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036-044
- Zusammenfsg.
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Once established in a genome, certain gene sets can force their maintenance to the genome by killing the host organism that has threatened their persistence. This form of programmed cell death, called post-disturbance killing or genetic addiction, might be a general principle in symbiosis of genetic elements within a genome. A simple example is provided by a gene complex coding for a restriction enzyme and a cognate modification methyltransferase, such as EcoRI gene complex. The descendants of cells that lose the restriction-modification gene complex are unable to modify a sufficient number of recognition sites in their chromosomes to protect them from lethal attack by the remaining molecules of restriction enzyme. In general, a gene is defined essential to an organism when its loss leads to death. However, since its first appearance in the genome as a 'dispensable' gene, from outside or from inside, the gene may have co-evolved with the host so that it can now program death upon its curing. The capacity of mitochondria to kill their host cell, upon disturbance, may have stabilized their endosymbiosis at the initial stage of evolution of the eukaryotic cells. Symbiosis between genes through mutual addiction may be already built in the apparatus known as the chromosome, in which genes align hand in hand. Death of one gene by a chromosomal double-stranded breakage would readily lead, by degradation, to death of the remainder of the resident genes and, therefore, to death of the cell. This scenario of genetic addiction or symbiosis-through-death may provide a paradoxical answer to the question of how symbiosis can ever evolve from interaction between genetic elements with potentially different interests.
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