Cloning may invoke an image of an army of identical cows or sheep churned out factory-style, but in actuality, the process is much more laborious.
The term "cloning" generally applies to a process more technically known as somatic cell nuclear transfer. What that means is that the DNA from the cell of an adult animal (take cows, for example), called the "donor," is extracted from the cell (usually a skin cell taken in a biopsy) and inserted into an egg cell from another cow. The egg cell has had its nucleus removed so that it will read and duplicate the DNA of the donor cell.
The newly created embryo is then zapped with electricity so that it starts multiplying, until it becomes a blastocyst (a small clump of cells that forms after an egg is fertilized), which is then implanted into a surrogate mother. The resulting newborn will be an identical genetic replica to the donor cow.
Cows have been cloned more than other animals because obtaining eggs from the cow is slightly easier than for swine, said geneticist Bill Muir of Purdue University, an author of a 2002 National Academy of Sciences report on the scientific concerns of animal biotechnology.
This process differs from other methods of artificial breeding, such as in vitro fertilization, in that it uses adult cells, instead of embryos.