the Reproduction — one of the fundamental properties of all living organisms. Reproduction supported the continued existence of the species through the replacement of successive generations. Under favorable conditions, the species could greatly increase their numbers, to expand into new territory. In the process of reproduction can occur in organisms with different properties than in the previous generation, which is an important source of variability. There are two types of reproduction of living beings. In one case, individuals of the new generation originate from one original specimen. Different forms of asexual and vegetative reproduction. In the second case, individuals of the child generation appear with the participation of two organisms, the parent generation: maternal and paternal. This sexual reproduction. the
the Asexual reproduction in unicellular organisms takes place by dividing their bodies into two or more number of children organisms, multicellular or through education-special cells — spores (e.g. mosses, ferns), or budding (e.g., Hydra). the
the Vegetative reproduction is carried out by separation from the original organism of a part, which gives rise to a new individual. This reproduction is generally inherent in higher plants. The natural way it occurs with the help of special organs (tubers, bulbs, rhizomes). Man can artificially vegetatively propagated plants by cuttings, layering, a variety of vaccinations. Sexual reproduction is very widespread in nature among plants and among animals. In this case, the two organisms — maternal and paternal — are produced by specialized sex cells. Combining then in one cell, germ cells give rise to new organism. Female sex cells are called egg malesperm. Sex cells are produced in special organs of sexual reproduction. the Egg consists of a core, large amount of cytoplasm with nutrient stores and shell, which sometimes has a very complex structure. The egg is devoid of the capacity for active movement. the Sperm also has a core. In the cytoplasm there is very little, the shell is thin but dense. In addition, the spermatozoa of animals are equipped with various formations, allowing them to travel extensively. So, the sperm cells of mammals can distinguish a head, in which is placed a core, and the neck with a ponytail, which serve for movement.
The beginning of the sexual cells of animals do not give the differentiated cells, which undergo a number of successive changes. Formation of female gametes is called ovogenesis male spermatogenesis . Both processes are schematically depicted in the figure. 10.
Cytologically both processes are similar and lead to the fact that in the nuclei of germ cells remains half the chromosomes than in the original cells of a given organism (n instead of 2n). This happens in the following way. The process begins with enhanced reproduction of the original normal cells by mitosis (breeding area). The number of cells increases dramatically. Then they cease to share, but hard to grow (growth area). Especially the increase in size of the future egg. At this time in their cytoplasm accumulate reserve nutrients. Finally, there comes a maturation of germ cells (zone of maturation), in which the number of chromosomes in the germ cells is reduced. During maturation, each cell divides twice, forming four cells. When spermatogenesis these 4 cells become the 4 sperm. When ovogenesis only one of the cells becomes the egg, and 3 others into so-called directional bull and later killed. Division in the zone of maturation, leading to the formation or 4 sperm, or one egg and 3 directional calf, is called a meiosis. It consists of two consecutive divisions. In the prophase of the first meiotic division homologous chromosomes of each pair come together and closely adjacent to each other; in the metaphase, such pairs are in the Equatorial plane of the cell, and in anaphase the homologous chromosomes of each pair diverge to opposite poles of the dividing cell. The result from one diploid cells are two cells with half the normal number of chromosomes. This set of chromosomes is called haploid. The second division in meiosis occurs in the usual way with the division of each chromosome in half (mitosis). As a result of 2 haploid cells arise also 4 haploid cells. The maturation of germ cells ends. Mature cells ready for fertilization.
the Fertilization – the process of merging egg and sperm into one cell, which is called zygote. The sperm penetrates the egg, their cytoplasm is mixed, and the nuclei fuse into one nucleus of the zygote. Thus, the zygote restores the diploid set of chromosomes. In this set one homologous chromosome of each pair introduced into the zygote by the egg and the other sperm. Therefore, the child organism that develops from a zygote, to the same extent provided with hereditary information from both the paternal and the maternal body. This circumstance is the reason of the huge importance of sexual reproduction among plants and among animals. By sexual reproduction can occur with organisms, combining useful traits of father and mother. Such organisms more viable. In agricultural practice the man has been very widely used this feature of sexual reproduction.