Reproduction is based on cell division. The period of a cell’s vital activity from the moment of its origin to the moment of its division into two daughter cells is called the cell cycle. During this period, a number of events occur that ensure the growth, development and reproduction of the cell. The duration of cell cycles in different tissues, even in the same organism, is different and varies widely. It can be less than one hour in the fragmenting cells of vertebrate embryos, or it can be as long as a whole year, as, for example, in the liver cells of an adult. The cell cycle consists of interphase and division.

This is the phase of the life cycle between two cell divisions. It is characterized by active metabolic processes, the synthesis of proteins, nucleic acids, carbohydrates, lipids, the accumulation of nutrients by the cell, an increase in the number of all its organelles, growth and an increase in volume. There are three consecutive phases in the interphase: presynthetic — G, synthetic — S and post-synthetic — G2. The presynthetic phase G is characterized by intensive metabolic processes. During this period, the cell actively synthesizes organic substances, the number of all organoids increases in it: chloroplasts, mitochondria, lysosomes, vacuoles with cell juice, etc. The endoplasmic reticulum and Golgi apparatus are increasing in size. All types of RNA are actively synthesized in the nucleus, ribosomes are formed and assembled in the nucleolus. Intensive cell growth occurs. The synthetic S phase occurs in the middle of the interphase and is characterized by DNA reduplication. As a result, a doubled number of DNA molecules are formed in the cell. Before the start of the S-phase, each chromosome corresponds to one DNA molecule, and after reduplication, one chromosome consists of two DNA. Next, the cell enters a short post-synthetic G2 phase. Intensive biosynthesis of substances also continues here, and the energy reserve of the cell increases due to the synthesis of ATP. At this time, the centrioles of the cell center are doubled. The cell is preparing for division. The duration of the interphase depends on the type of cells and averages at least 90% of the total time of the cell cycle. This time most often depends on the G-phase, the duration of which varies very widely. It can be practically absent when cells divide rapidly, for example, during zygote fragmentation. But it can be a very large amount — almost the entire life of the body. For example, adult nerve cells are in phase G, interphase all their lives and no longer divide. The interphase ends, and the cell enters the next period of the cell cycle, the division stage.

The ability to divide is the most important property of a cell. As a result of division, two new cells arise from one cell. One of the main properties of life, self—reproduction, is already manifested at the cellular level. The most common method of cell division is mitosis, the indirect division of a cell. Mitosis is the process of formation of two daughter cells with a set of chromosomes identical to the original mother cell. Mitotic division leads to an increase in the number of cells, ensures the growth of the body, regeneration or replacement of cells during their aging. In some organisms, mitosis underlies their reproduction asexually. Cell division consists of two sequential processes: karyokinesis is the division of the nucleus, or mitosis proper, and cytokinesis is the division of the cytoplasm. In the process of karyokinesis, the main, most important event occurs — the redistribution of chromosomes, i.e. DNA molecules, ensuring the uniform transfer of hereditary information between two daughter cells. During cytokinesis, the cytoplasm and its organoids are distributed more or less evenly between the two daughter cells. However, this event does not occur with the same precision as the process of karyokinesis. The events occurring in mitosis can be seen in a light microscope on fixed preparations. Modern methods of phase contrast microscopy and micrography have made it possible to observe this process in a living cell. Currently, the cell cycle and mitosis are being studied on separate isolated cells. A cell population derived from a single parent cell is called a clone.

Chromosomes play an important role in the cell cycle. Chromosomes are carriers of the hereditary information of a cell and an organism contained in the nucleus. They not only regulate all metabolic processes in the cell, but also ensure the transfer of hereditary information from one generation of cells and organisms to another. The number of chromosomes corresponds to the number of DNA molecules in a cell. The increase in the number of many organoids does not require precise control. During division, the entire contents of the cell are distributed more or less evenly between the two daughter cells. The exception is chromosomes and DNA molecules: they must double and be precisely distributed among newly formed cells. The cells of each organism contain a specific set of chromosomes, which is called a karyotype. Each type of organism has its own karyotype. The chromosomes of each karyotype differ in shape, size, and set of genetic information. The human karyotype, for example, consists of 46 chromosomes, the fruit fly of drosophila has 8 chromosomes, and one of the cultivated wheat species has 28. The chromosome set is strictly specific for each species. Karyotype studies of various organisms have shown that cells can contain single and double sets of chromosomes. Double, or diploid (from Greek. diploos — double and eidos – type), a set of chromosomes characterized by the presence of paired chromosomes that are the same in size, shape and nature of hereditary information. Paired chromosomes are called homologous (from Greek. homois — the same, similar). For example, all human somatic cells contain 23 pairs of chromosomes, i.e. 46 chromosomes are represented as 23 pairs. In drosophila, 8 chromosomes form 4 pairs. Paired homologous chromosomes are very similar in appearance. Their centromeres are located in the same places, and the genes are arranged in the same sequence. In some cells or organisms, there may be a single set of chromosomes, which is called haploid (from Greek. haploos — single, simple, and eidos — type). Paired chromosomes are absent in this case, i.e. there are no homologous chromosomes in the cell. For example, in the cells of lower algae plants, the set of chromosomes is haploid, whereas in higher plants and animals, the set of chromosomes is diploid. However, the germ cells of all organisms always contain only a haploid set of chromosomes. The chromosomal set of cells of each organism and species as a whole is strictly specific and is its main characteristic. Source: Petrosova Renata Armenakovna “Reproduction of organisms”