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:
“Such poverty as we have today in all our great cities degrades the poor, and infects with its degradation the whole neighborhood in which they live. And whatever can degrade a neighborhood can degrade a country and a continent and finally the whole civilized world, which is only a large neighborhood.”
—George Bernard Shaw (1856–1950)
“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)
“In her present ignorance, woman’s religion, instead of making her noble and free, by the wrong application of great principles of right and justice, has made her bondage but more certain and lasting, her degradation more hopeless and complete.”
—Elizabeth Cady Stanton (1815–1902)