One thing Dolly and her clonal colleagues could clarify is how DNA changes through an animal's life. As we've mentioned, scientists thought that chromosomes undergo irreversible maturation as the body develops, making them unable to revert to the primitive stage of embryonic chromosomes. By the time a cell is fully specialized -- to be a muscle or nerve cell -- most of its genes are no longer functioning. Thus, nerve cells don't make hair or skin proteins, even though they have the genetic instructions to do so. Dolly is apparently baa-ing proof that this is not a problem. Chromosomes can apparently revert to a primitive stage.
Chromosomes also change through time when markers called telomeres are added to their ends. Telomeres are apparently a way of counting and controlling how many times a cell can reproduce. That could mean that Dolly will die young as her chromosomes follow their programming and stop reproducing.
Then again, maybe not. According to new research, the function of telomeres is more complicated -- see "Scientists Rethinking..." in the bibliography.
Finally, there's the question of mutations -- the accumulated flaws that occur as DNA reproduces under assault from various chemicals and radiation. Most of these breaks are repaired by normal cell processes. Usually, the rest don't cause problems because they are on genes that function only during development, and not after maturity.
For that very reason, nobody knows exactly how chromosomes change as an animal grows up. "We have no idea of how they really change during this process of growth and specialization," says Philip Leder of the Howard Hughes Medical Institute at Harvard University. But since mutations are usually harmful, they could be a limiting factor on Wilmut's cloning technique.
Another limiting factor, at least in the early stages, is efficiency. Of 227 egg cells that received new genetic material, only one developed. Maybe Wilmut was lucky -- and happened to succeed with a technique that works only extremely rarely. Then again, perhaps his scientific followers -- who are already trying to cook up clones of their own -- will find better ways to bring the cloned cells into the world.
Time for a word from the skeptics. Are we taking this cloning business a bit too far? Is there less here than meets the eye?
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