>>I don't think the entropy idea really holds water because errors would
>>accumulate from one generation to the next as quickly as from one
>>mitosis to the next. Just as lethal mutations are selected out in a
>>species so they would be selected out in a group of cells.
>>This is a very good argument. One would think that deleterious
>but silent mutations could accumulate from one generation to
>another until an entire species is wiped out. But that does not
>happen.
I don't see how easily deleterious but silent mutations could accumulate
until an entire species is wiped out. Mutations, as matter of fact,
happen in continuation in every living species, beeing this the basis
of evolution, but I can not see entire species wiped out.
Moreover, at species level, the mutations select them selves,
the deleterious ones making the organism less competitive and less
able to reproduce and the opposite happens for the favorable mutations.
This is the basis for the evolution where mutations play a very
important role and I see no need to discuss further.
At individual organism the situation is a little different
beeing not always easy for all the kind of cells to be replaced
when some damage reduce their efficiency, especially in determinate
kind of cells and in the adulthood. Replacement can be effective
only in open growth system, as is the case of species, but not in
controlled growth system, as the individual organisms.
Moreover, the mutation could be favourable for the single cell
and deleterious for the organism (e.g. cancer mutations), making
completely lose the sense of a selection from one mitosis to the next,
even if the cells of the individual were free to replicate at will.
>The death clock argument is the first suggested mechanism of aging that
>has ever made sense to me because it explains why there are no imortal
>mutants - they die of cancer before we notice they are immortal.
Normal cells, should not develop in cancer-like form even when
their clock is not terminated; cells are inhibited in the growth
by the contact with the surrounding cells. This contact makes the cell
to "understand" its position and role in the organism. When for some
mutation the information on the contact inhibition is lost in the
cell, it could become cancer-like.
Bye,
-- Vincenzo
