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Twelve years ago, the world was startled by the announcement that Dolly, a seven month old lamb that had no father and that was an exact copy of her six-year-old DNA-donor "mother" had been cloned. Quickly, Dolly became a celebrity.
Her woolly face graced the covers of magazines and splashed across the screen of television receivers. No one doubted that Dolly's birth was a technological achievement, but for some the news of her birth evoked visions of Aldous Huxley's Brave New World. Now that a higher-order mammal had been cloned, some worried, how long would it be before a human would be cloned? Or should humans even be cloned?
While governments and churches took up the question of human cloning, cloning experiments continued among the scientific community. Since Dolly, several other mammals have been cloned, including cattle, mice, and rhesus monkeys.
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Technically, the term "clone" means that the process must produce genetically identical offspring. This means that the cloned animal, like Dolly, is an exact copy of the single animal that donated the entirety of its genes. In contrast, the offspring of sexually-initiated reproduction, which so far includes all of us, gains half of its genes from a mother and half from a father.
In humans, this union of the father's and mother's genetic material takes place during fertilization, when a sperm cell combines with an egg cell (or oocyte). To aid fertilization, the egg cell contains proteins (called cytoplasmic proteins) that direct genes in both the egg and sperm cells to begin the process of embryo creation.
In normal fertilization, the egg cell's cytoplasmic proteins turn off adult genes being expressed in the sperm and reprogram other genes into embryonic development. This crucial function of the egg is what enables cloning.
After fertilization, rapid cell division, known as cleavage, takes place. By the end of the third day, sixteen cells are present. In the fourth or fifth day following fertilization, continuing cleavage has caused the egg to develop into a hollow sphere of cells called a blastocyst. On about the sixth day, the blastocyst attaches to the wall of the uterus in a process called implantation.The developing embryo becomes a fetus during about the eighth week, and the pregnancy continues toward birth (Tortura and Grabowski, 199).
Sometimes a fertilized ovum splits in an early stage of development. This produces identical, or monozygotic, twins that are genetically identical to each other, but not to either parent. Each sibling has genes from both father and mother. Because of this, they are genetically identical to each other and are always of the same sex, but, technically, are not clones.
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Preexisting techniques laid much of the theoretical and practical groundwork for cloning. Since 1978, relatively infertile couples have been able to have fertilization occur outside the body in a process called in vitro fertilization (IVF). In IVF, the mother is first given follicle-stimulating hormone so the release of more than one secondary oocyte, which is called superovulation, will occur. A surgeon then makes a small incision near the umbilicus (or belly button), and secondary oocytes are harvested from the stimulated follicles.
The oocytes are placed in a solution containing the male's sperm. When fertilization occurs, the fertilized ova are placed in another dish and monitored for cleavage. Upon reaching eight or sixteen cells, one or more ova are placed in the uterus for implantation.
As in the IVF process, cloning techniques require that egg cells, or oocytes, be obtained from a donor. Unlike in the IVF process, fertilization does not occur in cloning, but a maturing ovum is produced and placed in a surrogate, or host, mother for implantation.
In the absence of fertilization, cloning methods must include a means to introduce and fuse genetic material into the egg cell. This is the core process of today's cloning technologies.
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Next: Cloning Technology