Dolly the sheep was the first mammal cloned from an adult cell. She born on July 5, 1996, and was created five months earlier. In 2016, four sheep that were cloned from Dolly seem to be in perfect health.
The famous Dolly the sheep was created at the University of Edinburgh by embryologist Karen Walker and embryologist Bill Ritchie. Dolly was created by nuclear transfer. This was done by starting with an oocyte (an unfertilized egg) and removing its chromosomes. That cell was then fused to an enucleated egg, and activated. After it started growing, it was transferred to a surrogate mother.
The result was Dolly. She was the not only the first mammal cloned from an adult cell, but also the first adult clone ever. Dolly’s birth was normal, and a veterinarian assisted it. The cells that made this unique sheep were derived from mammary tissue, so the scientists named Dolly the sheep after Dolly Parton.
Obviously, Dolly the sheep got a lot of attention from the media. At first, she appeared normal (and even gave birth to several sheep herself). When Dolly was 5 years old, she developed osteoarthritis By age six, she had the type of joint and lung problems that are reminiscent of old age.
The scientists discovered Dolly had a disease called jaagseikte, which caused a large tumor in her lung. She had to be euthanized. Researchers discovered something interesting when they looked at Dolly’s telomeres. Telomeres are structures at the end of DNA that shorten with each replication. This creates shorter telomeres as the body ages. Dolly’s telomeres surpassed her chronological age.
The implication was that the “clock” on her cells had been ticking for longer than Dolly had been alive. It was unclear if this situation was unique to Dolly, or if it meant that all mammals cloned from adult cells would have the same problem.
In July 2007, four sheep that were derived from the same mammary gland cell that gave rise to Dolly were born. They were named Debbie, Denise, Dianna, and Daisy. For whatever reason, these four clones are doing just fine.
In 2016, these sheep clones turned nine years old, surpassing Dolly’s age by three years. Only one of the four sheep is showing signs of arthritis (something that is not unusual for sheep of that age.)
If these healthy, cloned, sheep live to be ten years old, they will have reached a milestone for farm animals. At that time, they will be humanely euthanized (and their bodies will be studied). There are no plans to make any more clones like Dolly.
Image by Neil Turner on Flickr.
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