Cell Cycle Ii - Mitosis

The cell cycle is a process whereby a cell progresses from one cell division to the next. The cell cycle in eukaryotic cells consists of two parts: interphase and M phase. The interphase, in turn, comprises three phases: G1, S and G2. The M phase, on the other hand, comprises two stages: mitosis and cytokinesis. Here we will refer mainly to the M phase. Keep in mind that a diploid cell has two copies of each chromosome, one from the father and one from the mother. Such diploid number is represented as 2n. In humans, the somatic cells have 46 chromosomes (23 pairs), this is the diploid number (2n = 46), whereas the sex cells or gametes have a single set of chromosomes (23 in total). Such haploid number is represented as n. At the end of G2 the cell has grown and its genetic material has doubled, this is, it has gone from diploid (2n) to tetraploid (4n), and it is ready to start the M phase, consisting of mitosis and cytokinesis. Mitosis is the separation of the duplicated cellular genome (4n) into two identical sets of chromosomes (2n). Cytokinesis is the division of one cell into two daughter cells. Mitosis takes place only in eukaryotic cells and is divided into: Prophase, Prometaphase, Metaphase, Anaphase and Telophase However, mitosis is a continuous and dynamic process; its division into phases is for descriptive purposes. At prophase, the nucleosomes are densely packed forming chromosomes. Each chromosome has already been replicated and consists of two sister chromatids joined by a protein called cohesin. The two centrosomes move toward opposite poles of the cell, where they organize the microtubules that form the spindle. At prometaphase the nuclear membrane is fragmented. Proteins bind to centromeres and form two kinetochores, one attached to each chromatid. Kinetochores bind to microtubules and the chromosomes begin to align. At metaphase, the kinetochores and the spindle's microtubule align the chromosomes at the metaphase plate, located in the central plane of the nucleus. At this point, a checkpoint verifies that the kinetochores are arranged in pairs. In this way, the cell ensures that in the transition to anaphase the two copies of each chromosome are directed toward opposite centrosomes. At anaphase, a protein called separin releases the sister chromatids from the cohesin and thus they are pulled apart. The kinetochores pull brother chromosomes, which are not sister chromatids anymore, to opposite centrosomes, guided by the spindle's microtubules. The telophase seems to be the reverse of prophase and prometaphase, because new nuclear membranes are formed from parental nuclear membrane fragments. The chromosomes are unpacked and nucleosomes extend partially. The spindle fibers disperse. The cytokinesis is a process which generally begins simultaneously with telophase. However, technically, it is not part of mitosis but an additional process in which the cell divides to complete cell division. In animal cells, a ring of a protein called actin in the cell membrane shrinks the midplane, where the metaphase plate was located, and the cell divides into two daughter cells, each having a nucleus. In plant cells, a cell wall is synthesized between the two daughter cells. The M phase ends with the division of a cell into two, and each daughter cell is ready to start a new cell cycle. The cell cycle is clearly, completely, and didactically presented in: Mendoza, Biologia II, Trillas, Mexico Refer to the webpage http://www.mendoza-sierra.org There you may find several answered questionnaires.