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In DNA Cloning (a.k.a. "recombinant DNA technology," "molecular cloning,"or "gene cloning") only the DNA of a cell is replicated. A DNA from an organism is transferred to a self-replicating genetic element such as a bacterial plasmid. In other words, a small piece of the DNA strand is removed and united with a plasmid which reproduces itself to create multiple copies of the same DNA code. This plasmid is also known as a vector. This copied DNA can then be propagated in a foreign host cell. After it is introduced into a suitable host cell, the recombinant vector can then be reproduced along with the host cell DNA

Reproductive cloning is a technology used to generate an animal that has the same nuclear DNA as another currently or previously existing animal. Human cloning also falls into this category. Dolly was created by reproductive cloning technology. In a process called "somatic cell nuclear transfer" (SCNT), the DNA information from the nucleus of a donor adult cell is copied into a cell whose nucleus (thus also its genetic material) has been removed. Chemicals or electric current are used to stimulate cell division. Once the cells start dividing and the embryo reaches a suitable stage, it is planted into the uterus of a female host where it develops until birth.

There is one important fact to know about reproductive cloning. Even though the DNA from the nucleus in the cloned cells is identical to the original cells, the whole animal (or human) is not identical. This is because some DNA is stored in the mitochondria of the cell which is not cloned and is thus unique. The DNA in mitochondria is believed to play an important role in the aging process.

Therapeutic cloning is like reproductive cloning, except that the embryos are not allowed to develop fully. The purpose of therapeutic cloning is to extract the stem cells from the embryos and study them. When the egg has been cloned and divided for 5 days, the stem cells are extracted from it. The embryos are destroyed due to the extraction process, which raises ethical concerns.

But why the need for stem cells? The answer lies in the composition of these cells. Stem cells are unspecialized cells which can transform into any of the 220 cell types that are in the human body. Many researchers hope that one day stem cells can be used to serve as replacement cells to treat heart disease, Alzheimer's, cancer, and various other diseases.

In November 2001, scientists from Advanced Cell Technologies (ACT), a biotechnology company in Massachusetts, announced that they had cloned the first human embryos for the purpose of advancing therapeutic research. They had taken cells from a woman's ovaries and removed their nucleus. Skin cells were then inserted into the ovary cells to serve as the new nucleus. The cells began dividing, but the results were limited in success. The process was carried out with eight eggs and only one of them successfully divided into six cells before stopping.