Human Embryo Cloning
Embryo cloning, otherwise known as "artificial twinning", works by mimicking the natural processes that causes twins to develop.
This technique of cloning involves fertilizing an egg with a sperm. The fertilized egg which is called a zygote, develops into a blastula which is a hollow mass of cells. The blastula further divides itself into more cells, with the number of cells doubling with each division.
A chemical is then added to remove the "zona pellucida" covering and provides nutrients to the cells to promote cell division. The blastula divides into individual cells as the covering is removed. Each cell is deposited on a dish and coated with an artificial zona pellucida and allowed to divide and develop. This encourages the cell to develop into a separate embryo with the same DNA as the original.
The ultimate result of the process described above is the formation of monozygotic twins, more commonly known as identical twins.
Reproductive Cloning
In adult DNA cloning, DNA is removed from the female's ovum and replaced with DNA from an adult animal. After which, the fertilized ovum, also known as a "pre-embryo", is implanted in the womb and allowed to develop into a new animal.
The most famous example of this cloning technique is Dolly the sheep. Dolly was cloned by fusing the DNA in the cell of a mature 6-year-old sheep with the ovum of a ewe (female sheep) that had its nucleus removed. An electric pulse was used to stimulate the cell. Out of 227 attempts at cell fusion, only 29 began to divide. These were all implanted in ewes. Thirteen of the ewes became pregnant, but only one lamb, Dolly, was born.
At this stage in time however, reproductive cloning for humans is still considered to be too dangerous and far too controversial to be carried out.
Therapeutical Cloning
Therapeutic cloning is also known as "biomedical cloning" or "research cloning".
The initial stages of this procedure are identical to that of adult DNA cloning.
First, the nucleus of a human ovum is removed. This is followed by the injection of the nucleus of a human cell. This process is collectively known as "somatic cell nuclear transfer".
Stems cells are then removed from the pre-embryo to be developed into organs that are ultimately implanted in the human DNA donor. The pre-embryo dies in the process.
Although therapeutic cloning and adult cloning are very similar, therapeutic cloning only aims to create healthy "replacement" organs for a patient. It does not attempt to create a newborn