The Importance of Mitosis & Apoptosis (OCR A Level Biology): Revision Note

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

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