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)
