Understanding human sexual reproduction can aid in understanding human asexual reproduction, especially in terms of the natural biological processes of methylation and regulation (Irving 2003a, pp. 1-42).
Following fertilization (sexual reproduction) the early human embryo grows and develops by means of multiplying its cells, and by means of various biochemical processes, including methylating and demethylating the DNA in each of those cells-part of the critical natural process called regulation. That is, the DNA in each cell of the organism is "allowed to speak" or is "silenced" by adding or removing these methylation bars-depending on what products, tissues, or organs the embryo or fetus needs to grow and develop at any point in time. These products then cascade throughout growth and development. The more specialized (or differentiated) a cell, the more methylated or silenced its DNA becomes. This is one way that the programming of the DNA of a cell is naturally accomplished. By adulthood the DNA in many of the cells of the human being have been almost completely silenced by the insertion of methylation bars-such as in human skin cells (Carlson 1999, pp. 49; Lewin 2000, pp. 678, 914, 950; Strachan and Read 1999, pp. 193ff; O'Rahilly and Müller 2001, p. 39).
In human asexual reproduction many of these processes operate in reverse to reprogram, or de-differentiate, the DNA in a cell. For example, in cloning by somatic cell nuclear transfer, one can begin with a highly specialized or differentiated human cell (such as a skin cell - in which some or even most of the DNA in that cell's nucleus has been silenced) and then incrementally remove the methylation bars on that DNA to allow it to speak until the DNA in that cell is in the same state of differentiation as the single-cell totipotent zygote - resulting in a new, single-cell human organism, a single-cell embryo, or human being (Commonwealth of Australia 2001; Brinsden 1999, p. 421; Campbell et al. 1997, pp. 18-19; Council of Europe 1998, p. 2; Geraedts et al. 2001, pp. 145-150; German National Ethics Council 2004, pp. 18, 19, 41; Irving 2005, pp. 1-36; National Institutes of Health 1998a; National Institutes of Health 1998b, p. A-3; Robertson 1994, p. 6; Strachen and Read 1999, pp. 508-509). That is, one begins with just a human cell, but ends up with a new single-cell organism, a human being.
This is roughly what happened with the reproduction of Dolly the sheep (Wilmut 1997, pp 810-813; Wilmut 1988, p. 138). Quoting human molecular geneticists Strachan and Read: "For the first time an adult nucleus had been reprogrammed to be totipotent once more, just like the genetic material in the fertilized oocyte from which the donor cell had ultimately developed" (Strachan and Read 1999, pp. 508-509). And as documented above, a fertilized oocyte is a single-cell human being - a human embryo - a single-cell human organism at Stage One of embryonic development.
Despite certain claims to the contrary (NAS 2002a, b; Weissman 2003; West 2001 and 2007), the technique referred to as nuclear transfer just described is always cloning-regardless of why it is performed, (Commonwealth of Australia 1986; Commonwealth of Australia 2001; Brinsden 1999, p. 421; Campbell et al. 1997, pp. 18-19; Council of Europe 1998, p. 2; Geraedts et al. 2001, pp. 145-150; Irving 2005; National Institutes of Health 1998a; National Institutes of Health 1998b, p. A-3; Robertson 1994, p. 6; Strachen and Read 1999, pp. 508-509). Furthermore, the cloned human embryo reproduced would not be "virtually genetically identical to the donor cell" because it would have a different genome due to the presence of foreign DNA from the extra-nuclear mitochondrial chromosomes left over from the enucleated oocyte used and due as well to the lack of original mitochondrial chromosomes from the donor cell used (Council of Research, Technology, and Innovation 1997, p. 5; German National Ethics Council 2004, p. 18; Irving 2001c, pp. 1-17; Irving 2006b, pp. 1-3; President's Council on Bioethics 2002; Strachan and Read 1999, pp. 508-509). Further, in normal cells there is critical intercommunication betwee the nuclear DNA and the mitochondrial DNA; such critical intercommunication would be altered in cells of the cloned embryo. Thus using stem cells from such cloned human embryos in therapies could cause serious immune rejection reactions, even in human donor recipients, as acknowledged by anti-cloning and pro-cloning policy makers alike (Kass 2001; Brownback 2003; Weldon 2005; Wilmut et al. 1997, pp. 810-813; Wilmut 1988; Silver 1997, p. 107; National Bioethics Advisory Commission 1997, p. 3; National Institutes of Health 1998, p. 3).
In addition to cloning by means of nuclear transfer, one may also clone by means of twinning (blastomere separation, blastocyst splitting, embryo splitting, embryo multiplication), for example, as happens in naturally occurring monozygotic identical twinning in vivo and artificially in vitro. Twinning can also take place with the separated cells of the early embryo (through the blastocyst stages) because such cells still exhibit a range of totipotency (American Medical Association 1994, pp. 1-9; American Society for Reproductive Medicine 2004, pp. S256-257; Carlson 1999, pp. 43-45, 73; Denker 2008, pp. 1656-1657; Illmensee et al. 2006 a, pp. 1112-1120; Institute of Medicine and National Research Council 1989, pp. 25, 102ff; Lewin 2000, p. 605; A. Liu 2005, pp. 369-378; O'Rahilly and Müller, pp. 23, 24, 37, 39, 136-137, 139; Schieve et al. 2004, pp. 1154-1163; Strachan and Read 1999, pp. 508-509). Twinning is also a common, and the most exact, form of cloning because the chromosomal DNA in the mitochondria in cells of twins are the same (Commonwealth of Australia 2001; Brinsden 1999, p. 421; Campbell et al. 1997, pp. 18-19; Council of Europe 1998, p. 2; Council of Research, Technology, and Innovation 1997, p. 5; Geraedts et al. 2001, pp. 145-150; German National Ethics Council 2004, p. 18; Irving 2006b, pp. 1-3; National Institutes of Health 1998a; National Institutes of Health 1998b, p. A-3; Robertson 1994, p. 6; Strachen and Read 1999, pp. 508-509).
Regulation not only helps one understand asexual reproduction; it is also involved in repairing an injury that has occurred to an organism, sometimes even repairing early genetic mutations. Regulation is the ability to heal a normal structure if parts have been removed or added (Carlson 1999, pp. 44-49). Thus, if successful, regulation could heal a damaged embryo whose totipotent cells have separated from it as well as revert the separated totipotent embryonic cells back to new human embryos, that is, new living human beings.
The question can arise as to when each of the twins begins to exist as an individual. Considering twinning from the standpoint of regulation, a human embryo is normally first produced sexually via fertilization (in vivo or in vitro). Scientifically it is known that this embryo has already been determined as a unique individual - both genetically and developmentally. He or she is a new human being. The developing embryo is also composed initially of totipotent cells and grows developmentally in total continuity with itself. If these totipotent cells of the embryo are damaged, the embryo could die, or regulation could set in to heal the damaged embryo and restore it to wholeness. However, if these totipotent cells of the embryo are actually separated from the intact embryo, then these separated cells too could die, or regulation could set in and possibly revert these totipotent cells to new human embryos. Thus the first twin would be the original human embryo produced sexually (in vivo or in vitro) and would begin to exist as an individual at the beginning of fertilization (penetration of the oocyte by the sperm). As the embryo begins to develop it could be healed (if damaged) by regulation, which reverses the degree of methylation of the DNA in each remaining cell of the damaged embryo back to that required by the number of cells that remain in the intact embryo. The second twin is the new human embryo produced asexually (in vivo or in vitro) from totipotent cells that have separated from the original embryo, and this twin begins to exist as an individual when regulation is successfully completed and the DNA of the separated cells has been reprogrammed back to that of a new single- or multiple-cell embryo (Irving 2003a, pp. 1-42).
The same considerations of regulation can be applied to questions about the fusion of two early human embryos to form a single human/human chimera. In this case the original sources of the cells, or sometimes pronuclei, are from two or more human embryos. If two human embryos fuse to make one organism, that organism is not a human being. It would have 92 nuclear chromosomes per cell. Both original embryos have died. If this chimeric organism undergoes regulation, ejects all excess chromosomes, and reduces the number and proper mixture (male and female) of nuclear chromosomes to 46, then it could theoretically result in the formation of a new human embryo. But that embryo would not be the same individual as either of the original embryos that fused. Or one can form a new human-human chimeric embryo using the male and female pronuclei from different already - existing human embryos reproduced either sexually or asexually. The same basic mechanisms would operate in the formation of human/non-human chimeras, e.g., human/mouse (Chang et al. 2004, pp. 960-962; Hutton 2007; Illmensee et al. 2006b, pp. 1248-1260; Weissman et al. 1988, pp. 1632-1639; Weissman 2003; Weissman 2005).
There are many different kinds techniques that can be used to clone human beings: twinning (blastomere separation and blastocyst/embryo splitting, embryo multiplication); somatic cell nuclear transfer, or SCNT; germ line cell nuclear transfer, or GLCNT; "hemi-cloning;" pronuclei transfer; parthenogenesis; mitochondria transfer; the use of artificially constructed sperm, oocytes, and embryos; nano-cloning; and many other reproductive genetic engineering techniques. Many of these cloning techniques are being considered or have already been used in IVF as infertility treatments (American Fertility Society 1986, p. 27S; American Medical Association 1994, pp. 1-9; American Society of Reproductive Medicine 2000, pp. 873-876; American Society for Reproductive Medicine 2004, pp. S256-257; Commonwealth of Australia 1986; Commonwealth of Australia 2001; Barr 2003; Escriba et al. pp. 149-161; German National Ethics Council 2004, pp. 18, 19, 41; Gordon and Ruddle 1981, pp. 1244-1246; Hao et al. 2006, p. S513; Parens and Knowles 2003; Illmensee et al. 2006a, pp. 1112-1120 ; Illmensee et al. 2006b, pp. 1248-1260; Institute of Medicine and National Research Council 1989, pp. 25, 102ff; Irving 2004a, pp. 1-31; Irving 2008b, pp. 1-10; Katagiri et al. 2004, p. S10; Yoko Kato et al. 1999, p. 1823; A. Liu 2005, pp. 369-378; H.C.C. Liu et al. 2004, p. S308; H. Liu et al. 2005a, p. S368; H. Liu et al. 2005b, p. S370; National Science Foundation and U.S. Dept. of Commerce 2002; Neri et al. 2004, p. S281; Neri et al. 2005a, pp. S400-S401; Neri et al. 2005b, p. S384; New Zealand 2003; Schieve et al. 2004, pp. 1154-1163; Tesarik et al. 2003, pp. 677-681; The Twins Foundation 1994; Valiotis et al. 1993, p. 48; Wolfson 2003, pp. 376-396).
In sum, the immediate product of both sexual and asexual human reproduction is a new, living, genetically unique, single-cell human being, human embryo, human organism, human individual, who immediately directs his/her own specifically human functions, activities, and development. Thus, there is a great deal at stake in debates involving the human embryo; for example, abortion, the use of abortifacients (drugs and devices that kill the new embryo before implantation), IVF and other artificial technologies (ARTs), prenatal genetic diagnosis, human embryo research, human cloning, human embryonic stem cell research, human genetic engineering, and drug and biological/chemical testing and development. Further, arguments for "delayed personhood" have been reversed and then transferred to end of life issues, such as euthanasia, physician-assisted suicide, organ transplantation, withholding/withdrawing food and hydration (Irving 1995). Consequently, the Church has addressed these related issues quite seriously in her teachings.