MITOSIS

Made by Naila Memon

MITOSIS: An Introduction Mitosis is the process by which a eukaryotic cell

separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokines is, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly equal shares of these cellular components. Karyokinesis and cytokinesis together define the mitotic (M) phase of the cell cycle—the division of the mother cell into two daughter cells, genetically identical to each other and to their parent cell. This accounts for approximately 10% of the cell cycle.

Prophase During this first mitotic stage, the nucleolus

fades and chromatin (replicated DNA and associated proteins) condenses into chromosomes. Each replicated chromosome comprises two chromatids, both with the same genetic information. Microtubules of the cytoskeleton, responsible for cell shape, motility and attachment to other cells during interphase, disassemble. And the building blocks of these microtubules are used to grow the mitotic spindle from the region of the centrosomes.In short the chromatin condenses into chromosomes; centrioles move toward opposite poles of the cell.

PROPHASE

Metaphase The nuclear membrane disappears completely. In animal cells, the two pair of centrioles align at

opposite poles of the cell. Polar fibers (microtubules that make up the

spindle fibers) continue to extend from the poles to the center of the cell.

Chromosomes move randomly until they attach (at their kinetochores) to polar fibers from both sides of their centromeres.

Chromosomes align at the metaphase plate at right angles to the spindle poles.

Chromosomes are held at the metaphase plate by the equal forces of the polar fibers pushing on the centromeres of the chromosomes.

METAPHASE

Anaphase When every kinetochores is attached to a cluster of

microtubules and the chromosomes have lined up along the metaphase plate, the cell proceeds to anaphase then proteins that bind sister chromatids together are cleaved. These sister chromatids now become separate daughter chromosomes, and are pulled apart by shortening kinetochores microtubules and move toward the respective centrosomes to which they are attached. Next, the non kinetochores microtubules elongate, pulling the centrosomes (and the set of chromosomes to which they are attached) apart to opposite ends of the cell. The force that causes the centrosomes to move towards the ends of the cell is still unknown, although there is a theory that suggests that the rapid assembly and breakdown of microtubules may cause movement .

ANAPHASE

Telophase In Telophase, the chromosomes are cordoned off

into distinct new nuclei in the emerging daughter cells

The polar fibers continue to lengthen. Nuclei (plural form of nucleus) begin to form at

opposite poles. The nuclear envelopes of these nuclei are

formed from remnant pieces of the parent cell's nuclear envelope and from pieces of the endomembrane system.

Nucleoli (plural form of nucleolus) also reappear. Chromatin fibers of chromosomes uncoil. After these changes, Telophase/mitosis is largely

complete and the genetic "contents" of one cell have been divided equally into two.

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TELOPHASE

Cytokinesis Cytokinesis is the division in two of the

cytoplasm. It occurs near or after the end of nuclear division. In animals, the cell membrane folds in all around the cell. From the outside, the cell looks like a half-deflated balloon that has an invisible thread looped around it; as cytokinesis proceeds the thread is increasingly tightened. Really, the tightening is by protein fibers of the cytoskeleton, orientated in various directions just below the cell membrane.

In Plant cells , the Golgi body manufactures a plate (middle lamella) of vesicles in a plane between the two daughter-nuclei. This plate spreads out from the center of the cell, eventually abutting against the cell membrane all round the cell in that plane. The cell then lays down a cell wall, replacing callose with cellulose and pectin.

CYTOKINESIS

Importance of mitosisMitosis plays a very vital role in maintenance of

chromosomal set and its importance is following :1.Growth. The number of cells within an organism increases by mitosis and this is the basis of growth in multicellular organisms.2.Cell Replacement. Cells are constantly sloughed off, dying and being replaced by new ones in the skin and digestive tract. When damaged tissues are repaired, the new cells must be exact co function of cells.3.Regeneration. Some pies of the cells being replaced so as to retain normal animals can regenerate parts of the body, and production of new cells are achieved by mitosis.4.Vegetative Reproduction. Some plants produce offspring which are genetically similar to themselves. These offspring are called clones. It is a method of multiplication in unicellular organisms means asexual reproduction.

Errors in mitosis The cell goes through dramatic changes in ultra

structure, its organelles disintegrate and reform in a matter of hours, and chromosomes are jostled constantly by probing microtubules. Chromosomes may become damaged. An arm of the chromosome may be broken and the fragment lost, causing deletion. The fragment may incorrectly reattach to another, non-homologous chromosome, causing translocation. It may reattach back to the original chromosome, but in reverse orientation, causing chromosomal inversion. Or, it may be treated erroneously as a separate chromosome, causing chromosomal duplication. The effects of these genetic abnormalities depend on the specific nature of the error and can range from no noticeable effect at all to organism death. Tumors are caused, They could be benign and malignant. Cancer is essentially a disease related to mitosis, involving a breakdown in the regulation process such that there is uncontrolled cell replication.