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Text File  |  1992-09-02  |  700b  |  18 lines
  1. In common usage, the period of deterioration
  2. of the physical condition of a living
  3. organism that leads to death; in biological
  4. terms, the entire life process. Three current
  5. theories attempt to account for ageing. The
  6. first suggests that the process is
  7. genetically determined, to remove individuals
  8. that can no longer reproduce. The second
  9. suggests that it is due to the accumulation
  10. of mistakes during the replication of DNA at
  11. cell division. The third suggests that it is
  12. actively induced by pieces of DNA that move
  13. between cells, or by cancer-causing viruses;
  14. these may become abundant in old cells and
  15. induce them to produce unwanted proteins or
  16. interfere with the control functions of their
  17. DNA.
  18.