|
It's like monks copying books by hand -
sometimes a spelling error creeps in; the monk who
copies the new manuscript also copies the spelling
error and maybe adds another error. As the
mistakes accumulate, the cat (the host body that
the cells live in) shows signs of ageing.
How does the cell know how many time it has
divided? The genes (lengths of DNA) are stored as
strings called chromosomes in the cell nucleus.
DNA is like an alphabet; the gene is like a word
and the chromosome is like a sentence. At the ends
of each chromosomes is a telomere - these are like
strings of full stops. In young kittens, the
telomeres are relatively long and there are lots
of full stops in each telomere. Each time the cell
divides, the telomere gets shorter as a full stop
is lost off of the end. This is part of the ageing
process and seems to be why animals grow old. The
telomeres are also like a clock, telling the
chromosome in the cell how old the cat is.
In effect, a cell taken from a cat knows how
old it is. For instance, a kitten might be cloned
from a cell from a ten year old cat. Although the
kitten is newborn, all of its cells are saying
that they are ten years old. Most likely the clone
will age faster than a kitten produced in the
normal way.
It seems like premature ageing but it isn't,
those cells are already older than the host body
and older cells don't copy themselves as
efficiently. Scientists are already seeing this in
Dolly the sheep - when Dolly was three years old,
all her cells were six years old because they had
already aged three years in her 'mother' before
being passed on to the clone (I forget how old
Dolly's 'mother' actually was, but hopefully you
get the idea).
At present, the only
way to overcome the problem of premature ageing in
clones is to clone very young animals or to take a
fertilized egg which has begun to divide and to
split it into several parts (the same way nature
creates identical twins and triplets).
|
