1. Cloning requires an egg cell, and an adult donor cell.

The (unwanted) chromosomes are removed from the egg cell and discarded.

The nucleus, containing the DNA to be cloned, is removed from the donor cell.

2. The donor nucleus is inserted into the empty egg cell, a process called somatic cell nuclear transfer (SCNT).

Afterwards the egg contains a full (adult) set of chromosomes as if it had been fertilised normally.

3. A pulse of electricity, or a chemical 'shock', kick-starts the development process, and the embryo begins to grow.

4. Cell division begins. The subsequent development of the embryo depends upon how successfully the donor nucleus has 're-programmed' the egg.