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Cell Division. Mitosis and Meiosis. Cell As a Part of a Tissue. Cell

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Cell Division. Mitosis and Meiosis. Cell As a Part of a Tissue. Cell http://www.bristol.ac.uk/phys‐pharm/media/teaching/ The cell cycle G0 phase (“NO DIVISION”) Cell Division. Mitosis Mitosis and meiosis. TIME Cell as a part of a tissue. Main text: Vander’s Human Physiology textbook, Movies from Lodish et al “Molecular Cell Biology” TIME The cell cycle 1. All cells originate from the single 0 hours G0 phase source: the fertilized oocyte. 2. All cells except for male and Mitosis female sex cells (sperms and 23 hours TIME oocytes) are produced by the Preparation for process of cell division called cell division M Organelles G2 mitosis. 20 hours double Organelles G1 Fundamental feature of mitosis: S double Organelles both descendant cells contain the double same genetic material (DNA) as the DNA synthesis TIME parent cell. (replication) 13 hours Cell 1 Cell 2 As a result of mitosis each descendant cell gets two copies of Each separate DNA molecule forms a each chromosome, one maternal chromosome. and one paternal, exactly as in the In all cells (exc. sperms and oocytes) parent cell. there are TWO copies of each chromosome: one from the father and the other from If the parent cell had 2 x N the mother. chromosomes, Maternal chromosome Paternal chromosome At the beginning of cell division each both descendant cells will have chromosome duplicates and forms sister also 2 x N each. chromatids connected at the centromere. 1 Before dividing the cell must not only duplicate, WHAT SHOLD THE CELL DO NEXT? but also assemble DNA into a manageable form. • Dissolve nuclear membrane. • Split chromosomes evenly and pull them to the opposite sides of the cell • Split cytoplasm and organelles into two separate lots, build a wall between the two new cells • Restore the nuclear membranes. At the beginning of cell division DNA is coiled into rather short and thick “trunks” visible under light microscope. It is this form of DNA which was originally identified as chromosomes. …phase Inter… (something in between) Pro… (before) Meta… (in the middle…) Ana… (thereupon ???) Telo… (completion) centrioles move to opposite sides Metaphase Anaphase Telophase Chromatids are Chromatides Cell cytoplasm is End of Prophase lined up at the separate and cut in two with a equatorial tightening belt Interphase 1. Chromosomes are clearly visible become individual plane made of contractile 2. Nuclear envelope disintegrates daughter (G2) proteins (actin & 3. Spindle fibres originating from chromosomes myosin) centrioles attach to centromeres 2 The cell cycle 0 hours G0 phase Cells get “old” – cell senescence (“normal life”) Even in dividing cells mitotic activity may fall with time Mitosis and eventually they may start dying. Checkpoint-2 One of the reasons for that is accumulation of small damages to TIME DNA acquired throughout life. M The other reason for a permanent arrest of cell division is the shut-down of the telomerase, an enzyme which elongates the ends G of the DNA of the chromosomes. 2 G1 S “Cell lines” are “immortalised” clones of various cell types. They are essentially “tumour-like” cells in which many essential regulatory processes are disabled or lost. TIME However they may still resemble their “ancestors” is some ways and are widely used in research. Checkpoint-1 Why cell division has to be so quick? Control of the cell cycle Cells which Cells which have may divide specialised G0 phase “perpetually” (differentiated) and 0 (“normal life”) seldom or never divide Blood marrow Nerve cells, cells, Skeletal muscle cells Skin cells, Cyclins are Cells which line M specialised proteins up the G which control cell intestine division. 2 G1 In dividing cells Movie: stem cells in intestine S concentration of cyclins Stem cells: undifferentiated cells with “indefinite” potential for self- progressively increases replication which can get transformed (differentiated) into other cells which before mitosis, then then loose this ability. rapidly drops Currently much research is going into the possibilities of using stem cells for medical purposes (to replace damaged cells, for example in MI). How can signals from outside Control of the cell cycle (such as growth factors) control cell division? G0 phase 1. Growth factors via special intermediates act on certain 0 -non-dividing state genes in the cell’s nucleus 2. Activation of these genes leads to production of proteins which control cell Proteins which M trigger cell division division Cells may be prompted to G enter the mitotic cycle by 3. These proteins perform external stimuli, for various functions which are 2 G1 example molecules known as required to make the cell growth factors or hormones. S Hormones can also direct enter mitosis. mitosis towards a more 4. As a result, the cell specialised, differentiated cell type. divides. 3 Meiosis: type of cell division which Remember: cell division is a tightly reduces the number of chromosomes by controlled process! half. CANCER: 2 copies of each A disease of uncontrollable cell division. chromosome Many cancers are caused by mutations in genes which encode for proteins involved Why do in control of cell division. we need 1 copy of each meiosis? chromosome Movie: cancerA8010006mov 2 copies of each chromosome To reduce the number of Meiosis chromosomes the cell duplicates them once, but divides two times. In the first round pairs of chromosomes get split between the two cells. In the second round daughter chromatides split. Key points -1: Key points -2: 1. A continuously dividing cell “loops” through G1, S,G2 7. In mitosis one cell divides in two and both phases and mitosis. daughter cells have identical sets of 2. A cell may get out of this cycle (enter G0 phase) and stop dividing. chromosomes to the parent cell. 3. It can be prompted to get out of G0 and start 8. Formula of mitosis: (2n 2) / 2. dividing again by external factors such as growth 9. All cells in our body except for sperms and factors. oocytes are produced as a result of mitosis. 4. Cyclins are specialised proteins which control cell 10.Meiosis is a specific type of cell division division. Their concentration increases before cell division and then drops rapidly. used ONLY to produce sperms and oocytes. 5. Some cells are able to divide indefinitely in non- Formula of meiosis: (2n 2) / 4. differentiated state. They act as a source of new 11. Sperms and oocytes carry only ONE copy of cells in some tissues and are called stem cells. each chromosome. 6. Cancer is the disease of uncontrollable cell division 4 5.
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