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Apoptosis is programmed cell death that occurs in multi-cellular organisms. Cells undergo
50 mitotic divisions (Hayflick constant) and then undergo a series of biochemical events
that leads to an orderly and tidy cell death. This is to ensure no hydrolytic enzymes are
The sequence of events in Apoptosis:
Enzymes break down the cell cytoskeleton
The cytoplasm becomes dense, with organelles tightly packed
The cell surface membrane changes and small bits called blebs form
Chromatin condenses and the nuclear envelope breaks. DNA breaks into fragments
The cell breaks into vesicles that are taken up by phagocytosis. The cellular debris
is disposed of and does not damage any other cells or tissues. Apoptosis occurs
Apoptosis is controlled by cell signals. The signals include cytokines made by cells of the
immune system, hormones, growth factors and nitric oxide. Nitric oxide can induce
apoptosis by making the inner mitochondrial membrane more permeable to hydrogen ions.
Proteins are released into the cytosol and they bind to apoptosis inhibitor proteins and allow
the process to take place.
Apoptosis and Development
There is extensive division and proliferation (growth by the rapid multiplication of parts)
of a particular cell type by pruning through programmed cell death. The cells shrink,
fragment and are phagocytosed so that the components are reused and no harmful
hydrolytic enzymes are released.
Apoptosis is regulated during development and different tissues use different
signals for inducing it. It weeds out ineffective or harmful T lymphocytes during the
development of the immune system.
During limb development, apoptosis causes the digits to separate from each other. The rate
of cells dying should balance the rate of cells produced by mitosis. If the rates are not
Not enough apoptosis leads to the formation of tumours
Too much apoptosis leads to cell loss and degeneration
Cell signalling plays a crucial role in maintaining the right balance.
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Stretch and Challenge
1. Pros: Boys and girls have a lower chance of getting HPV virus. Cons: Boys
and girls have to be vaccinated before sexual maturity so every person
experiencing or after sexual maturity will not be affected.
1. Nitric oxide can induce apoptosis by making the inner mitochondrial
membrane more permeable to hydrogen ions. Proteins are released into the
cytosol and they bind to apoptosis inhibitor proteins and allow the process
to take place.