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1. Each cell divides into two daughter cells and an equal distribution of chromosomes results in the formation of two nuclei, which are identical in both quantity and quality.
2. The number of chromosomes present in a cell remains the same in the daughter cells.
Importance of Nucleic acids. Nucleic acids are found in chromosomes and are stored in large quantities in the nucleolus. Many changes in mitosis are due to the behavior of nucleic acids. During interphase, chromosomes are invisible but the nucleolus is clearly seen.
In prophase and metaphase, the nucleic acid of nucleolus is added to chromosomes, hence they become visible and the nucleolus disappears. The nucleic acids also bring about the division of centromere and separation of chromatids, because chromatids begin to lose their nucleic acid and are drawn apart during anaphase and telophase.
Regulation of cell cycle progression.
The progression of cells through the division cycle is regulated by extracellular signals from the environment, as well as by internal signals that monitor and coordinate the various processes that take place during different cell cycle phases. The regulator factors of organism belong to different levels of organism organization: intracellular, organ, tissue, and organism.
Intracellular factors are: genes (for example p53, Cdc2, p21,), some proteins (for example MCM proteins, kinase, cyclins), and some other substances (for example cyclic monophosphates). Extra cellular factors of organ and tissue levels are growth factors such as epidermal growth factor, thrombocytes growth factor). On the level of organisms the regulation of cell cycle progression realize by endocrine system (hormones), nervous system, immune system.
As a whole, system of regulation is very complex. Here is description some events of cell cycle progression regulation only on the example of cell cycle checkpoints. Several checkpoints function on ensures that complete genomes are transmitted to daughter cells. One major checkpoint arrests cells in G2 in response to damaged or unreplicated DNA. The presence of damaged DNA also leads to cell cycle arrest at a checkpoint in G1. Another checkpoint, in M phase, arrests mitosis if the daughter chromosomes are not properly aligned on the mitotic spindle. In mammalian cells, arrest at the G1 checkpoint is mediated by the action of a protein knows as p53, whit is rapidly induced in response to damaged DNA. The arrest at the G2 phase is mediated by factor that called maturation-promoting factor.
Meiosis
Meiosis is indirect reduction type of division of sex cells predecessors, which is accompanied with crossing over between homologous chromosomes.
Meiosis is the biological process that take place during the period of sex cells maturing. Diploid cells named gametocytes 1 (oocyte 1 or spermatocyte 1) are divided by an indirect way. In result the cells with a haploid number of chromosomes are formed.
The meiosis includes first (reduction) and second (equation) meiotic division (figure 22.9).
Figure 22.9. Phase of meiotic division (membrane and cytoplasm not shown):
I - 1st division: 1 - leptotene, 2 - zygotene, 3 - pachytene, 4 - diplotene, 5 - diakinesis, 6 - metaphase I, 7 - anaphase I, 8 - telophase I.
II - 2nd division: 9 - interphase II, 10 - metaphase II, 11 - anaphase II, 12 - metaphase.
First meiotic division.
The first division is preceded with an interphase. synthesis DNA takes place in interphase.
prophase I of meiotic division differs from a prophase of mitosis. prophase I of meiotic division consists of five stages: leptotene, zygotene, pachytene, diplotene and diakinesis (figure 22.10). Leptotene is the stage of thin threads. The beginning of chromatin condensation takes place. During zygotene is passing conjugation of homologous chromosomes takes place and due to synoptonemal complex (figure 22.11) tetrads (bivalents) are formed. There is crossing over between homologous chromosomes during pachytene (figure 22.12). The exchange of chromosomal material takes place during crossing over. Diplotene and diakinesis is characterized of homologous chromosomes divergence. Therefore prophase of the first division is the most complex and long stage of meiosis.
Figure 22.10 prophase I of meiotic division consists of five stages: leptotene, zygotene, pachytene, diplotene and diakinesis
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