apoptosis rough draft - Compared with the life of a multicellular organism cells are cheap Regulated cell death apoptosis is used for a variety of

apoptosis rough draft - Compared with the life of a...

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Compared with the life of a multicellular organism, cells are cheap. Regulated cell death - apoptosis - is used for a variety of processes by such organisms, from the sculpting of limbs during development to the destruction of infected cells to prevent the spread of infection. It is clearly an extremely important process and yet it is relatively poorly understood; only in the last decade or so has significant progress been made in understanding the signaling cascades involved, and even now many of the details remain unclear. It does seem, however, that the default state of many cells is suicidal, and that this fate is only averted by the detection of extracellular 'survival signals'. As our knowledge increases, however, it seems that our understanding of the scale and influence of apoptosis can only increase. One particularly interesting speculation is that caspases - the family of proteases responsible for cell execution in apoptosis - may have several other important functions, including a role in the regulation of cell-cycle progression. In this essay, I will outline the basic mechanisms of apoptosis, before considering how it is or may be involved in the larger regulation of the cell life cycle. Apoptosis: Step-by-Step Death The term 'apoptosis' was originally coined to describe the morphological characteristics of a certain type of cell death, in contrast to the uncontrolled process of necrosis. In the latter, cells swell and burst, spilling their contents across neighboring cells with the potential to induce a damaging inflammatory response; in the former, cells shrink and their contents condense, leading to membrane blabbing. Their nuclear contents also condense, and DNA is degraded to give rise to a 'ladder' structure. Cells undergoing apoptosis identify themselves to their neighbors, most noticeably by exposing phosphatidylserine (PS) on the outer leaflet of the plasma membrane,
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when normally it is maintained in the inner leaflet. This acts as a signal for neighboring cells to remove the remnants of the dying cell by phagocytosis to be recycled. In mammalian cells, there are two main pathways for apoptosis: Signaling via mitochondria, and signaling via 'death-receptors' such as CD95 or Fas. There is a certain amount of cross-talk between the pathways, and both result in the destruction of the same apoptotic substrates. They are both discussed below; however, first it would be prudent to introduce the major players in apoptosis: Caspases. Caspases are so named because all known examples possess an active-site cysteine residue, and cleave protein substrates after aspartic acid residues. This can have two main effects, as shown in figure two; it may lead to loss of function, or it may lead to gain of function. Over a dozen caspases are known in mammals, and each has distinct effects. A given caspase's specificity is determined by the four residues amino-terminal of the cleavage site, and such cleavage has been used to explain many of the characteristic features of apoptotic cells. For example, caspase-3 activates CAD (caspase-activated DNase) by cleavage of an inhibitory subunit (ICAD). CAD
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