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When an egg has been fertilised, and thus becomes a zygote, it undergoes mitosis, dividing to become two cells, then four, then eight etc. In a human embryo, up to the eight-cell stage all the cells are identical and have the potential to become any kind of cell required by the body. These eight cells are known as embryonic stem cells.
Mitosis occurs in the natural process of growth, but many plants and some animals also use mitosis as a method of reproduction known as asexual reproduction.
The strawberry plant sends a runner out from the main stem and, where this touches the ground, new plants develop from buds on the runner. Spider plants regularly send out drooping branches, which, if they touch the ground, will root and produce new plants. The single-celled amoeba is one of a small number of animals that reproduce asexually. The new plants or animals produced are exact copies of the parents and are known as clones.
Since identical twins have both developed from the same fertilised egg, they can be regarded as naturally occurring clones.
Cloning is a natural process, but scientists have also developed methods of producing clones in an artificial way. Many plants are produced by layering, cuttings and by grafting and budding. Plantlets can also be grown from one original plant by tissue culture. Just a few cells of the plant are treated with hormones in a special environment and many thousands of plants can be produced quickly, disease free and all year round.
Cloning of animals has also been done. It is quite common, for example, to use the sperm from a prize bull to artificially inseminate selected cows, which have been given hormones to make them produce lots of eggs. The fertilised eggs develop into embryos each of which can be divided to create several identical embryos. These cloned embryos are then implanted into other cows, or they can be frozen and used later. Cloning was also used in the case of Dolly the sheep, which was the first mammalian clone produced from one adult cell.