Three kinds of cloning have been used: molecular cloning for genetic research, in which "scientists must make millions of identical copies of genes of molecular size in order to have sufficient material for testing," cell cloning, which produces cell-lines with the same properties, and nuclear-cell transplantation, which puts "nuclei from the cells of early preimplantation embryos into unfertilized eggs from which the nuclei have been removed" (Cohen and Tomkin 13-14). The first two kinds of cloning are often called research cloning. The third kind is called reproductive cloning. Once the fertilized egg, stimulated by electricity, has divided into eight cells, it is introduced to the uterus so that nidation, or attachment to the uterine wall, can be accomplished. That procedure has been used to reproduce cattle with genetically desirable traits, and it was used to produce the famous sheep Dolly in 1996.
Reproductive cloning is the kind is very controversial. Another kind of research cloning, called therapeutic cloning, is also controversial. All cloning technology anticipates discarding or destroying eggs that do not make it through the process of nidation and gestation. In so-called therapeutic cloning, "development of the organism is typically halted during an early (blastocyst) stage when the original cell has divided into eight cells. Then, stem cells . . . are extracted for use in scientific research" (Johnson 30).
For those who believe that human life begins at fertilization, the destruction of fertilized eggs basically disposes of the issue and makes cloning morally suspect. The US Conference of Catholic Bishops objects to therapeutic cloning because it deliberately cuts off the life of a developing embryo for the purpose of extracting embryonic stem cells for research and, in theory at least, regenerative therapy: "In moral terms, we think that creating human lives solely to destroy them is more problematic than cloning f...