OCR A Level Biology

Revision Notes

6.1.7 The Importance of Mitosis & Apoptosis

The Importance of Mitosis & Apoptosis

  • Mitosis is the type of cell division that produces identical new cells for processes such as growth, cell replacement and tissue repair
  • Apoptosis is programmed cell death (sometimes referred to as natural cell death)
  • In apoptosis, old cells that have already undergone a large number of mitotic cell divisions (approximately 50 divisions, although this depends on the cell type) are systematically taken through various processes leading to cell death
  • These processes include:
    • The DNA of the cell becoming denser and more tightly packed
    • The nuclear envelope of the cell’s nucleus breaking down and chromatin condensing
    • Vesicles forming that contain hydrolytic enzymes
    • Phagocytes engulfing and digesting the cell via phagocytosis

The importance of mitosis and apoptosis in controlling body plan development

  • By constantly replacing and destroying cells throughout the early development of an organism, mitosis and apoptosis are both key mechanisms controlling the development of body form
  • Apoptosis is important in development as, in some cases, some cells that are produced (by mitosis) earlier on in development may no longer be needed
  • As a result, these cells are destroyed (by apoptosis) as part of the development of the organism
  • For example, structures like fingers and toes first develop as a single combined unit and are then separated later via programmed cell death (apoptosis) of the cells in between the digits

The control of mitosis and apoptosis

  • Mitosis is controlled by various different genes that are categorised into two distinct groups:
    • Proto-oncogenes are genes that stimulate cell division
    • Tumour-suppressor genes are genes that reduce cell division
  • Tumour-suppressor genes can also stimulate apoptosis in cells with damaged DNA that cannot be repaired
    • This protects the body as it ensures that any cells that are genetically damaged (and that could, therefore, lead to cancer) are destroyed
  • During the cell cycle (in cells due to undergo mitosis) there are various ‘checkpoints‘ that need to be passed to ensure that damaged cells are not produced
  • These controls ensure that the cell is prepared for the mitosis phase of its cell cycle and that any DNA damage is repaired
  • These controls are regulated by two groups of proteins, known as cyclins and cyclin-dependent kinases (CDKs), that regulate the progress of the cell through the cell cycle
    • Cyclins act as regulators
    • CDKs act as catalysts (once activated by cyclins)
    • For example, CDKs that have been activated by cyclins will catalyse the phosphorylation of particular target proteins, which can either activate or inactivate them
    • This ensures the cell cycle progresses from one stage to the next
    • Different cyclins are produced at different stages of the cell cycle in response to internal molecular signals
  • The genes that control the cell cycle and apoptosis are able to respond to:
    • Internal cell stimuli
    • External cell stimuli

Examples of internal cell stimuli

  • Internal factors that affect apoptosis and the cell cycle include:
    • Irreparable genetic damage
    • RNA decay
    • Internal biochemical changes that lead to cell changes or cellular injury (e.g. oxidative reactions)
    • Production of cyclin D
  • These factors can all initiate apoptosis in cells that are undergoing cell stress

Examples of external cell stimuli

  • External factors that affect apoptosis and the cell cycle include:
    • The presence of cell signalling molecules such as cytokines from the immune system, hormones and growth factors
    • Viruses and bacteria, harmful pollutants or ultraviolet light can affect the delicate balance of mitosis and apoptosis by damaging or destroying cells faster than they can be repaired or replaced
  • Cells often respond to such stressful stimuli by activating pathways to increase their chance of survival, or by initiating apoptosis
    • For example, a cell will often begin by defending itself and trying to recover from the stressful stimulus by counteracting any damage caused to it
    • However, if the stressful stimuli remain, cell death pathways are activated (i.e. apoptosis is initiated)
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