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Cell divison

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Cell divison

  1. 1. CELL DIVISION
  2. 2. Cells arise from pre-existing cells, and cell division leads to an increase in cell number. There are two types of cell division in eukaryotic cells - Meiosis - Mitosis Before any cell division the amount of DNA in the cell must double = semi-conservative replication. Prokaryotic cells divide by binary fission The DNA in a bacterium is often held in a singular circular chromosome (plasmid)This needs to replicate to form an identical copy
  3. 3. BINARY FISSION Bacteria divide by binary fission – dividing in half. Single chromosome replicates itself. Chromosomes move so that they are at either end of the cell. As the process continues the cell membrane pinches inwards and two new identical cells are formed. Binary fission occurs at a rapid rate – which is important when cloning DNA or other forms of genetic manipulation.
  4. 4. MITOSIS All cells from the first time they are formed go through what is called a cell cycle.There are three stages 1. Mitosis – division of the nucleus 2. Cytokinesis – division of cytoplasm and formation of two new cells 3. Interphase a) time of growth (G1) b) DNA replication (S) c) preparation for division (G2)
  5. 5. Mitosis accounts for the division of a cell to form two identical “daughter” cells.This process can be divided into four phases. Prophase  in the nucleus chromatid fibres become tightly coiled and visible Each chromosome is duplicated forming two identical chromatids joined at the centromere At end of prophase the nucleolus and nuclear membrane disappear and the chromosomes move into the cytoplasm  Microtubules appear as spindle fibres and centrioles move towards the poles of the cell.
  6. 6. Metaphase Spindle fibres radiate from the poles towards cells equator. Chromosomes become aligned along the metaphase plate Microtubules become attached to the centromeres (spindle).
  7. 7. Anaphase  Centromeres of the chromosome divide and chromatids are drawn apart  Each daughter chromosome is drawn towards opposite poles of the cell by the microtubules Telophase  Daughter cells form nuclei and nucleoli reappear Chromosomes uncoil to form chromatin and cytoplasm begins to pinch in Cell begins to cleave (divide) and microtubules act as a ligature = cytokinesis
  8. 8. Pray Mitosis Ain’t Tested
  9. 9. DIVISION IS CONTROLLED BY INTERNAL AND EXTERNAL FACTORS It is vital that cells control timing of their division In a mature human, red blood cells divide continually while nerve and muscle cells do not divide. The cell cycle is controlled by a set of molecules in the cytoplasm of the cell and “cycle” in concentration
  10. 10. INTERNAL FACTORS There are two main types of molecules 1. Enzymes – kinases – Cdks (cyclin dependant kinases) 2. Cyclin Cyclin combines with kinase to form a complex protein called Mitosis Promoting Factor (MPF) An increase in MPF is the stimulus to initiate cells to divide
  11. 11. The control of the cell cycle is checked at three check points 1. At the end of G1 in which the cell growth is assessed 2. At the end of G2 where progress of DNA replication is assessed before cell proceeds to mitosis 3. Progress of mitosis is checked during metaphase, M
  12. 12. COMPARE CELL CYCLE AND A WASHING MACHINE  The function of the washing machine is to take in water and detergent, wash the clothes, rinse them, and spin them dry.  The essential processes, such as DNA replication and mitosis and cytokinesis, are triggered by a central cell-cycle control system. By analogy with a washing machine, the control system is drawn as an indicator that rotates clockwise, triggering essential processes when it reaches specific points on the outer dial.  Feedback is essential
  13. 13. EXTERNAL FACTORS Physical and / or chemical triggers external to the cell which impact on the internal factors controlling division. Nutrient dependence If certain nutrients are unavailable in the extracellular fluid, a cell will not divide. Anchorage dependence Cells will only divide if they are attached to a substrate
  14. 14. Density dependence Cells which are in close contact with each other (membranes touching) inhibit cell division. Hormones can also regulate cell division Growth hormone (GH) stimulates cells in the liver to release a growth factor Sex hormones (FSH) stimulates gamete production Cytokinins stimulate plant cell division
  15. 15. If cells are deprived of growth factors they will not divide. They are thought to stop at the G1 checkpoint and are said to be in the G0 phase. Mature nerve and muscle cells do not divide and are said to be in G0 phase. An example of a growth factor which has been identified is PDGF ( platelet derived growth factor).This stimulates division of cells termed fibroblasts which are important for clotting of blood.
  16. 16. Growth factors travel in the blood of animal and the sap of plants.They become attached to the specific receptor molecules in the plasma membrane of the target cells. These target cells relay messages across the membrane and to a series of proteins called “relay proteins” These relay proteins activate the process of transcription and in turn translation to produce a protein that stimulates cell division. Growth – inhibiting factors have the opposite effect producing non-functional proteins inhibiting cell division.
  17. 17. Growth factors in plant cells are called “auxins” Synthetic auxins such as “2,4,5T” and “glycol” are active ingredients of “Round- up” They stimulate cells of plants to divide rapidly thereby causing death of a plant.
  18. 18. CARCINOGENS AND CELL DIVISION Mutations are permanent changes to DNA and the rate is increased by radiation, mutagenic chemicals and heat. Carcinogens are mutagenic substances that induce mutations that control normal cell division, leading to the development of oncogenes. Cancer cells have faulty cell cycles Cancers tend to be associated more with ageing as more and more damage accumulates in DNA over time
  19. 19. ANIMATIONS/VIDEOS  Mitosis  Mitosis  Mitosis video real  MitosisAnimation
  20. 20. CANCER It appears that between four and seven mutations in a single cell may cause a cancer These mutations interfere with the production of MPF’s, cyclins and kinases. There are a relatively small numbers of genes which control the development of cancers which are called oncogenes. Oncogenes are faulty counterparts of normal genes that control functioning in the cell.
  21. 21. A cancer is an abnormal growth of cells which invades other tissues. Several environmental factors can increase the rate of cancer.These factors are referred to as mutagens or carcinogens. Asbestos fibre Cigarette smoke Free radicals – found in food additives and personal care products (soap, skin lotions)!!!  electromagnetic radiation – x-rays / UV rays  mobile phones (microwaves)
  22. 22. New products and substances are now tested for their “mutagenicity” by exposing microbes grown on an agar plate. Types of cancer Benign growth Growth which remains at its original site – generally not life threatening unless in a confined space (skull) Slow growing and removed surgically
  23. 23. Malignant growth Grow quickly and cells split off and spread (metastasis) to other parts of the body and can lead to secondary tumours.

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