Where do male sex cells mature? Development and structure of germ cells. Hayflick limit and telomeres
reproduction- the most important property of all living things. A species that reproduces only asexually can flourish for quite a long time if it lives in relatively constant conditions. If changes occur in its habitat that cause the death of individual individuals, it is very likely that all individuals will die, because they are very similar genetically.
At sexual maternal and paternal organisms produce specialized sex cells -. Female non-motile gametes are called eggs, male non-motile gametes are called sperm, and motile gametes are called spermatozoa. These germ cells fuse to form a zygote, i.e. fertilization takes place. Sex cells, as a rule, have a half set of chromosomes (), so that when they merge, a double (diploid) set is restored, a new individual develops from the zygote. During sexual reproduction, offspring are formed by the fusion of haploid nuclei. Haploid nuclei are formed as a result of meiotic division.
Meiosis leads to a halving of the genetic material, due to which the amount of genetic material in individuals of a given species remains constant in a number of generations. During meiosis, several important processes occur: random segregation of chromosomes (independent dissection), exchange of material between homologous chromosomes (crossing over). As a result of these processes, new combinations of genes arise. Since the zygote nucleus after fertilization contains the genetic material of two parental individuals, this increases the genetic diversity within the species. If the essence and biological significance of the sexual process is the same for all organisms, then its forms are very diverse and depend on the level of development, habitat, lifestyle and some other features.
sexual reproduction found in all plant groups. Mosses grow in sods. Male and female plants are side by side. Rainwater helps the spermatozoa to get to the tops of the female plants, where they merge with the eggs, a zygote is formed, from which a spore box sitting on a long leg develops. In germ cells, they develop on the outgrowth formed as a result of spore germination. On the underside of the growth, the female organs are archegonia, while the male organs are antheridia. In a humid environment, the germ cells merge, the zygote gives rise to an embryo, from which a young one grows. In flowering plants, the most difficult sexual reproduction- double fertilization. Pollen (male sex cells) falls on the stigma of the pistil (female reproductive organ) and germinates. The sperm travel along the pollen tube to the. Sperm enter the embryo sac. One merges with the egg and gives rise to the embryo, the second sperm merges with the central cell and gives rise to the endosperm - the reserve nutrients.
sexual reproduction has very big advantages in comparison with asexual. The essence of sexual reproduction is the combination in the hereditary material of the descendant of genetic information from two different sources - parents. Fertilization in animals can be external or internal. Fusion produces a zygote with a double set of chromosomes.
In the nucleus of the zygote, all chromosomes become paired: in each pair, one of the chromosomes is paternal, the other is maternal. The daughter organism that develops from such a zygote is equally equipped with the hereditary information of both parents.
The biological meaning of sexual reproduction consists in the fact that the emerging organisms can combine the beneficial traits of the father and mother. Such organisms are more viable. Sexual reproduction plays an important role in the evolution of organisms.
One young but principled biology teacher once gave a student a "F" for confusing gametes for gonads. At first glance, everything is absolutely correct: gametes are sex cells, and gonads are sex organs. But after all, the student looked straight to the root: the function of the gonads, or genital organs, is the creation of germ cells for the purpose of fertilization and procreation. So, probably, it would be possible not to put a deuce.
Indeed, is it possible to do without gametes at all, and reproduce without germ cells? About reproduction in private, or why gametes are needed, we will consider below.
In nature, reproduction alone is possible, sometimes it happens, but only in certain species and under certain circumstances. This phenomenon is called parthenogenesis. With the help of parthenogenesis, aphids and some lizards can reproduce. Much more well-known is hydra budding, in which one multicellular animal turns into two different creatures.
It is important to note that the hydra is a primitive creature, so this type of reproduction can be considered natural for it. And then, if possible, the hydra tries to use sexual reproduction. As for lizards, under conditions of parthenogenesis, only the female is capable of reproduction.
From an unfertilized egg, which begins to split (out of "hopelessness"), a daughter is obtained, genetically identical to her mother. Therefore, without exaggeration, parthenogenesis can be called cloning.
Of course, only females can reproduce like this, because they can bear offspring. Male individuals would be happy to do the same, but males are not capable of bearing, and even more so, of giving birth to offspring.
There is also copying of the genome of the parent individual in plants. Everyone knows the reproduction of currants by layering, and garden strawberries - by mustaches. But over time, the genetic material is depleted, the harvest becomes smaller, and the lizards, which for a long time have not been able to find at least some gentleman, lose the battle under the sun.
Therefore, gametes (eggs and spermatozoa) are needed in order to provide future offspring with healthy and complete genetic material, taken equally from the father and from the mother. It was sexual reproduction that allowed the emergence of a wide variety of characters, made possible appearance evolution, and has provided us with the most different types useful to man plants and animals.
The formation of germ cells and fertilization are perfect mechanisms for the maintenance and development of life on our planet. But let us now turn to the human germ cells. What is their uniqueness, and how do they differ from ordinary cells, of which there are many billions in us?
Differences
A person who has studied biology can give several answers to the question of how human germ cells differ from somatic (that is, from ordinary cells). And in every case, he will be right. So, spermatozoa are mobile cells with flagella, which are nowhere else in the body, and eggs determine the menstrual cycle, which no other cell in the body does either. All this is correct.
But the main difference between gametes and other highly differentiated cells from which our body is built is the half and “shuffled” content of hereditary material. What does it mean?
It is known that the human karyotype contains 46 chromosomes. Of these, 22 chromosomes are paired, that is, in total, a person has 44 chromosomes that encode a variety of body proteins. They are called autosomes.
Even in the karyotype, there are 2 unpaired chromosomes, which are called sex. In women, these chromosomes are the same - XX, and in men they are different - XY. Therefore, each cell of the body, with the exception of gametes, contains 46 chromosomes (autosomes + sex chromosomes). This set is called double, or diploid, because 22 pairs of autosomes can be arranged in each cell.
Since the male and female gametes are joined together, the total number of chromosomes should be 46. Therefore, the germ cells contain a half, or haploid set. How many autosomes are in human germ cells? Of course, 23. A paired set is formed during fertilization.
In addition to the haploid set, gametes differ from somatic cells in a special type of sexual reproduction of cells, which is called meiosis. If somatic cells simply divide in half, having previously doubled their genetic material, then during meiosis, the genetic material, on the contrary, is halved. Of course, human germ cells are formed under special conditions, and this process is much more complicated than the division of somatic cells.
Meiosis is characteristic of germ cells not only in that it allows you to prepare a separate “key” - a spermatozoon and a “lock” - an egg, but also during meiosis, there are two very important procedures that allow you to avoid the accumulation of mutations and refresh the genetic material.
This is the so-called independent distribution, when during meiosis the chromosomes diverge quite randomly along different poles of the cell, and crossing over.
Crossing over is the process of the exchange of hereditary material between chromosomes within one diploid pair, which have not yet been divided into haploid sets. After crossing over, new combinations of hereditary material arise, which randomly, as a deck of cards is shuffled before being dealt, falls into one gamete. In human germ cells, these processes are required.
As a result, the diversity, and consequently, the inheritance of traits, increases significantly. After the fusion of haploid gametes, a normal, diploid zygote is formed - or a fertilized egg. In the zygote, there are already two sets of parental chromosomes, and a new individual with a unique genetic code is born. This is how human germ cells differ from other human cells in the main.
But even in sexual reproduction, with its perfect shuffling and replacement of genetic material, over time, if it is not updated, certain problems arise. The fact that the genetic material is gradually depleted, mutations accumulate and diseased individuals arise, mankind has long known.
In some not very developed isolated cultures of northern peoples, where there is much inbreeding, often an honored guest passing through the tribe had to share a bed with the wife of the plague host. It was the only way to throw healthy genetic material into the tribal gene pool dying from mutations.
Structure
Primary knowledge about the structure of germ cells was obtained in ancient times, when trying to create various varieties of agricultural crops, since it is much easier to study plant sources of hereditary material, and you can ignore numerous prohibitions, including those of a religious nature.
We have already learned that the human germ cells contain half the number of chromosomes ready to join together. For fertilization to occur, the sperm must be able to convey genetic information to the egg, and the egg must accept the healthiest and most active sperm into its “womb”.
The structure of human germ cells is the best way to show how they are adapted to this function. Consider very briefly how the human sperm and egg are arranged.
Sperm
The spermatozoon is a very small and very mobile gamete. Many millions of spermatozoa are produced daily in the testicles, and they mature every 2 months. From the moment of puberty to old age, they are produced continuously, and at each moment of time a man has a large number of mature spermatozoa. Unlike a woman, a man is always ready for fertilization.
The male spermatozoon consists of a head, neck, intermediate section and flagellum.
The head of the spermatozoon contains the most important thing - the haploid number of chromosomes, from above the head is covered with a special cap - the acrosome, like a frontal armor. The acrosome contains special enzymes that help the sperm dissolve the surface of the egg before fertilization. Thus, the acrosome is a "chemical weapon".
The short neck of the sperm contains axial structures, or centrioles. Centrioles are made up of microtubules that form the flagellum or rigid scaffold of all cells.
As for the intermediate section, it contains a huge number of mitochondria, or energy accumulators - batteries. The sperm must constantly beat its tail, and the mitochondria must supply a large amount of energy to move the sperm towards the egg. The main thing that he must do without fatigue is to go from the vagina to the place of fertilization (sometimes very far, to the ovary).
Then the spermatozoa accumulate near the egg, make their movements for some time, while orienting themselves in a certain place on its surface. After that, the penetration of spermatozoa through the many membranes of the egg begins, and at this stage of “opening unknown doors”, the strongest wins.
Egg
The difference between oogenesis and spermatogenesis lies in the fact that a man begins to produce spermatozoa only after puberty, and in girls, all eggs occur even in the prenatal period. talking plain language, each woman is born with a completely limited set of eggs, and not a single new egg occurs in a woman after birth.
Unlike men, eggs (gametes) in women are not released constantly, but cyclically, and this process is called the ovarian-menstrual cycle. Every 28 days, if fertilization does not take place, then the prepared uterine epithelium is rejected as unnecessary in the form of menstruation, and everything starts all over again.
If we trace the development of the egg by day, it turns out that the most primary follicle, in which the immature oocyte is located, gradually matures, increases in size, and eventually turns into a mature follicle, or Graafian vesicle. It is visible to the naked eye, as it rises above the surface of the ovary and is about one centimeter across.
After that, the mature follicle bursts, and the egg comes out. This is how ovulation happens. During ovulation, the egg is a so-called second-order oocyte. It is this second-order oocyte that contains the haploid set of chromosomes.
In the process of maturation of oocytes in the ovary, the first division of meiosis occurs in a woman, and the second division of meiosis (with the final shuffling of the genetic material) occurs after fertilization. Gametes in humans are formed as a result of meiosis, in contrast to the simple division of somatic cells, which is called mitosis.
Of course, knowledge about what happens to the sperm and egg during maturation could only be obtained in a society where the scientific worldview dominates, there are prerequisites for making discoveries, and there is a material and technical base.
At present, this knowledge is successfully applied in practice. In vitro fertilization has become possible, and soon the time will come when there will be no hereditary diseases, and science will learn how to renew genetic material, and this will save the world from cancer. We hope that our descendants will be able to live to see this truly bright future.
20. Formation of sex cells. Meiosis
Remember!
Where does sex cells form in the human body?
What set of chromosomes do gametes contain? Why?
Specialized cells are required for sexual reproduction. gametes containing a single (haploid) set of chromosomes. When they merge (fertilize), a diploid set is formed, in which each chromosome has a pair - a homologous chromosome. In each pair of homologous chromosomes, one chromosome comes from the father and the other from the mother.
In animals, the process of formation of germ cells - gametogenesis- proceeds in special organs - the sex glands (gonads). In most animals, male sex cells (spermatozoa) are formed in the testes, female gametes (eggs) in the ovaries. The development of the oocyte is called ovogenesis or oogenesis, and spermatozoa spermatogenesis.
The structure of germ cells.
Oocytes- These are relatively large fixed cells of a rounded shape. In some fish, reptiles and birds, they contain a large supply of nutrients in the form of yolk and range in size from 10 mm to 15 cm. The eggs of mammals, including humans, are much smaller (0.1–0.3 mm) and the yolk is practically do not contain.
Spermatozoa - small mobile cells, in humans their length is only about 60 microns. In different organisms, they differ in shape and size, but, as a rule, all spermatozoa have a head, neck and tail, which ensures their mobility. In the head of the sperm cell there is a nucleus containing chromosomes, and an acrosome - a special vial with enzymes necessary to dissolve the egg cell membrane. Mitochondria are concentrated in the neck, which provide the moving sperm with energy (Fig. 63).
Rice. 63. Mammalian sperm: A - electronic photography; B - structure diagram
Spermatozoa were first described by the Dutch naturalist A. Leeuwenhoek in 1677. He also introduced this term - sperm (from the Greek. sperma- seed and zoon- a living being), i.e. a living seed. The mammalian egg was discovered in 1827 by the Russian scientist K. M. Baer.
The formation of germ cells. The development of germ cells is divided into several stages: reproduction, growth, maturation, and in the process of spermatogenesis, the formation stage is also distinguished (Fig. 64).
Rice. 64. Human gametogenesis
Rice. 65. Phases of meiosis
breeding stage. At this stage, the cells that form the walls of the gonads actively divide by mitosis, forming immature germ cells. This stage in men begins with the onset of puberty and lasts almost a lifetime. In women, the formation of primary germ cells is completed even in the embryonic period, that is, the total number of eggs that a woman will mature during her reproductive period is determined already at an early stage in the development of the female body. At the stage of reproduction, the primary germ cells, like all other cells of the body, are diploid.
growth stage. At the growth stage, which is much better expressed in oogenesis, there is an increase in the cytoplasm of cells, the accumulation of necessary substances and DNA reduplication (doubling of chromosomes).
maturation stage. The third stage is meiosis. Meiosis- This is a special way of cell division, leading to a halving of the number of chromosomes and to the transition of the cell from a diploid state to a haploid one.
Future gametes at the stage of maturation divide twice. Cells that begin meiosis contain a diploid set of already doubled chromosomes. In the process of two meiotic divisions from one diploid cell four haploid are formed.
Meiosis consists of two successive divisions preceded by a single duplication of DNA carried out during the growth stage. In each division of meiosis, four phases are distinguished, which are also characteristic of mitosis (prophase, metaphase, anaphase, telophase), but they differ in some features (Fig. 65).
Prophase of the first meiotic division ( prophase I) is much longer than the prophase of mitosis. At this time, the doubled chromosomes, each of which already consists of two sister chromatids, spiralize and acquire a compact size. Then the homologous chromosomes are arranged parallel to each other, forming the so-called bivalents or tetrads, consisting of two chromosomes (four chromatids). An exchange of the corresponding homologous regions (crossing over) can occur between homologous chromosomes, which will lead to the recombination of hereditary information and the formation of new combinations of paternal and maternal genes in the chromosomes of future gametes (Fig. 66).
By the end of prophase I, the nuclear envelope is destroyed.
IN metaphase I homologous chromosomes in pairs in the form of bivalents, or tetrads, are located in the equatorial plane of the cell, and spindle threads are attached to their centromeres.
IN anaphase I homologous chromosomes from a bivalent (tetrad) diverge towards the poles. Consequently, only one of each pair of homologous chromosomes gets into each of the two resulting cells - the number of chromosomes is halved, the chromosome set becomes haploid. However, each chromosome still consists of two sister chromatids.
Rice. 66. Crossing of chromosomes and exchange of homologous regions
IN telophase I cells are formed that have a haploid set of chromosomes and a double amount of DNA.
After a short period of time, the cells begin the second meiotic division, which proceeds like a typical mitosis, but differs in that the cells involved in it are haploid.
IN prophase II the nuclear envelope breaks down. IN metaphase II Chromosomes line up in the equatorial plane of the cell, spindle fibers connect to the centromeres of chromosomes. IN anaphase II centromeres connecting sister chromatids divide, chromatids become independent daughter chromosomes and diverge to different poles of the cell. Telophase II completes the second division of meiosis.
As a result of meiosis, from one initial diploid cell containing doubled DNA molecules, four haploid cells are formed, each chromosome of which consists of a single DNA molecule.
During spermatogenesis at the maturation stage, as a result of meiosis, four identical cells are formed - the precursors of spermatozoa, which at the formation stage acquire the characteristic appearance of a mature spermatozoon and become mobile.
Meiotic divisions in oogenesis are characterized by a number of features. Prophase I ends in the embryonic period, that is, by the time the girl is born, her body already has a complete set of future eggs. The remaining events of meiosis continue only after puberty in the female. Every month, in one of the woman's ovaries, one of the cells that has stopped in its division continues to develop. As a result of the first division of meiosis, a large cell is formed - the precursor of the egg and a small, so-called polar, body, which enter the second division of meiosis. At the stage of metaphase II, the precursor of the egg ovulates, that is, it leaves the ovary in abdominal cavity from where it enters the oviduct. If fertilization occurs, the second meiotic division is completed - a mature egg and a second polar body are formed. If the fusion with the sperm does not occur, the cell that has not finished dividing dies and is excreted from the body.
Polar bodies serve to remove excess genetic material and redistribute nutrients in favor of the egg. Some time after division, they die.
Significance of gametogenesis. As a result of gametogenesis, germ cells are formed containing a haploid set of chromosomes, which makes it possible to restore the number of chromosomes characteristic of the species during fertilization. In the absence of meiosis, the fusion of gametes would double the number of chromosomes in each successive generation resulting from sexual reproduction. This does not happen due to the existence of a special process - meiosis, during which the diploid number of chromosomes ( 2n) is reduced to haploid (1 n). Thus, the biological role of meiosis is to maintain the constancy of the number of chromosomes in a number of generations of the species.
Review questions and assignments
1. Compare the structure of male and female germ cells. What are their similarities and differences?
2. What determines the size of the eggs? Explain why mammalian eggs are among the smallest.
3. What periods are distinguished in the development of germ cells?
4. Describe how the maturation period (meiosis) proceeds in the process of spermatogenesis; ovogenesis.
5. List the differences between meiosis and mitosis.
6. What is the biological meaning and significance of meiosis?
Think! Execute!
1. The organism developed from an unfertilized egg. Are his hereditary characteristics an exact copy of the characteristics of the mother's body?
2. Explain why there are two terms for male germ cells: sperm (for example, in angiosperms) and spermatozoa.
Work with computer
Refer to the electronic application. Study the material and complete the assignments.
Repeat and remember!
Human
sex cells. Sperm formation in men begins at puberty. The duration of all four phases of spermatogenesis is about 80 days. For a lifetime, a huge amount of spermatozoa is formed in a man's body - up to 10 10.
Despite the fact that a lot of eggs are laid in the female embryo, only a few of them mature. During the reproductive period, that is, when a woman is capable of childbearing, about 400 eggs are finally formed.
The development of germ cells (ovogenesis and spermatogenesis) determines the health of the future generation. Smoking, the use of alcoholic beverages, drugs can have an irreversible effect on the developing sex cells, which will later lead to infertility or the birth of a child with hereditary or congenital disorders.
Gamete (gamete): a germ cell (sperm or egg) containing a haploid set of chromosomes, that is, having one copy of each of the chromosomes.
With sexual reproduction, offspring usually have two parents. Each parent produces sex cells. Sex cells, or gametes, have a half or haploid set of chromosomes and result from meiosis. Thus, a gamete (from the Greek gamete - wife, gametes - husband) is a mature reproductive cell containing a haploid set of chromosomes and capable of merging with a similar cell of the opposite sex to form a zygote, while the number of chromosomes becomes diploid. In a diploid set, each chromosome has a paired (homologous) chromosome. One of the homologous chromosomes comes from the father, the other from the mother. The female gamete is called egg, the male is sperm. The process of gamete formation is collectively called gametogenesis.
In embryos of all vertebrates, at an early stage of development, certain cells are isolated as precursors of future gametes. Such primary germ cells migrate to the developing gonads (ovaries in females, testis and males), where, after a period of mitotic reproduction, they undergo meiosis and differentiate into mature gametes. In germ cells, before meiosis, additional genes are activated that regulate the pairing of homologous chromosomes, recombination and separation of recombined homologous chromosomes in the anaphase of the first division.
Oocytes develop from primary germ cells, which at an early stage of development of the organism migrate to the ovary and turn into oogonia there. After a period of mitotic reproduction, oogonia become first order oocytes, which, having entered the first division of meiosis, linger in prophase I for a time measured in days or years, depending on the type of organism. During this delay, the oocyte grows and accumulates ribosomes, mRNA, and proteins, often using other cells, including surrounding helper cells. Further development(oocyte maturation) depends on polypeptide hormones (gonadotropins), which, by acting on the supporting cells surrounding each oocyte, induce them to induce the maturation of a small part of the oocytes. These oocytes complete the first division of meiosis, producing a small polar body and a large second-order oocyte, which later enters the metaphase of the second meiotic division. In many species, the oocyte remains at this stage until fertilization initiates the completion of meiosis and the start of embryonic development.
The sperm is usually a small and compact cell that is highly specialized for the function of bringing its DNA into the egg. While in many organisms the entire pool of oocytes is formed at an early stage of female development, in males, after the onset of puberty, more and more germ cells enter meiosis, with each first-order spermatocyte giving rise to four mature spermatozoa. Sperm differentiation occurs after meiosis, when the nuclei are haploid. However, since cytokinesis is not completed during mitotic division of mature spermatogonia and spermatocytes, the descendants of one spermatogonium develop as
The structure of male and female germ cells determines the performance of their most important function - the implementation of generative reproduction. It is characteristic of representatives of both plants and animals. Features of the structure of germ cells will be discussed in our article.
Gametes: the relationship of structure and function
The specialized cells that carry out the process are called gametes. Male and female germ cells - sperm and eggs - have a haploid, i.e., a single set of chromosomes. This structure of germ cells provides the genotype of the organism, which is formed when they merge. It is diploid, or double. Thus, the body receives half of the genetic information from the mother, and the other part from the father.
Despite the common features, the structure of sex and animals differs in many respects from each other. This primarily concerns certain places their formation. So, in angiosperms, sperm are located in the anthers of the stamen, and the egg is located in the ovary of the pistil. Multicellular animals have special organs - glands, in which the formation of germ cells occurs: eggs - in the ovaries, and sperm - in the testes.
The process of formation of germ cells
The structure and development of germ cells is determined by the course of gametogenesis - the process of their formation, which proceeds in several stages. During the reproduction phase, the primary gametes divide several times by mitosis. In this case, a double set of chromosomes is preserved. In individuals of different sexes, this stage has its own differences. So, in male mammals, it begins from the moment of onset and lasts until old age. In females, the division of primary germ cells occurs only during intrauterine development of the fetus. And until the onset of puberty, they remain dormant.
The growth phase is next. During this period, the primary gametes increase in size, DNA replication (doubling) occurs. An important process is also the storage of nutrients, because they will be needed for subsequent divisions.
The last stage of gametogenesis is called the growth phase. In its course, the primary germ cells divide by reduction division - meiosis. Its result is four haploid cells formed from primary diploid ones.
spermatogenesis
As a result of the formation of male germ cells, i.e., spermatogenesis, four identical and complete structures are formed. They have the ability to reproduce. The structure of the male germ cell, or rather its feature, lies in the emergence of specific adaptations. In particular, it is a flagellum, with the help of which the movement of male gametes occurs. This process occurs in the last additional phase of formation, which is characteristic only for the process of spermatogenesis.
Ovogenesis
The structure of female germ cells, as well as the process of their formation (ovogenesis), has a number of characteristic features. During meiosis, the cytoplasm is distributed unevenly between future cells. Only one of them eventually becomes an egg capable of giving rise to a future life. The remaining three turn into directional bodies and are destroyed as a result. The biological meaning of this process is to reduce the number of mature female reproductive cells capable of fertilization. Only under this condition can a single egg cell receive required amount nutrients, which is the main condition for the development of the future organism. As a result, during the time when a woman is capable of giving birth to children, only about 400 germ cells can form. While in a man this figure reaches several hundred million.
The structure of male germ cells
Spermatozoa are very small cells. Their size barely reaches a few micrometers. In nature, such sizes, of course, are compensated by their number. The structure of the germ cells of the male body has its own characteristics.
The spermatozoon consists of a head, neck and tail. Each of these parts performs specific functions. In the head is a permanent cell organelle of eukaryotes - the nucleus. It is the carrier of genetic information enclosed in DNA molecules. It is the nucleus that provides the transmission and storage of hereditary material. The second component of the sperm head is the acrosome. This structure is a modified Golgi complex and secretes special enzymes that can dissolve the shells of the egg. Without this, the fertilization process will be impossible. The neck contains mitochondrial organelles that provide tail movements. Centrioles are also located in this part of the spermatozoa. These organelles play an important role in the formation of the spindle during cleavage of the fertilized egg. The tail of spermatozoa is formed by microtubules, which, using the energy of mitochondria, ensure the movement of male germ cells.
The structure of the eggs
The female sex cells are much larger than the spermatozoa. Their diameter in mammals is up to 0.2 mm. But the same figure for lobe-finned fish is 10 cm, and for the herring shark - up to 23 cm. Unlike male germ cells, the eggs are immobile. They are round in shape. The cytoplasm of these cells contains a large supply of nutrients in the form of yolk. In the nucleus, in addition to DNA, which carries genetic information, there is another nucleic acid - RNA. It contains information about the structure of the most important proteins of the future organism. The yolk may be located unevenly in the egg. For example, in the lancelet it is in the center, and in fish it occupies almost the entire surface, shifting the nucleus and cytoplasm to one of the poles of the cell. Outside, the egg is reliably protected by membranes: yolk, transparent and outer. It is they that have to be dissolved by the acrosome of the sperm head in order to carry out the fertilization process.
Types of fertilization
The structure and functions of germ cells determine the implementation of the process of fertilization - the fusion of gametes. As a result of this process, the genetic material of the gametes combines in a single nucleus, and a zygote is formed. She is the first cell of a new organism.
Depending on the place of passage of this process, the external (external) and the first type are distinguished outside the body of the female. It usually occurs in aquatic habitats. Examples of organisms in which external fertilization occurs are members of the fish class. Their females throw their eggs into the water, where the males sprinkle it with seminal fluid. The number of eggs of such animals reaches several thousand, of which not so many individuals survive and grow. Most of them are eaten by aquatic animals. But for all mammals, internal fertilization is characteristic, which occurs inside the female body with the help of specialized males. At the same time, the number of eggs ready for fertilization is small.
The structure of the male, female germ cells and the reproductive system of plants differs significantly from that of animals. Therefore, the process of fusion of gametes occurs differently. Male germ cells of plants do not have a tail and are not capable of movement. Therefore, fertilization is preceded by pollination. This is the transfer of pollen from the anther of the stamen to the stigma of the pistil. It occurs with the help of wind, insects or humans. Once on the stigma of the pistil in this way, the sperm descend along the germ tube into its expanded lower part - the ovary. There is an ovum. When the gametes fuse, the seed embryo is formed.
The concept of parthenogenesis
The structure of germ cells, in particular female ones, makes possible one of the unusual forms of generative reproduction. It's called parthenogenesis. Its biological essence lies in the development of an adult organism from an unfertilized egg. Such a process is observed in life cycle daphnia crustaceans, during which sexual and parthenogenetic generations alternate. The female sex cell contains enough nutrients to give rise to a new life. However, during parthenogenesis, new combinations of genetic information do not appear, which means that the appearance of new characters is also impossible. However, parthenogenesis is of great biological importance, since it makes the process of sexual reproduction possible even without the presence of an individual of the opposite sex.
Phases of the menstrual cycle
In the female body, germ cells are not always ready for fertilization, but only at certain times. During this physiological process, cyclic regular changes in the functions of the reproductive system occur in the body. This process is regulated by the humoral system. The duration of this cycle is 21-36 days with an average of 28. This period can be divided into three phases. In the first (menstrual), which lasts about the first 5 days, the uterine mucosa is shed. This is accompanied by rupture of small blood vessels. On the 6-14th day, under the influence of the pituitary gland, a follicle is released, in which the egg matures. The mucous membrane of the uterus during this period begins to recover. This is the essence of the postmenstrual phase. From the 15th to the 28th day, the formation of adipose connective tissue - the corpus luteum occurs. It acts as a temporary endocrine gland that produces hormones that delay the maturation of follicles. In the period from the 17th to the 21st day, the probability of fertilization is highest. If this does not happen, the germ cell is destroyed and the mucous membrane exfoliates again.
What is ovulation
On the 14th day menstrual cycle the structure of the female germ cell changes somewhat. The egg breaks the follicular membrane and exits the ovary into the fallopian tube. It is there that its maturation ends. This process is called ovulation. This is a very important period during which the uterus acquires the ability to accept a fertilized egg.
Chromosomal set of germ cells
Eggs and sperm have a single set of genetic information. For example, in humans, germ cells contain 23 chromosomes each, and the zygote - 46. When gametes merge, the body receives half of the genes from the mother, and the second part from the father. This also applies to gender. Chromosomes are divided into autosomes and one pair of sex chromosomes. They are designated in Latin letters. In humans, female cells contain two identical sex chromosomes, while male cells contain different ones. Sex cells contain one of them. Thus, the sex of the unborn child depends entirely on the male body and on the type of chromosomes that the spermatozoon carries.
Functions of germ cells
The structure of the female germ cell, like the male, is interconnected with the functions that they perform. As part of the reproductive system, they perform the function of generative reproduction. Unlike asexual reproduction, in which the integrity of the organism's genetic information is preserved, sexual reproduction ensures the creation of new traits. This is necessary condition the emergence of adaptation, and hence the entire existence of living organisms.
