Mitosis & its phases Prophase,Metaphase, Anaphase , Telophase and Cytokinesis | chapter cell cycle | скачать в хорошем качестве

Mitosis & its phases Prophase,Metaphase, Anaphase , Telophase and Cytokinesis | chapter cell cycle |

Mitosis In 1880s, a German biologist Walther Flemming observed that in a dividing cell, nucleus passes through a series of changes which he called mitosis. Mitosis is the type of cell division in which a cell divides into two daughter cells, each with the same number of chromosomes as were present in parent cell. Mitosis occurs only in eukaryotic cells. In multicellular organisms, the somatic cells undergo mitosis. Prokaryotic cells undergo a process similar to mitosis called binary fission. They do not undergo proper mitosis. Why? Phases Of Mitosis The process of mitosis is complex and highly regulated. There are two major phases i.e. the division of nucleus known as karyokinesis; and the division of cytoplasm known as cytokinesis. A. Karyokinesis: The division of nucleus is further divided into four phases i.e. prophase, metaphase, anaphase and telophase. Prophase Normally, the genetic material in nucleus is in a loose thread-like form called chromatin. At the onset of prophase, chromatin condenses into highly ordered structures called chromosomes. Since the genetic material has already been duplicated earlier in S phase, each chromosome is made of two sister chromatids, bound together at the same centromere. Each chromosome also has kinetochore at centromere. Kinetochore is a complex protein structure that is the point where spindle fibers attach. Prokaryotes do not have proper nucleus and do not form spindles during division. That is why their division is not called mitosis. There are two centrioles (collectively called a centrosome) close to nucleus (recall from chapter Each centriole duplicates and thus two daughter centrosomes are formed. Both centrosomes migrate to the opposite poles of cell. Here, they give rise to microtubules by joining tubulin proteins present in cytoplasm. The microtubules thus formed are called spindle fibres. Complete set of spindle fibres is known as mitotic spindle. By this time, nucleolus and nuclear envelope have degraded, and spindle fibres have invaded the central space. In highly vacuolated plant cells, nucleus has to migrate to the centre of cell before prophase. The cells of plants lack centrioles. So, spindle fibres are formed by the aggregation of tubulin proteins on the surface of nuclear envelope during prophase. Metaphase When spindle fibres have grown to sufficient length, some spindle fibres, known as kinetochore fibres, attach with the kinetochores of chromosomes. Two kinetochore fibres from opposite poles attach with each chromosome. Chromosomes arrange themselves along the equator of cell forming a metaphase plate. A number of other fibres (non-kinetochore) from the opposite centrosomes attach with each other. iii. Anaphase When a kinetochore spindle fibre connects with the kinetochore of chromosome, it starts to pull toward the originating centrosomes. The pulling force divides the chromosome’s sister chromatids and they separate. These sister chromatids are now sister chromosomes, and they are pulled apart toward the respective centrosomes. The other spindle fibres (non-kinetochore) also elongate. At the end of anaphase, cell has succeeded in separating identical copies of chromosomes into two groups at the opposite poles. iv. Telophase Telophase is a reversal of prophase. A new nuclear envelope forms around each set of separated chromosomes. Both sets of chromosomes, now surrounded by new nuclear envelopes, unfold back into chromatin. Nuclear division is completed, but cell division has yet one more step to complete. B. Cytokinesis: Cytokinesis is the division of cytoplasm. In animal cells, cytokinesis occurs by a process known as cleavage. A cleavage furrow develops where the metaphase plate used to be. The furrow deepens and eventually pinches the parent cell into two daughter cells. Cytokinesis in plant cells occurs differently. Vesicles derived from the Golgi apparatus move to the middle of cell and fuse to form a membrane-bounded disc called cell plate or phragmoplast. The plate grows outward and more vesicles fuse with it. Finally, membranes of cell plate fuse with plasma membrane and its contents join the parental cell wall. The result is two daughter cells, each bounded by its own plasma membrane and cell wall ( #mitosis #stagesofmitosis