The structure of male, female sex cellsstipulates the fulfillment of their most important function - the realization of generative reproduction. It is characteristic of representatives of both plants and animals. Features of the structure of the sex cells will be considered in our article.
Gametes: relationship between structure and functions
Specialized cells that carry out the processgenerative propagation, are called gametes. Male and female sex cells - spermatozoa and ovules - have a haploid, i.e., a single set of chromosomes. This structure of the sex cells provides the genotype of the organism, which is formed when they merge. It is diploid, or double. Thus, half of the genetic information the body receives from the mother, and the other part - from the father.
Despite common features, the structure of germ cellsplants and animals in many ways different from each other. This primarily concerns certain places of their formation. Thus, in angiosperms of spermatozoa, stamens are located in the anthers, and the ovule is in the pestle ovaries. Multicellular animals have special organs - glands, in which the formation of sex cells: egg cells - in the ovaries, and spermatozoa - in the testes.
The process of formation of germ cells
The structure and development of germ cells is determinedthe course of gametogenesis - the process of their formation, which takes place in several stages. During the breeding phase, the primary gametes are divided several times by mitosis. At the same time, a double set of chromosomes is preserved. In individuals of different sex, this stage has its differences. Thus, in mammalian males, it begins with the onset of puberty and lasts until old age. In females, the division of primary sex cells occurs only during intrauterine development of the fetus. And before puberty, they remain at rest.
The growth phase is the following. During this period, the primary gametes increase in size, DNA replication (duplication) occurs. An important process is also the storage of nutrients, because they will be necessary for subsequent divisions.
The last stage of gametogenesis is called the phasegrowth. In its course, primary sex cells are divided by reduction division - meiosis. Its result is four haploid cells, formed from primary diploid cells.
As a result of the formation of male sex cells,spermatogenesis, four identical and full-fledged structures are formed. They have the ability to fertilize. The structure of the male sexual cell, more precisely its peculiarity, consists in the emergence of specific adaptations. In particular, it is a flagella 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 of the process of spermatogenesis.
The structure of female sex cells, like the process of theirformation (oogenesis), has a number of characteristic features. When oocytes mature during meiosis, the cytoplasm is distributed unevenly between future cells. Only one of them as a result becomes an egg, capable of giving rise to a future life. The remaining three turn into directing bodies and as a result are destroyed. The biological meaning of this process is to reduce the number of mature, capable of fertilizing female sex cells. Only under this condition will a single egg be able to obtain the necessary amount of 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 men this figure reaches several hundred million.
The structure of male germ cells
Spermatozoa are very small cells. Their size barely reaches several micrometers. In nature, such dimensions are naturally compensated by their quantity. The structure of the sex cells of the male body has its own characteristics.
The sperm consists of a head, neck and tail. Each of these parts performs certain functions. The permanent cell organelle of the eukaryotic nucleus is located in the head. It is the carrier of the genetic information contained in DNA molecules. It is the nucleus that ensures the transfer and storage of hereditary material. The second component of the sperm head is the acrosome. This structure is a modified complex of Golgi and gives off special enzymes that can dissolve the shells of the egg. Without this, the fertilization process will be impossible. In the neck there are organelles of mitochondria, which provide movement of the tail. In this part of the spermatozoa there are also centrioles. These organelles play an important role in the formation of the fission spindle during the crushing of a fertilized egg. The tail of spermatozoa is formed by microtubules, which, using the energy of the mitochondria, provide the movement of the male sex cells.
Structure of eggs
Female sex cells are much largerspermatozoa. Their diameter in mammals is up to 0.2 mm. But the same index in fist-fish is 10 cm, and in the herring shark - up to 23 cm. Unlike male sex cells, egg cells are immobile. They have a rounded shape. In the cytoplasm of these cells there is a large quantity of nutrients in the form of yolk. In the nucleus, besides DNA carrying 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 unevenly distributed in the egg. For example, in a lancelet it is in the center, and in fish occupies almost the entire surface, shifting the nucleus and the cytoplasm to one of the poles of the cell. Outside, the egg cell is securely protected by shells: vitelline, transparent and external. It is they who have to dissolve the acrosome of the sperm head for the process of fertilization.
Types of Fertility
The structure and functions of germ cells determinethe process of fertilization - the fusion of gametes. As a result of this process, the genetic material of gametes is united in a single nucleus, and a zygote is formed. It is the first cell of a new organism.
Depending on the place of passage of thisprocess distinguish between external (external) and internal fertilization. The first type is carried out outside the female body. This usually occurs in an aquatic environment. Examples of organisms that undergo external fertilization are representatives of the fish class. Their females spawn in water, where the males and water it with seminal fluid. The number of eggs of such animals reaches several thousand, of which not many individuals survive and grow. Most of them eat aquatic animals. But for all mammalian animals is characterized by internal fertilization, which occurs inside the female body with the help of specialized male copulatory organs. At the same time, the number of ovules ready for fertilization is small.
The structure of the male, female sex cell andreproductive system of plants is significantly different from that of animals. Therefore, the process of fusion of gametes is different. 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 process of transferring pollen from the anther of the stamen to the stigma of the pistil. It occurs with the help of wind, insects or a person. Once in this way on the stigma of the pistil, the sperm are lowered along the embryonic tube into its enlarged lower part - the ovary. There is an egg. When the gametes merge, a seed germ is formed.
The concept of parthenogenesis
The structure of germ cells, in particular female,makes possible one of the unusual forms of generative reproduction. It is called parthenogenesis. Its biological essence lies in the development of an adult organism from an unfertilized ovum. Such a process is observed in the life cycle of daphnia crustaceans, during which the sexual and parthenogenetic generations alternate. The female sex cell contains enough nutrients to give birth to a new life. However, in case of parthenogenesis, no new combinations of genetic information arise, which means that the appearance of new signs is also impossible. However, parthenogenesis has an important biological significance, since it makes possible the process of sexual reproduction even without the presence of an individual of the opposite sex.
Phases of the menstrual cycle
In the female body, the sex cells are not alwaysready for fertilization, but only in certain phases of the menstrual cycle. During this physiological process, the body undergoes cyclic, regular changes in the functions of the reproductive system. 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, there is a rejection of the mucous membrane of the uterus. 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 ripens. The mucous membrane of the uterus during this period begins to recover. This is the essence of the post-menstrual phase. From the 15th to the 28th day, fatty connective tissue, the yellow body, forms. It acts as a temporary gland of internal secretion, which produces hormones that delay the maturation of follicles. In the period from the 17th to the 21st day, the probability for fertilization is highest. If this does not happen, the germ cell is destroyed and the mucosa again exfoliates.
What is ovulation
On the 14th day of the menstrual cycle, the structurethe female sex cell somewhat changes. The ovary ruptures the follicular membrane and exits the ovary into the fallopian tube. This is where her maturation comes to an end. This process is called ovulation. This is a very important period during which the uterus acquires the ability to receive a fertilized egg.
Chromosomal set of reproductive cells
Egg cells and spermatozoa have a single setgenetic information. For example, in a human, the sex cells contain 23 chromosomes and the zygote 46. When gametes merge, half the genes the organism receives from the mother, and the second part from the father. This also applies to sex. Among the chromosomes, autosomes and one pair of genital are distinguished. They are denoted by Latin letters. In a human, female cells contain two identical sex chromosomes, and male cells have different chromosomes. Sex cells contain one of them. Thus, the sex of the unborn child depends entirely on the male organism and on the type of chromosomes that the sperm carries.
Functions of germ cells
The structure of the female sex cell, like that of the male,is interconnected with the functions they perform. Being part of the reproductive system, they carry out the function of generative reproduction. Unlike the asexual, in which the integrity of the body's genetic information is preserved, sexual reproduction provides the creation of new characteristics. This is a necessary condition for the emergence of adaptation, and hence the entire existence of living organisms.</ p>