ageing
In common usage, the period of deterioration of the physical condition of a living organism that leads to death; in biological terms, the entire life process.
Three current theories attempt to account for ageing. The first suggests that the process is genetically determined, to remove individuals that can no longer reproduce. The second suggests that it is due to the accumulation of mistakes during the replication of DNA at cell division. The third suggests that it is actively induced by fragments of DNA that move between cells, or by cancer-causing viruses; these may become abundant in old cells and induce them to produce unwanted proteins or interfere with the control functions of their DNA.
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