In a message dated 5/1/00 9:54:53 PM Pacific Daylight Time,
jonkc@worldnet.att.net writes:
> <CurtAdams@aol.com> Wrote:
>
> >Mice cells can go forever in culture;
>
> I assume you mean cancerous mouse cells, the same is true of humans cells.
I was referring to explanted normal cells. Normal mice
explants almost always transform (more or less become
cancerous) after grown in culture for a while. Human
cells never transform in culture. The transforming
mutations, statistically, must happen; apparently human
cellular self-screening is nearly flawless in culture.
(I.e., cells with pre-cancerous mutations kill themselves)
It's obviously not *that* good in vivo, although it's
obviously very good. One hypothesis is that human cancer
arises when cell are subject to certain strong growth
signals, which overide the suicide genes, permitting
pre-cancerous mutants.
>> human cells always have a finite lifespan.
>That was true until about a year ago when the Geron corporation found a way
>non cancerous human cells could be immortalized by lengthening their
telomeres.
I was thinking of explants, not engineered cells.
>>I think you may get more divisions prior to transformation, but that's
>>not a telomerase issues; it just means longer-lived species have better
>>cancer control
>Or it could mean telomerase issues have something to do with cancer control.
I do find the "standard explanation" for the benefit of our limiting telomeres
compelling; an absolute and fairly short limit to cell replication means
there's just not enough generations for mutations of small effect to add up
to a cancerous cell. If the surveilance systems can catch most of the
big mutation (probably a fairly limited set; knock out gene A, constituitively
activate B, etc.) then cancer becomes nearly impossible. There has to
be some benefit to turning off telomerase, or it'd just be on everywhere.
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