Reproductive cloning is producing an animal with genetic material in a lab. The most common method of reproductive cloning is somatic cell nuclear transfer, which involves taking an egg from an animal and removing the genetic material. Scientists take a cell from the animal to be cloned and remove the cell's nucleus. The nucleus (which contains the DNA) is inserted into a donor egg. The egg then receives an electric charge, which sort of glues everything together and promotes reproduction. After the egg has reproduced enough to be considered an embryo, it's implanted in a female. Hopefully, the female carries the embryo to term and gives birth to the cloned animal. Although some people worry about mass cloning of various species, the process is more difficult than it sounds. After more than a decade of attempting to clone various animals, scientists now admit that cloning as a method of reproduction is highly impractical [source: Kiem].
Despite significant progress, for example, scientists have had trouble cloning monkeys. Monkeys have a regular reproductive cycle, so researchers can easily predict when to extract eggs. That's the easy part, however. Scientists faced several challenges after extracting a viable egg, including growing a viable embryo without losing chromosomes and getting a female monkey to carry the embryo to term [source: Graham].
Likewise, cloning dogs has proven surprisingly difficult. Dogs don't ovulate on a regular, predictable cycle, which makes egg extraction difficult. When dogs do ovulate, the eggs are only viable for a few hours, meaning scientists have to extract the eggs fairly quickly [source: Kiem]. Scientists have to remove the genetic material from the extracted egg and insert a nucleus from a cell taken from the animal being cloned. Dogs' eggs are coated with fat, making it fairly difficult to remove the genetic material. Some dogs have been cloned, but again, it's not easy.
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