Unlike asexual cells, gametes are characterized by a low level of metabolic processes.
In anaphase II, the centromeres of chromosomes divide and the chromatids diverge to the poles of the cell due to the shortening of the filaments of the spindle.
During telophase II, chromosomes despiralize, the fission spindle disappears, and nucleoli and nuclear envelope are formed. Telophase II ends with the division of the cytoplasm. Thus, as a result of the second meiotic division, the number of chromosomes remains the same as after the first, but the amount of DNA, due to the difference of chromatids to daughter cells, is halved.
Thus, after two consecutive meiotic divisions of the mother diploid cell, four haploid daughters are formed, each of which has the same set of genes, but individual genes of different daughter cells may be in different states (represented by different alleles). That is, the resulting daughter cells may differ in hereditary information.
Biological significance of meiosis. If the number of chromosomes did not decrease during meiotic divisions, it would double in each subsequent generation when the germ cell nuclei merge. Due to meiosis, mature germ cells receive a haploid set of chromosomes. At fertilization the diploid set inherent in this kind of organisms is restored. This preserves a constant set of chromosomes (karyotype) and the amount of nuclear DNA for each species.
The exchange of sites between homologous chromosomes (the process of crossover), as well as the independent divergence of homologous chromosomes to different daughter cells, contributes to hereditary variability, as new combinations of different states (alleles) of certain genes appear. Of each pair of homologous chromosomes (maternal and paternal) that are part of the chromosome set of diploid organisms, the haploid set of germ cells contains only one. It can be paternal, maternal, paternal with a maternal plot or maternal with a paternal plot.
Germ cells perform the function of transmitting hereditary information from individuals of the paternal generation to offspring. Compared with asexual (somatic) cells, they have a half (usually haploid) set of chromosomes, which ensures their reproduction in the fertilized egg of a typical set of chromosomes (usually diploid). Unlike asexual cells, gametes are characterized by a low level of metabolic processes.
Eggs differ from sperm in larger size because they mostly contain a supply of nutrients needed for embryo development and can be covered with various shells. For example, in birds, the egg is additionally covered with a thick protein shell, two thin subshells, a hard limestone shell, and an outer thin cuticular layer. These shells perform a protective function, and protein is also a source of water for the developing embryo.
The size of the egg depends on the amount of spare nutrients in its cytoplasm. For example, in placental mammals, the embryo of which receives nutrients from the mother through the placenta, the size of the egg (without shell) varies from 50 μm (voles) to 180 μm (sheep), the diameter of the human egg is 90 μm.
If the cytoplasm of the egg accumulates a significant supply of nutrients (yolk), its size (without shells) can reach several centimeters: 50-70 mm (sharks), 80 mm (ostriches). Given the shells, the size of such eggs is even larger. Thus, the African ostrich egg length can be more than 150 mm and weigh 1.5-2 kg.
Sperm are smaller than an egg, ranging in length from 10 to 800 microns, but can sometimes reach 8,000 microns (some crustaceans). Sperm move mainly through the flagella. The structure of these flagella is similar to the structure of flagella of different types of eukaryotic cells, they provide translational movement in a liquid medium. Sperm with flagella are characteristic of different groups of organisms (green and brown algae, ferns, plauns, horsetails, mosses, chordates, etc.).
Consider the structure of mammalian sperm. They have a short head that contains the nucleus. At its anterior end is a special organelle – the acrosome, which is formed by elements of the Golgi complex. The acrosome ensures the penetration of the sperm into the egg, releasing enzymes that dissolve the shell of the latter, as well as provides its activation – the transition from rest to development. Behind the head is the neck, and behind it – a short intermediate section. It contains a centriole surrounded by mitochondria, which generate energy for the flagellum, located at the rear end of the https://123helpme.me/ethan-frome/ cell.
In some higher plants (most gymnosperms, angiosperms), some algae (red) and fungi, as well as in some groups of animals (roundworms, crayfish, etc.) sperm do not have flagella.
Immune system: concepts and functions. Abstract
The function of the immune system is to recognize genetically foreign antigens and specifically respond to them. Its main purpose is to neutralize and destroy those antigens that stimulate the immune response
Formation of immune reactions. There are forms of specific immune reactions (responses):
Antibody production Immunological memory Immunological tolerance (selective non-response to a given antigen on re-encounter). Occurrence of allergy – hypersensitivity to a specific antigen.
Production of antibodies – proteins that belong to a class of immunoglobulins, the synthesis of which is stimulated after the receipt of antigen. Antibodies interact with this antigen. But immune reactions often in the course of their implementation damage and destroy their own cells and non-cellular structures of the body. This type of immune reaction is called an allergy.
Allergy is a prerequisite for the pathological immune process. Allergens are substances that cause allergic reactions. There are external allergens (exotic allergens): food, chemicals, drugs, odors). Internal allergens (endo-allergens) are the body’s own tissues mainly with altered properties that occur due to the formation of toxic substances in pathological processes (burns or frostbite, the action of toxic substances, bee stings, ionizing radiation).
Manifestations of allergies: urticaria, edema, redness, local or general increase in t, itching, pain, narrowing of the airways (asthma), allergic eczema, cough.
Allergy is determined by hereditary predisposition of the body, metabolic disorders, activity with. in. with., the development of neuroses, malnutrition.
Ways to prevent allergies: tempering, exercise, healthy living, no harmful environmental influences, avoid contact with allergens.
Immunological memory is the body’s ability to respond rapidly and enhanced to re-introduced antigen. Immunological memory is retained by T lymphocytes, which have a much longer lifespan than B lymphocytes. The presence of immune memory is due to the increased content of B-lymphocytes that carry the corresponding receptors (T-memory lives for decades).
Possible causes of suppression of the immune system
The immune system is represented by central and peripheral organs. The central ones include: red bone marrow, thymus. To the peripheral: lymph nodes, spleen, tonsils, appendix.
The cause of suppression of the immune system is the occurrence of immunological insufficiency – a congenital or acquired defect of the immune system, which is manifested by the inability of the body to carry out humoral or cellular immune responses.
Failure can be primary or secondary.
a) primary – due to congenital defects of the immune system (genes, chromosomal mutations, intrauterine infections). b) secondary – acquired insufficiency. The reasons can be: radiation, chemicals, viruses, aging, intoxication (burns, malignant tumors, uremia), poor nutrition, frequent hypothermia or overheating, excessive sun exposure.
AIDS – the syndrome was first described in 1981 by an American researcher. In 1983, the pathogen was isolated for the first time.
HIV is a human immunodeficiency virus. Ways of HIV transmission – from mother to child, through blood, when using infected medical instruments, sexual.
The AIDS virus affects T lymphocytes.
Cute – stimulates the heart, accelerates heart rate.
Along with the nervous system, the heart is affected by substances carried by the blood.
Adrenal hormones: adrenaline and noradrenaline and Ca ions – increase and accelerate heart rate (thyroxine).
Acetylcholine and K ions – reduce the frequency and strength of heart contractions.
In addition, an increase in blood CO2 levels or a decrease in O2 concentration leads to an increase in heart rate.
The action of humoral factors is closely related to nervous regulation.
The work of the heart is definitely affected by the cerebral cortex, ie conditioned reflexes (when excited).
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Lesson plan-summary on the stated topic with multimedia support (media presentation is attached), the purpose of which is to form students’ ideas about the age characteristics of nutrition, caloric content of food; provide information on the necessary components of food; teach to follow a complete diet; continue to develop healthy lifestyle skills
Development of a lesson “Nutrition” on the basics of health in 6th grade
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Author: Shevchuk Tamara DenisovDolzhnost: teacher of social disciplines.
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