Cloning and Telomerase

[Carebear123]

I learned in my grade 11 biology class that Telomerase is the enzyme responsible for creating telomeres. It is present in blastulas because telomeres need to be created in order for cells to divide as rapidly as needed for the growth of a fetus. However it's not found in adult cells. Its obvious I don't have a lot of biology background but this is what I'm wondering:
How come when adult cells were cloned from animals like Dolly, they were able to grow into a baby. If adult cells lack the enzyme, how come when we clone the cell, it is able to divide so many times into a fetus? Where is the enzme coming from?
Hopefully I explained well enough,
Thanks!

[blcr11]

Cloning “cells” is not the same thing as cloning an animal. When you clone an animal—when you make a Dolly—you do an in vitro fertilization between isolated egg and sperm, and then implant the embryo into a live animal where it develops as if it were a natural fetus. The telomerase gene is present in the nascent embryo and will be expressed during development in the same way it would if the embryo had arrived the usual way.

The telomerase gene is never really lost, by the way. It is repressed and so not expressed in mature, differentiated cells, but the gene is still there. You can take cells from at least some kinds of differentiated tissues and get them to grow in culture for some number of cell divisions. These cells can be “cloned” in the sense of isolating an outgrowth from a single parental cell, but this is not the same thing as cloning an animal. Primary cell cultures will die off after some number of divisions, in part because the lack of telomerase results in ever shortening chromosomes that eventually can’t function normally. One of the steps required for the in vitro immortalization of cell lines is the de-repression of telomerase to enable the telomeres to be continually restored.

They are learning how to do more with differentiated cells. Recently, there were reports of people being able to make stem cells out of differentiated cells. Up until very recently this was thought to be at least difficult if not impossible to do. I get suspicious, though, whenever some pundit declares something to be impossible. Almost certainly someone, some day, will prove them wrong--with the possible exception of cold fusion, perhaps.

[snowcapk]

Carebear, another thing to consider is that there is no evidence that telomere shortening results in complete loss of telomeres (and thus loss of genes at chromatid ends) within the lifetime of a normal sheep, or even twice the normal lifetime. In other words, even if the telomerase gene were completely lost, there may well be enough telomere sequence to protect chromatid ends for, say, 3 generations. Without knowing the telomere length, there is no reason to believe that telomere shortening would limit the number of cell divisions enough to prevent an NT clone like Dolly from developing into a healthy adult. (BTW, Dolly did not die of cloning-related causes, but of a retrovirus that is common in sheep kept indoors. The same virus claimed other "control" sheep born concurrently at the same facility.)