Apoptosis
3.3
KEY IDEAS In this topic, you will learn that: ✚
apoptosis is a natural, regulated process of programmed cell death internal or external signals can lead a cell to apoptose.
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The normal functioning of an organism involves the process of interphase and the reproduction of cells through mitosis and cytokinesis. When a cell is damaged and cannot be repaired, or is not required anymore, its death needs to be regulated so that it doesn’t damage any other cells in the process. If a cell is exposed to toxins or extreme temperatures or is damaged, it can swell and eventually lyse, meaning the membrane breaks and the contents are released. This can cause the surrounding cells to become damaged and also die. This process of swelling Extrinsic pathway and breaking apart is called necrosis. TNF-α To prevent this from happening, Death cells need to have a safe, organised receptor cell death pathway. Programmed cell death (apoptosis) is important when there is an excess of cells; for Caspases activated example, when an infection is under Mitochondrion control and high levels of white blood Cytochrome c cells are no longer needed, or when released the body needs to remove a virusAPOPTOSIS infected cell. Intrinsic pathway The signal that starts the process of apoptosis can come from an FIGURE 1 Apoptosis can be activated through an intrinsic or internal (intrinsic) or external extrinsic pathway. (extrinsic) source (Figure 1).
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Intrinsic pathway to apoptosis
intrinsic pathway
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a biochemical pathway occurring completely within a cell that triggers apoptosis
cytochrome c
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a small protein on the inner membrane of mitochondria, which is involved in the electron transport chain of aerobic cellular respiration
OXFORD UNIVERSITY PRESS
The intrinsic pathway to cell death involves internal processes. These may be in response to cell damage that is irreparable.
Mitochondrial damage Cytochrome c is a small protein found on the inner membrane of mitochondria, where it plays a part in aerobic cellular respiration. When a mitochondrion is damaged, cytochrome c leaks into the cytoplasm, where it can activate apoptosis.
DNA damage A cell’s DNA is regularly checked by a multitude of repair enzymes. If the DNA is damaged, the protein p53 attempts to repair the damage and blocks the cell cycle from progressing, giving the DNA more time to repair. If the DNA cannot be repaired, p53 initiates apoptosis in the cell by activating the mitochondria to release cytochrome c into the cytoplasm.
CHAPTER 3 THE CELL CYCLE
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