Cloning is the production of one or more individual plants or animals that are genetically identical to another plant or animal. Nature itself is the greatest cloning agent. In about one of every 75 human conceptions, the fertilized ovum splits for some unknown reason and produces monozygotic (identical) twins. Each has a genetic makeup identical to the other.
Two very different procedures have been referred to as “cloning”:
· Embryo cloning has been successfully carried out for years on many species of animals. Some limited experimentation has been done on human embryos.
· Adult DNA cloning has been used to clone a sheep, but was not tried on humans until December 1998.
Human embryo cloning starts with a standard in vitro fertilisation procedure. Sperm and an egg cell are mixed together on a glass dish. After conception, the zygote (fertilised egg) is allowed to develop into a blastula (a hollow mass of cells). The zygote dividesfirst into two cells, then four, then eight… A chemical is added to the dish to remove the “zona pellucida” covering. This material provides nutrients to the cells to promote cell division. With the covering removed, the blastula is divided into individual cells, which are deposited on individual dishes. They are then coated with an artificial zona pellucida and allowed to divide and develop. The experiment by Sillman showed that the best results could be obtained by interrupting the zygote at the two-cell stage. Many of these pairs of zygotes were able to develop to the 32-cell stage, but no further. They might well have had the potential to develop further and even mature into a viable foetus, except that the original ovum was defective and would have died anyway. For ethical reasons, the researchers selected embryos which had no possibility of ever maturing into foetuses, and thus into newborn babies.

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