In February 1997, a sheep called Dolly became the most famous example of her species, briefly even becoming a TV celebrity. The reason for her fame is that she was the first mammal to be “created” by a process called somatic cell nuclear transfer, or in other words the first man-made clone (man-made to be distinct from identical twins, who are natural clones). The process leading to her birth required a mature oocyte (a unfertilised egg) from one female sheep and an ordinary cell from the udder of a second sheep. First the nucleus (the part containing the DNA) was removed from the oocyte. This was done using a special microscope as although oocytes are quite big compared to other cells, they are still too small to see with the naked eye. Then the nucleus from the udder cell was inserted into enucleated oocyte. Thus, Dolly had three “mothers”: the donor of the oocyte, the donor of the udder cell and the sheep that carried the developing embryo to term. No father was involved. Although this process was, and remains, very inefficient it was the first proof that the genes from an adult mammalian cell can be “epigenetically reprogrammed” back to the state of the embryonic stem cells that can develop into any other type of cell. Subsequently the same process has been applied to other species and may have medical uses in generating cells that could repair tissues damaged by injury or disease.
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