There are 3 main methods of cloning, which are briefly outlined here in three different sections. It is recommended that you read from top to bottom, because certain definitions are explained in the first section.
Process of Nuclear Transfer
Nuclear transfer is the process of creating an embryo by fusing an adult animal cell, in Dolly's case, an udder cell from a sheep, into an enucleated egg cell.
are still unsure of why an electric pulse sent to the two cells causes
them to meld together and activates development in the egg, even though
they have been carrying out this step for years in the process of cloning.
However, they have learnt that the shock doesn't fully mimic the activation
process of a sperm, which could explain why very few embryos survive beyond
the first few days of formation.
The Process of Nuclear Transfer (Microinjection)
Microinjection can be said to be a subset of nuclear transfer (fusion). While nuclear transfer (fusion) is fusing an entire adult animal cell into an egg cell whose nucleus has been removed, the microinjection technique uses only the adult cell nucleus.
Artificial twinning, is the process of splitting an embryo into two or more embryos.
* An interesting point to note is that no one really knows why an embryo splits in the process of natural twinning, from which natural identical twins are formed.
So far, only animal cloning has been done, and success rates are far from high. Human cloning would probably be more difficult than sheep or cattle cloning, because the cells of human embryos start producing proteins at a relatively early stage. Thus, there would not be as much time for the egg cytoplasm to reprogram a transplanted nucleus. However, the successful 1998 cloning of mice, which also start producing proteins at an early embryonic stage, strongly indicated that this problem can be overcome in humans.