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Apoptosis (or cell suicide) : Process and TypesBY: Amna Adnan | Category: Biology | Submitted: 2010-08-07 22:21:53
Apoptosis is the programmed cell death in which cells before dying undergo series of events. Through this way, the unnecessary cells are removed from the body or the cells which cause harm to the body. Apoptosis usually occurs during the embryonic stages when the cells are growing and developing. It can also occur in adult cells which are affected through some injury or when the tissues need to be remodeled. Aging is another factor which causes apoptosis. Process of apoptosis is also celled as cell suicide because cells use cellular machinery to kill themselves. It takes place only in multicellular organisms. It is a normal thing when the cell functions and if there is an incomplete process of apoptosis; it may lead to the development of malignant and benign tumors.
What Triggers the Process of Apoptosis:-
For every process which occurs in the body, there is some reason behind it. There are numerous causes which make the process of apoptosis to take place. The most important cause is the DNA damage. When the body of the person is exposed to ionizing radiations like x-rays, ultraviolet radiations or chemotherapy medications for the treatment of cancer then apoptosis can occur. Another factor which triggers apoptosis is the corticosteroids. On the surface of the every cell there is a special type of protein called as Fas protein, it also causes the cell suicide.
How Apoptosis Takes Place:-
There are several steps involved in the programmed cell death;
1) When the unnecessary enzymes start activating in the cell, they eat up the proteins due to which cell starts becoming round.
2) DNA present inside the nucleus starts separating and eventually it shrinks down.
3) There is a nuclear membrane around the nucleus, when the apoptosis starts, it degrades and cell's nucleus becomes without the outer layer.
4) Due to the absence of the nuclear membrane, the DNA molecule starts rupturing into small fragments. These fragments are not in a particular size.
5) As nucleus is no longer protected that is why it breaks down into many pieces along with the uneven pieces of DNA molecule.
6) Due to the breakage inside the cell, cell itself starts degrading through the process of blebbing.
7) Blebbing converts the cell into mall pieces which are eaten by other small cells known as phagocytes.
Types of Apoptosis:-
There are three types of apoptosis;
Apoptosis can occur through internal signals. Internal signals mean when there is damage to the cell internally. Internal damage triggers a protein called BAX. This protein pricks the mitochondrial membrane. Mitochondria are the power house of the cell and they provide energy to the cell to perform various functions. Due to the puncturing of mitochondrial wall, cytochrome c releases from it and binds to the Apaf-1. This binding makes the production of apoptosomes which trigger apoptosis. These apoptosomes cause the formation of capsases. Capsases breakdown the structure of the cell and DNA destroys ultimately.
External signals also cause the apoptosis to occur. The Fas proteins on the surface of the cell bind to another protein called TNF or tumor necrosis factor which will in turn trigger the signal in cytoplasm and will activate capsase 8. This enzyme will lead to the formation of more capsases which will eventually breakdown the cell and destroy the DNA.
Apoptosis Inducing Factor:-
Apoptosis inducing factor also causes the cell suicide. This type of apoptosis usually occurs in the neurons which are the responsible for conducting the nerve impulses. Other cells can also undergo AIF. It is a protein present inside the mitochondrial intermembrane space. When the apoptosis starts, cell receives a signal of death and in response to this; AIF is released in the cytoplasm. This protein reaches to the nucleus and destroys the DNA molecule. When DNA will be killed then there will be no activity taking place inside the cell and cell will die automatically.
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