Goodbye, Dolly - Euthanasia to first cloned sheep.
Dolly, the famous cloned sheep has been put to death after being diagnosed with a progressive lung disease and follows earlier reports that she was prematurely aging, including developing arthritis. [DETAILS & DISCUSSIONS]
EXCERPTS FROM SLASHDOT : You see there's this very important substance called telomeres attached to the end of chromosomes. As cells divide, the telomere caps become shorter, and eventually the cells stop dividing and either malfunction or die. It stands to reason that, if you start off with an adult cell, you already have shortened telomeres and will therefore have a reduced lifespan. Telomeres aren't just any substance. They're DNA. The end of a chromosome has short repeating sequences of a few base pairs (ex: AATTAATT, etc.) which are not all replicated when a cell duplicates its genome and divides. This presumably acts as a molecular "clock" for the organism to keep track of its "age," but this is pretty controversial and unsubstantiated.
The mechanics of it are simple -- at the end of the DNA strand in each nucleus is a length of DNA that codes for nothing, called a telomere. With each division, the telomere gets cut back. Eventually, the cell divides one last time and the telomere isn't long enough, so the cut goes right into part of the DNA that codes for something, thus causing an error catastrophe ultimately resulting in death.
But take hope. In the 1980's, while studying cancer cells (in fish, I think), researchers found this chemical called telemorase, which rebuilds the telomeres. [For which one of the more recent nobel prizes was awarded] Ever since, researchers have been working feverishly on telemorase treatments, which we should start seeing in the next decade or two.