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life and death of a cell

an excerpt from The Machinery of Life

by David S. Goodsell

2nd ed., Springer, 2009

The major discovery made by the earliest cells is the ability to maintain order when challenged by the inevitable decay into equilibrium. We might imagine two ways to face this great challenge: immortality or planned obsolescence. An immortal organism would be completely resistant to environmental forces, or have powerful repair mechanisms to undo damage as it occurs. This obviously was too much to ask for the earliest cells, which were built from fragile organic materials. Instead, they developed along a path of planned obsolescence (see below). Molecules, cells, and organisms are all born perfect and new, they live for a minute, a year, or a century, and then they die ... but not before reproducing to create a new molecule, cell, or organism.

Cells that are gravely injured make a mess when they die. They swell and burst, and the contents of the cell spill out, get into inconvenient places and release their destructive components. The organism responds with immune cells that struggle to clean up the mess without damaging the healthy surrounding too much. To avoid this messy problem, our cells are programmed with a method to commit suicide, quickly and cleanly: programmed cell death.

All of our cells are preprogrammed with a method to die on command, if necessary. When a cell enters into programmed cell death, it disassem- bles its own molecular machinery in a safe and orderly manner and notify the immune system that it is ready to be recycled. In the illustration on the right (magnification is about one million times), a cytotosic T-cell at the top has given the signal for the cell at the bottom to die. Proteins on the T-cell surface are recognized by death receptors (A) on the cell surface, beginning the process that leads to death. As part of the process, the BID protein (B) forms a pore in the surface of mitochondria (shown at the bottom), releasing cytochrome (C) into the cytoplasm. This is the signal to assemble an apoptosome (D), which then activates initiator caspases (E), the ultimate executioners of programmed cell death. These then activate more caspases (F), launching an orderly campaign to attack key proteins throughout the cell. For instance, caspases make a cut in the protein gelsolin (G), converting it into an active form that disassem- bles actin filaments.

programmed cell death

 2010-07-11 

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