Oh, pick me. I wrote a paper on this for my genetics class.
Therapeutic cloning is used to create stem cells genetically identical to a particular person. The stem cells are then used to grow tissues or repair organs. Because the cells have the same DNA, rejection of the cells is very unlikely (the only foreign matter are the cell-surface proteins from the donor egg).
Therapeutic cloning as currently practiced requires the harvesting of embryos. A donor egg is collected from a rabbit or cow and its nucleus removed or inactivated. The nucleus from one of the patient’s cells is then inserted into the egg cell and prodded into multiplying.
The various chemicals in the egg cell cause the new nucleus to develop as an embryo rather than the type of cell it came from (such as a skin cell). When the embryo has developed into a blastocyst, the stem cells are harvested from it.
Because an embryo is created, a life is created. It is therefore immoral to harvest it for cells.