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Lecture 17

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Western University
Biology 1001A
Tom Haffie

Typical causes of cell death. - ● digesting itself via a lysosome ● volume too great for surface volume to support this will starve (unlikely because chlamy is a eukaryote) ● viruses ● environment conditions too severe ● cell might be preyed upon ● toxins ● mutational load Mechanism of plasmid toxin/antitoxin system as a possible origin for cell death genes. - ● Bacteria often suffer from infection by parasite plasmids ● The cells of course want to get rid of the plasmid. One way this occurs is when the cell divides, it is possible that the plasmid does not end up in one of the daughter cells ○ that cell without the plasmid is now selected for and is advantageous to the cells with plasmids thus will divide faster and overcome the population ● Plasmids don’t want this and thus they code for a toxinAND the antitoxin in their genes, however the antitoxin degrades at a faster rate (shorter half life) than the toxin ○ this means that if the cell were to somehow get rid of the plasmid, they would have antitoxin and toxin but the antitoxin would degrade faster, leaving just the toxin and the cell dying as a result ● Cells as a result try to get the gene that codes for the antitoxin and when they do, the plasmid responds by creating a new toxin. In response, the cell then again gets the antitoxin gene and we end up with an arms race Programmed cell death cascade in C. elegans. - ● The cell death mechanisms is a highly regulated and irreversible mechanism ● The proteins involved in this cascade include CED-9, which inhibits CED-4 in it’s normal state, CED-4, which activates CED-3, and CED-3 which is a caspase (protease) that digests (cuts) specifics proteins (proteases and nucleases) to activate them ● The death signal, which is usually developmentally related, is received by the death signal receptor on the plasma membrane which then inactivates CED-9 ● CED-9 releases CED-4 which can now activated CED-3, which then subsequently makes the cuts necessary to activate specific proteases and nucleases which kill the cell ● In this case, the death signal is external, but can come internally as well Possible evolutionary origin of programmed cell death genes. - ● It is thought that when endosymbiosis `took up the mitochondria bacteria, our cells also took up the toxin/antitoxin mechanism and put it under developmental control to commit suicide when needed Characteristics that make Drosophila an attractive model system. - ● Easy and cheap to work with ● Many mutants available, development and which genes are apart of the process is understood well ● Two organisms form from one genome, a larva and a fly and so cell death
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