Dolly the cloned sheep

Dolly the sheep was created by Ian Wilmut and a team of researchers at the Roslin Institute in Edinburgh in July 1996.  Now five years old the cloning research which produced Dolly has advanced into the technique of Nuclear Transfer.

What was especially remarkable about Dolly was that the donor cell came from the mammary gland of an unrelated sheep.  This adult cell had already differentiated and was genetically programmed to do all the things that mammary cells do (which are different from the things brain cells and blood cells do) and yet during the cloning process it had regained its totipotency as an unformed embryonic stem cell.

For quite some time there was scepticism about whether the donor cell was actually taken from the sheep's udder.  Perhaps the real donor was just a rogue embryonic stem cell.  Further experiments showed that this was not the case.  Tests on Dolly showed that she was displaying signs of cell ageing and that biologically she was six years old even though newly born. Scientists are still not sure what effect premature ageing will have on clones (which is another reason to be concerned about human cloning).