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Cell Cycle


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Cell Cycle

  1. 1. Cell Growth & The Cell Cycle <ul><li>Living things can be distinguished from non-living things by their ability to REPRODUCE </li></ul><ul><li>This characteristic is based on cells being ability to DIVIDE </li></ul>
  2. 2. What is DNA and where is it stored? <ul><li>The nucleus is a membrane bound organelle that contains the genetic information in the form of chromatin, highly folded ribbon-like complexes of deoxyribonucleic acid (DNA) and a class of proteins called histones . </li></ul>
  3. 3. Cell Cycle <ul><li>Cell division allows organisms to grow, develop, to replace dead cells and to repair tissue </li></ul><ul><li>This is a CONTINUAL PROCESS </li></ul><ul><li>The length of the cell cycle depends on the type of cell and external factors e.g. temp, O2 supply etc </li></ul><ul><li>Bacterial cells – 20 mins </li></ul><ul><li>Liver cells divide only once a year or only if the need arises e.g. injury </li></ul><ul><li>Skin cells – all the time! </li></ul><ul><li>Nerve & muscle cells don’t divide at all in a mature adult </li></ul>
  4. 4. The Cell Cycle <ul><li>Stages in the Cell Cycle: </li></ul>
  5. 5. Cell Growth and The Cell Cycle <ul><li>A eukaryotic cell cannot divide into two, the two into four, etc. unless two processes alternate: </li></ul><ul><ul><li>doubling of its genome ( DNA ) in S phase (synthesis phase) of the cell cycle; </li></ul></ul><ul><ul><li>halving of that genome during mitosis ( M phase ) </li></ul></ul><ul><li>The period between M and S is called G 1 ; that between S and M is G 2 . </li></ul>
  6. 6. <ul><li>For a new cell to be produced … </li></ul><ul><ul><li>The quantity of DNA must double – DNA replication </li></ul></ul><ul><ul><li>Must be copied EXACTLY </li></ul></ul><ul><li>Due to the brief flurry of cytological activity during cell division, the cycle is divided up into 2 parts: </li></ul><ul><ul><li>INTERPHASE (G1, S, G2 phases) </li></ul></ul><ul><ul><li>MITOTIC PHASE (M phase) </li></ul></ul>
  7. 7. So, the cell cycle consists of: <ul><ul><ul><ul><li>G 1 = growth and preparation of the chromosomes for replication </li></ul></ul></ul></ul><ul><ul><ul><ul><li>S = synthesis of DNA (and centrosomes ) </li></ul></ul></ul></ul><ul><ul><ul><ul><li>G 2 = preparation for </li></ul></ul></ul></ul><ul><ul><ul><ul><li>M = mitosis </li></ul></ul></ul></ul><ul><li>When a cell is in any phase of the cell cycle other than mitosis, it is often said to be in Interphase . </li></ul>
  8. 8. What is (and is not) mitosis? <ul><li>Mitosis is nuclear division plus cytokinesis , and produces two identical daughter cells during prophase, metaphase, anaphase, and telophase. </li></ul><ul><li>Interphase is often included in discussions of mitosis, but interphase is technically not part of mitosis, but rather encompasses stages G1 , S , and G2 of the cell cycle . </li></ul>
  9. 9. Interphase <ul><li>The cell is engaged in metabolic activity and performing its preparation for mitosis (the next four phases that lead up to and include nuclear division). </li></ul><ul><li>Chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleolus may be visible. </li></ul><ul><li>The cell may contain a pair of centrioles (or microtubule organising centres in plants) both of which are organisational sites for microtubules. </li></ul>
  10. 10. Prophase <ul><li>Chromatin in the nucleus begins to condense and becomes visible in the light microscope as chromosomes. </li></ul><ul><li>The nucleolus disappears. Centrioles begin moving to opposite ends of the cell and fibres extend from the centromeres. </li></ul><ul><li>Some fibres cross the cell to form the mitotic spindle. </li></ul><ul><li>The nuclear membrane dissolves, marking the beginning of metaphase. </li></ul><ul><li>Proteins attach to the centromeres creating the kinetochores . Microtubules attach at the kinetochores and the chromosomes begin moving. </li></ul>
  11. 11. Metaphase <ul><li>Spindle fibres align the chromosomes along the middle of the cell nucleus. This line is referred to as the metaphase plate. </li></ul><ul><li>This organisation helps to ensure that in the next phase, when the chromosomes are separated, each new nucleus will receive one copy of each chromosome. </li></ul>
  12. 12. Anaphase <ul><li>The paired chromosomes separate at the kinetochores and move to opposite sides of the cell. </li></ul><ul><li>Motion results from a combination of kinetochore movement along the spindle microtubules and through the physical interaction of polar microtubules. </li></ul>
  13. 13. Telophase <ul><li>Chromatids arrive at opposite poles of cell, and new membranes form around the daughter nuclei. </li></ul><ul><li>The chromosomes disperse and are no longer visible under the light microscope. </li></ul><ul><li>The spindle fibres disperse, and cytokinesis or the splitting of the cell may also begin during this stage. </li></ul>
  14. 14. Cytokinesis <ul><li>In animal cells, cytokinesis results when a fibre ring composed of a protein called actin around the centre of the cell contracts pinching the cell into two daughter cells , each with one nucleus. </li></ul><ul><li>In plant cells, the rigid wall requires that a cell plate be synthesised between the two daughter cells. </li></ul>
  15. 15. Remember …! <ul><li>P rophase </li></ul><ul><li>M etaphase </li></ul><ul><li>A naphase </li></ul><ul><li>T elophase </li></ul><ul><li>C yto k inesis </li></ul><ul><li>P ositive </li></ul><ul><li>M ental </li></ul><ul><li>A ttitude </li></ul><ul><li>T owards </li></ul><ul><li>C alvin K lein </li></ul>Cell Division Animation
  16. 16. DNA Replication <ul><li>Before a cell can divide, it must duplicate all its DNA. In eukaryotes, this occurs during S phase of the cell cycle . </li></ul><ul><li>Recap the steps in DNA replication …. </li></ul><ul><li>A portion of the double helix is unwound by a helicase . </li></ul><ul><li>A molecule of a DNA polymerase binds to one strand of the DNA and begins moving along it in the 3' to 5' direction, using it as a template for assembling a leading strand of nucleotides and reforming a double helix. </li></ul><ul><li>Because DNA synthesis can only occur 5' to 3', a molecule of a second type of DNA polymerase binds to the other template strand as the double helix opens. This molecule must synthesize discontinuous segments of polynucleotides (called Okazaki fragments). Another enzyme, DNA ligase I then stitches these together into the lagging strand . </li></ul>
  17. 17. DNA Replication is Semiconservative <ul><li>When the replication process is complete, two DNA molecules — identical to each other and identical to the original — have been produced. Each strand of the original molecule has remained intact as it served as the template for the synthesis of a complementary strand. </li></ul><ul><li>This mode of replication is described as semi-conservative : one-half of each new molecule of DNA is old ; one-half new . </li></ul>
  18. 18. Interphase: G1, S and G2 phases <ul><li>Lasts much longer than the M phase </li></ul><ul><li>Sometimes referred to as the ‘resting’ phase – this is UNTRUE as although it doesn’t look like much is happening, in biochemical terms, this is a very active period of CELL GROWTH & METABOLISM </li></ul><ul><ul><ul><ul><li>Protein synthesis takes place </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Cytoplasmic organelles are synthesised </li></ul></ul></ul></ul><ul><ul><ul><ul><li>The cell grows and replicates its chromosomes [only during S phase] </li></ul></ul></ul></ul>
  19. 19. <ul><li>Interphase is divided into 3 parts: </li></ul><ul><ul><ul><li>(1) G1 – First ‘Gap’ phase ( During this time the cell is very active, growing and carrying out metabolic processes ) </li></ul></ul></ul><ul><ul><ul><li>(2) S - DNA replication ( The 'S' stands for synthesis as during this phase DNA is synthesized in the process of replication. Each chromosome becomes two sister chromatids) </li></ul></ul></ul><ul><ul><ul><li>(3) G2 - Second ‘Gap’ phase ( In this period mitochondria and other organelles are divided so that each daughter cell will have an equal number of organelles) </li></ul></ul></ul>
  20. 20. Mitosis: the M phase <ul><li>Interphase is followed by M Phase which consists of mitosis and cytokinesis. </li></ul><ul><li>Mitosis is the division of the contents of the nucleus (PMAT), whilst cytokinesis (CK) refers to the division of the cytoplasm. </li></ul><ul><li>Cell division involves mitosis and cytokinesis. The growth of an organism and the replacement of its cells for tissue repair both depend on mitosis and cytokinesis.   </li></ul>
  21. 21. Control of the Cell Cycle <ul><li>A central mechanism is used to assess the status of the cell as it progresses through the cycle. This system works through 3 main checkpoints: </li></ul><ul><li>G1 Checkpoint: towards the end of the S phase.Size of the cell is assessed - if sufficient growth has occurred i.e. cell large enough for division, then S phase can proceed </li></ul><ul><li>G2 Checkpoint: the success of DNA replication is monitored. If successful the cell cycle will continue to mitosis </li></ul><ul><li>M Checkpoint: during metaphase prior to anaphase and telophase triggers exit from from mitosis and cytokinesis and entry into next G1 phase for daughter cells </li></ul>
  22. 22. Abnormal Cell Division : Cancer cells <ul><li>Normal cell development will break down if the control of cell division , cell growth or cell death fails </li></ul><ul><li>If cell division or cell growth fails, TUMOURS arise </li></ul><ul><li>These can either be benign - don’t cause serious problems and can be removed by surgery or malignant - enter the circulation, migrate and proliferate to form new tumours in new areas of the body. This is called METASTASIS </li></ul>
  23. 23. Causes of Cancer <ul><li>Somatic Cell mutations </li></ul><ul><li>Proliferation genes (proto-oncogenes -> oncogenes) </li></ul><ul><li>Anti-proliferation genes (also known as Tumour-suppressor genes) </li></ul>
  24. 24. Mitotic Index <ul><li>Fraction or percentage of cells in a given sample that contain condensed chromosomes i.e. the cells are undergoing mitosis and dividing </li></ul>
  25. 25. Chemotherapy <ul><li>Chemotherapy is the use of anti-cancer (cytotoxic) drugs to destroy cancer cells (including leukaemias and lymphomas). There are over 50 different chemotherapy drugs and some are given on their own, but often several drugs may be combined (this is known as combination chemotherapy ). </li></ul><ul><li>Chemotherapy may be used alone to treat some types of cancer. Sometimes it can be used together with other types of treatment such as surgery, radiotherapy, hormonal therapy, immunotherapy, or a combination of these. </li></ul>
  26. 26. How do chemotherapy drugs work? <ul><li>Chemotherapy drugs interfere with the ability of a cancer cell to divide and reproduce itself. As the drugs are carried in the blood, they can reach cancer cells all over the body. The chemotherapy drugs are taken up by dividing cells, including some normal cells such as those in the lining of the mouth, the bone marrow (which makes blood cells), the hair follicles, and the digestive system. Healthy cells can repair the damage caused by chemotherapy but cancer cells cannot and so they eventually die. </li></ul><ul><li>Chemotherapy drugs damage cancer cells in different ways. If a combination of drugs is used, each drug is chosen because of its different effects. Unfortunately, as the chemotherapy drugs can also affect some of the normal cells in your body, they can cause unpleasant side effects. However, damage to the normal cells is usually temporary and most side effects will disappear once the treatment is over. </li></ul><ul><li>Chemotherapy is carefully planned so that it destroys more and more of the cancer cells during the course of treatment, but does not destroy the normal cells and tissues. </li></ul>