U1 KA5 - PROTEIN CONTROL OF CELL DIVSION 3) Apoptosis Flashcards
What is apoptosis
The destruction of cells is brought about by a process known as programmed cell death or apoptosis.
- in a multicellular organism , growth and development involves not only the careful control of cell division by the cell cycle but also careful control of of the deliberate destruction of cells by apoptosis
Examples of apoptosis - where is it necessary
- apoptosis is essential during development of an organism to remove cells no longer required as development progresses: during human embryonic development , programmed cell death of the cells between the fingers and toes allow individual digits to form (rather than webbed digits)
- apoptosis is essential during metamorphosis : also important in this organisms which undergo metamorphosis such as the development of an adult frog from a tadpole
How is apoptosis triggered
Programmed cell death (apoptosis) is triggered by cell death signals
Where can cell death signals come from
These signals can come from outside of the cell(external ) or from within the cell(internal )
What can both types of death signal (internal and external ) result in
Both types of death signal result in the activation of caspases (protease enzymes) that cause destruction of the cell. Note that whether the starting point was intrinsic or extrinsic , both pathways lead to the same end point
It is important that the rate of apoptosis is carefully controlled. Why ?
It is important that the rate of apoptosis is carefully controlled:
- if the degradation enzymes called caspases that achieve apoptosis are overactive , then neurogenerative disease can result
- on the other hand , undereactive caspases can be a cause of tumour development
Explain the extrinsic pathway (external cell death )
- an example of an external death signal is the production of death signals from lymphocytes
- external death signal ligands bind to a surface receptor protein changing the conformation of its subunits . This change triggers a protein cascade within the cytoplasm which produces caspases.
Explain the intrinsic pathway (internal death signal )
- death signals can originate from within the cell: DNA damage is an example of an internal death signal.
- internal death signal resulting from DNA damage results in activation of p53- the tumour suppressor protein which can also activate a caspase cascade
How else may cells initiate apoptosis
Cells also may initiate apoptosis in the absence of cell growth factors
-Animal cells undergo apoptosis if they are deprived of survival factors released by other cells.
What is the caspase cascade
- both types of death signal result in the activation of protease enzymes called caspase : which act in cascades to cause the destruction of the cell
- caspase cascade involves post translational modification to proteins already present in cytoplasm. This allows rapid response . The PTM involves cleavage (subunits are removed and they can re assemble to form active caspases ) the caspase cascade involves inactive caspases becoming activated.
- each initiator caspase at the beginning of cascades activates several executioner caspases- these destroy the cell.
What is fragmentation of the cell
Once the cellular components have been destroyed, the cell can no longer function. The lack of intact cytoskeleton means that cells breaks into many small vesicles fragments. These are rapidly engulfed and digested by phagocytic white blood cells
Give different roles of apoptosis in a multicellular organism
Programmed cell death also prevents cells from surviving in environments where they should not be present.
• To kill cells that have been infected by pathogens.
• To prevent the survival of cells with damaged DNA (these have the potential to become cancerous).
• To eliminate white blood cells that would produce an immune response to the body’s own cells.
