Cloning is based on nuclear transfer, the same technique scientist have used for some years to copy animals from embryonic cells. Nuclear transfer involves the use of two cells.  The recipient cell is normally an unfertilized egg taken from an animal soon after ovulation.  These eggs begin developing once they are stimulated. The donor cell is the one to be copied. A researcher working under a high-power microscope holds the recipient egg cell by suction of the end of a fine pipette and uses an extremely fine micropippette to suck out the chromosomes, sausage shaped bodies that incorporate the cell's DNA.  The chromosomes are not enclosed in a distinct nucleus.  Then typically the donor cell, complete with its nucleus, is fused with the recipient egg.  Some fused start to develop like a normal embryo and produce offspring if implanted into the uterus of a surrogate mother.

     After the birth of Megan and Morgan showed that they could produce viable offsprings from embryo derived cultures, they filed for patents and started experiments to see whether offspring could be produced from more completely differentiated cultured cells.  They selected a pregnant adult because mammary cells grow vigorously at this stage of pregnancy, indicating that they might do well in culture.  They have stable chromosomes, suggesting that they retain all their genetic information.  The successful cloning of Dolly from the mammary derived culture and of other lambs from the cultured fibroblasts showed that Roslin protocol was robust and repeatable.  All the cloned offspring in their experiments looked, as expected, like the breed of sheep that donated the originating nucleus, rather than like their of surrogate mother or egg donor cells. Genetic tests proved that Dolly is indeed a clone of an adult.

    Although cloning by nuclear transfer is repeatable, it has limitations.  Some cloned cattle and sheep are unusually large, but this effect has also seen when embryo are simply cultured before gestation.  Perhaps the most important nuclear transfer is not yet efficient.  All the cloning studies found so far show a consistent pattern of deaths during embryonic and fetal development, with laboratories reporting only 1 to 2 percent of embryos surviving to become offspring.