Degradation
Multicellular organisms replace worn-out cells through cell division. In some animals, however, cell division eventually halts. In humans this occurs on average, after 52 divisions, known as the Hayflick limit. The cell is then referred to as senescent. Cells stop dividing because the telomeres, protective bits of DNA on the end of a chromosome required for replication, shorten with each copy, eventually being consumed, as described in the article on telomere shortening. Cancer cells, on the other hand, are not thought to degrade in this way, if at all. An enzyme called telomerase, present in large quantities in cancerous cells, rebuilds the telomeres, allowing division to continue indefinitely.
Read more about this topic: Cell Division
Famous quotes containing the word degradation:
“Gradually we come to admit that Shakespeare understands a greater extent and variety of human life than Dante; but that Dante understands deeper degrees of degradation and higher degrees of exaltation.”
—T.S. (Thomas Stearns)
“Self-alienation is the source of all degradation as well as, on the contrary, the basis of all true elevation. The first step will be a look inward, an isolating contemplation of our self. Whoever remains standing here proceeds only halfway. The second step must be an active look outward, an autonomous, determined observation of the outer world.”
—Novalis [Friedrich Von Hardenberg] (1772–1801)
“We found nothing grand in the history of the Jews nor in the morals inculcated in the Pentateuch.... I know of no other books that so fully teach the subjection and degradation of woman.”
—Elizabeth Cady Stanton (1815–1902)