6.1.7 The Importance of Mitosis and Apoptosis Flashcards
What is mitosis?
Mitosis is the type of cell division that produces identical new cells for processes such as growth, cell replacement and tissue repair
What is apoptosis?
Apoptosis is programmed cell death (sometimes referred to as natural cell death)
What happens during apoptosis?
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
What is the importance of both 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
How is mitosis controlled?
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
What can tumour-suppressor genes also do?
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
What do checkpoints do during mitosis?
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
What regulates the control through these checkpoints?
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
What are the genes that control the cell cycle and apoptosis able to respond to?
The genes that control the cell cycle and apoptosis are able to respond to:
Internal cell stimuli
External cell stimuli
What are some 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
What are some 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
