNA or RNA molecules coated with a protein coat. They are so small that they can only be seen with an electron microscope. They lack cytoplasm, mitochondria and ribosomes, so they are not able to synthesize the protein and energy necessary for their life. Once in a living cell and using foreign organic substances and energy, they develop normally. Eukaryotes
- organisms with larger typical cells containing all the main organelles: nucleus, endoplasmic reticulum, mitochondria, ribosomes, Golgi complex, lysosomes and others. Eukaryotes include all other plant and animal organisms. Their cells have a similar type of structure, which convincingly proves the unity of their origin.
1. What shapes are cells? What does this depend on?
The shape of the cells of our body is very diverse: flat, round, spindle-shaped, convoluted, have one or more processes or flagella, which depends on the location of the cells in the body and the functions performed by these cells.
2. Name the role of the kernel; cytoplasm; cell membrane.
Cytoplasm is the living contents of the cell and consists of organelles, inclusions and hyaloplasm. Hyaloplasm forms the internal environment of the cell and ensures the interaction of all parts of the cell with each other; the composition of the hyaloplasm determines the osmotic properties of the cell. Organelles (endoplasmic reticulum, Golgi complex, mitochondria, lysosomes) ensure the normal functioning of cells in particular and the body as a whole (see questions 7,8,9,10).
The cell membrane serves as the outer framework of the cell and limits the cell from the external environment; main functions: protective and transport, the membrane also provides communication between cells, is involved in the perception of signals from the environment and transmitting them to the cell (receptor), participates in the construction of special cell structures (flagella, processes, etc.)
3. What are the functions of the kernel? Which human cells do not contain it?
The nucleus is responsible for storing and transmitting hereditary information in the form of an unchanged DNA structure; regulation of all vital processes through the protein synthesis system. Most human cells have one nucleus; there are also multinucleated cells; red blood cells are non-nucleated.
4. How many chromosomes are there in human germ and somatic cells?
In humans, somatic cells contain a double set of chromosomes - 23 pairs (46 chromosomes); in the sexes - single (23 chromosomes).
5. What is the cytoplasm? What is its role in the cell?
See question 2.
6. Explain the significance for a cell of such a membrane property as semi-permeability?
Semi-permeability is the ability of living cells to allow some substances to pass through and not others. Water with some dissolved substances necessary to nourish the cells penetrate into the cell along a concentration gradient, and waste products are removed out, which ensures the maintenance of the constancy of the ionic and molecular composition in the cell.
7. Tell us about the structure and role of the endoplasmic reticulum in the cell.
The endoplasmic reticulum (ER) is a kind of labyrinth of many tiny tubules, vesicles, sacs of various shapes and sizes, the walls of which are formed by elementary biological membranes. There are 2 types of endoplasmic reticulum: agranular (smooth) and granular (granular, containing ribosomes on the surface of channels and cavities). EPS ensures the division of the cell cytoplasm into compartments that prevent the mixing of the chemical processes occurring in them; ensures the transport of substances both within the cell and between neighboring cells. Granular ER accumulates, isolates for maturation and transports proteins synthesized by ribosomes on its surface, synthesizes cell membranes; The smooth ER synthesizes and transports lipids, complex carbohydrates and steroid hormones, and removes toxic substances from the cell.
8. What functions does the Golgi complex perform? How is it built?
The Golgi complex (CG) is a system of flat sacs (cisternae), from which vesicles bud, and a system of membrane tubules connecting the complex with the channels and cavities of the smooth ER. In the CG tanks, products of synthesis, decay, and substances entering the cell, as well as substances that are removed from the cell, accumulate. The accumulated substances are packaged into vesicles and enter the cytoplasm, which are then either used to nourish the cell or removed outside.
9. Why are mitochondria called the “battery” of the cell?
The main function of mitochondria is the oxidation of organic substances, accompanied by the release of energy, which goes into the formation of ATP molecules, which serves as a universal cellular battery.
10. What organelles take part in the destruction and dissolution of parts of the cell that have lost their significance?
Such organelles are lysosomes.
11. Come up with and draw up a diagram of the “Structure of an animal cell.”
12. Remember how a human cell differs from a plant cell; mushroom; bacteria.
Unlike plant cells, animal and human cells do not have a cell wall, chloroplasts, or large vacuoles. The storage carbohydrate of plant cells is starch, and that of animal cells is glycogen. The method of nutrition of plant cells is autotrophic, and that of animal cells is heterotrophic.
Fungal cells have a cell wall made of chitin and large vacuoles. Most fungal cells are multinucleate, unlike animal cells, where most cells are mononucleate.
Bacterial cells, unlike human cells, do not have a formed nucleus and nucleoli, but have mesosomes, which replace other membrane organelles for bacteria. The shell of some bacteria contains a mucous capsule, which is not found in human cells. In human flagellar cells (spermatozoa), the flagella have a complex structure and contain microtubules; in bacteria, the flagella have a simple structure. In bacteria, cells are divided by binary fission, rarely by budding and conjugation; in humans, by mitosis, meiosis, and amitosis.
13. Why is a cell considered a structural and functional element of the body?
The body is made up of a large number of cells, each of which performs its own special function, but together they ensure the unified functioning of the body as a whole. Each cell of the body has the basic properties of living organisms as a whole: self-renewal, self-regulation and self-reproduction.
Cells, like the bricks of a house, are the building material of almost all living organisms. What parts do they consist of? What function do various specialized structures perform in a cell? You will find answers to these and many other questions in our article.
What is a cell
A cell is the smallest structural and functional unit of living organisms. Despite its relatively small size, it forms its own level of development. Examples of single-celled organisms are the green algae Chlamydomonas and Chlorella, the protozoa Euglena, amoeba and ciliates. Their sizes are truly microscopic. However, the function of the body cell of a given systematic unit is quite complex. These are nutrition, breathing, metabolism, movement in space and reproduction.
General plan of cell structure
Not all living organisms have a cellular structure. For example, viruses are formed by nucleic acids and a protein shell. Plants, animals, fungi and bacteria are made up of cells. They all differ in structural features. However, their general structure is the same. It is represented by the surface apparatus, internal contents - cytoplasm, organelles and inclusions. The functions of cells are determined by the structural features of these components. For example, in plants, photosynthesis occurs on the inner surface of special organelles called chloroplasts. Animals do not have these structures. The structure of the cell (the table “Structure and Functions of Organelles” examines all the features in detail) determines its role in nature. But all multicellular organisms have in common the provision of metabolism and interconnection between all organs.
Cell structure: table "Structure and functions of organelles"
This table will help you familiarize yourself in detail with the structure of cellular structures.
Cellular structure | Structural features | Functions |
Core | Double-membrane organelle containing DNA molecules in its matrix | Storage and transmission of hereditary information |
Endoplasmic reticulum | System of cavities, cisterns and tubules | Synthesis of organic substances |
Golgi complex | Numerous cavities from pouches | Storage and transportation of organic substances |
Mitochondria | Round, double-membrane organelles | Oxidation of organic substances |
Plastids | Double-membrane organelles, the inner surface of which forms projections into the structure | Chloroplasts provide the process of photosynthesis, chromoplasts give color to various parts of plants, leucoplasts store starch |
Ribosomes | consisting of large and small subunits | Protein biosynthesis |
Vacuoles | In plant cells these are cavities filled with cell sap, and in animals they are contractile and digestive cavities. | Supply of water and minerals (plants). ensure the removal of excess water and salts, and digestive - metabolism |
Lysosomes | Round vesicles containing hydrolytic enzymes | Biopolymer degradation |
Cell center | Non-membrane structure consisting of two centrioles | Spindle formation during cell cleavage |
As you can see, each cellular organelle has its own complex structure. Moreover, the structure of each of them determines the functions performed. Only the coordinated work of all organelles allows life to exist at the cellular, tissue and organismal levels.
Basic functions of the cell
A cell is a unique structure. On the one hand, each of its components plays its role. On the other hand, the functions of the cell are subordinated to a single coordinated operating mechanism. It is at this level of life organization that the most important processes take place. One of them is reproduction. It is based on a process. There are two main ways of doing it. So, gametes are divided by meiosis, all others (somatic) are divided by mitosis.
Due to the fact that the membrane is semi-permeable, various substances can enter the cell in the opposite direction. The basis for all metabolic processes is water. Upon entering the body, biopolymers are broken down into simple compounds. But minerals are found in solutions in the form of ions.
Cellular inclusions
Cell functions would not be fully realized without the presence of inclusions. These substances are a reserve of organisms for unfavorable periods. This could be drought, low temperature, or insufficient oxygen. The storage functions of substances in plant cells are performed by starch. It is found in the cytoplasm in the form of granules. In animal cells, glycogen serves as a storage carbohydrate.
What are fabrics
Cells that are similar in structure and function are united into tissues. This structure is specialized. For example, all cells of epithelial tissue are small and tightly adjacent to each other. Their shape is very diverse. This fabric is practically absent. This structure resembles a shield. Thanks to this, epithelial tissue performs a protective function. But any organism needs not only a “shield”, but also a relationship with the environment. To carry out this function, the epithelial layer has special formations - pores. And in plants, a similar structure is the stomata of the skin or lentils of the cork. These structures carry out gas exchange, transpiration, photosynthesis, and thermoregulation. And above all, these processes are carried out at the molecular and cellular level.
Relationship between cell structure and function
The functions of cells are determined by their structure. All fabrics are a clear example of this. Thus, myofibrils are capable of contraction. These are muscle tissue cells that carry out the movement of individual parts and the entire body in space. But the connecting one has a different structural principle. This type of tissue consists of large cells. They are the basis of the entire organism. Connective tissue also contains a large amount of intercellular substance. This structure ensures its sufficient volume. This type of tissue is represented by such varieties as blood, cartilage, and bone tissue.
They say that they are not restored... There are many different views on this fact. However, no one doubts that neurons connect the entire body into a single whole. This is achieved by another structural feature. Neurons consist of a body and processes - axons and dendrites. Through them, information flows sequentially from the nerve endings to the brain, and from there back to the working organs. As a result of the work of neurons, the entire body is connected by a single network.
So, most living organisms have a cellular structure. These structures are the building blocks of plants, animals, fungi and bacteria. The general functions of cells are the ability to divide, perceive environmental factors and metabolism.