Clone

Clone is a term with multiple usages in the biomedical sciences, referring both to a general concept in molecular genetics and to the application of this concept to whole organisms (particularly, but not exclusively, to higher animals). Generally, a clone is any organism (or group of organisms) that comes into existence with a DNA base sequence identical to that of an already-existing or previously existent organism. Although certain lower worms are naturally capable of forming clones, the use of this term with regard to animals usually refers to efforts by scientists to clone mammals and, more specifically, humans. Although potentially a new means of reproduction, cloning in humans is primarily viewed as a source of embryonic stem cells for research and potential transplantation. As cloning is not a natural occurrence for humans (except for the case of identical twins), scientists are developing specialized techniques to allow the cloning of humans. Although potentially useful in research and/or therapy, the process remains ethically controversial, particularly with religious conservatives.

Because many considerations of the ethical, legal, and social impact of cloning begin with fundamental discussions of how cloning is carried out, and by extension what a clone is and what respect and protections it ought to be afforded, a basic understanding of cloning and its differences from fertilization is crucial to evaluating the competing arguments.

Normal human development begins with fertilization: the fusion of egg and sperm to form a zygote. After four to five days of growth and differentiation, the zygote reaches the blastocyst stage (100 to 200 cells). If this development has taken place in utero, the blastocyst may implant on the uterine wall and, with the right conditions, develop into a newborn in due course. If fertilization is done in vitro (IVF), there is no reason the blastocyst must ever be implanted. Blastocysts have been maintained in media for up to 14 days. During this period, it is possible to extract embryonic stem cells from the blastocyst. Blastocysts have also been maintained indefinitely in cold storage where they might be later used as a source of embryonic stem cells or as part of an IVF procedure leading to pregnancy and, later, birth.

While there are several techniques that a scientist might use to create a clone, the common usage of cloning with respect to animals is a technique known as somatic cell nuclear transfer (SCNT). Cloning of mammalian species via SCNT is a complex, multistep process. First, an egg cell is obtained from a female (not unlike IVF). This cell is then enucleated (the nucleus is removed). Concurrently, a nucleus is removed from a somatic cell of the organism being cloned. This nucleus is placed into the newly enucleated egg cell. This cell is then stimulated to launch a cascade of biochemical events leading to cell division resembling that of a zygote arising from fertilization.

With respect to humans, there are two clear purposes to cloning—reproduction and therapy—giving rise to two labels for a fully defined cloning protocol that specifies the fate of the blastocyst generated by nuclear transfer. Reproductive cloning is cloning carried out with the intent to merge the practice of SCNT with IVF, eventually leading to pregnancy and birth. Therapeutic cloning is the practice of SCNT for the purposes of extracting embryonic stem cells from the resultant blastocyst.

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