In 1997, a baby sheep named Dolly introduced the world to reproductive cloning. She was a clone because she and her mother shared the same nuclear DNA; in other words, their cells carried the same genetic material. They were like Identical twins reared generations apart.

Scientists at the Roslin Institute in Scotland created Dolly by a process called nuclear transfer. Taking the genetic material from an adult donor cell, they transfered it into an unfertilized egg whose genetic material had been removed. In Dolly’s case, the donor cell came from the mammary glan of a six year old Finn Dorset ewe. The researchers then gave the egg an electric shock and it began dividing into an embryo.

One of the reasons Dolly’s creation was so astounding was that it proved to the scientific community that a cell taken from a specialized part of the body could be used to create a whole new organism. Before Dolly, almost all scientists believed that once a cell became specialized it could only produce other specialized cells. A heart cell ould only make heart cells and a liver cell could only make liver cells. But Dolly was made entirely from a cell extracted from her mother’s mammary gland, proving that specialized cells could be completely reprogrammed.

In many ways, Dolly was not like her mother. For example, her telomeres were too short. Telomeres are thin strands of protein that cap off the end of chromosomes, the structures that carry genes. Although no one is sure exactaly what telmores do, they seem to protect and repair our cells. As we age, our telomeres get shorter and shorter. Dolly revieved her mother’s six year old telomeres, so from birth, Dolly’s telomeres were shorter than the average lamb her age. Although Dolly appeared to be mostly normal, she was euthanized in 2004 at the age of six, after suffering from lung cancer and crippling arthritis. The average Finn Dorset sheep lives to age eleven or twelve.

ADDITIONAL FACTS

1. Since 1997 cattle, mice, goats and pigs have been sucessfuly cloned using nuclear transfer.
2. The success rate for cloning is very low for all species. Published studies report that about 1 percent of reconstructed embryos survive birth. But since unsuccessful attempts largely go unreported, the actual number may be much lower.
3. Before she died, Dolly was the mother of six lambs, all bred the old-fashioned way.
4. A group of Korean researchers claimed to have cloned an human embryo in 1998, but their experiment was terminated at the 4-cell stage, so there is no evidence of their sucess.