Cells are the fundamental structural and functional units within living organisms. All living organisms consist of one or more cells. With the exception of bacteria, all organisms are made of eukaryotic cells, which have a membrane-enclosed nucleus and organelles (e.g., mitochondria, endoplasmic reticulum, and ribosomes). The nucleus within each cell contains the hereditary information for the entire organism, encoded within DNA.

In multi-cellular organisms, cells differentiate and specialize. Specialized cells organize into tissues (e.g., muscle, blood, bone, fat, nerve), which make up organs (e.g., kidneys, heart, stomach, lung), which, in turn, comprise organ systems (e.g., respiratory, digestive, excretory). Genes that do not pertain to the functioning of each individual cell become inactive, or "turn off." For example, a kidney cell uses only the DNA needed to be a kidney cell. The remaining information is "turned off," but it is still present. There are more than 200 different types of cells (nerve cells, muscle cells, epithelial cells, blood cells, bone cells, etc.) among the human body's estimated 100,000,000,000,000 total cells.

In the simplest terms, cloning is the creation of a genetically identical copy of an original organism. Plants are relatively easy to clone. Most people are familiar with the use of cuttings or stem segments (such as the "eyes" of potatoes) to create new plants. Technically, the new plants are clones of the original, because they are genetically identical to the parent plant. In the wild, many different kinds of plants and animals use forms of reproduction that copy an exact genotype. This type of reproduction (which is seen in grasses, strawberry plants, sponges and flatworms, for example) often is referred to as asexual reproduction. Even fraternal twins can be thought of as clones, because they have identical sets of DNA.

Molecular biologists use the term "cloning" to refer to a variety of processes that involve making identical copies of part or all of a DNA molecule, a single cell type, or an entire organism. DNA cloning technology, also referred to as molecular cloning, recombinant DNA, or gene cloning, is a common practice in molecular laboratories today. A DNA fragment from one organism is introduced into a self-replicating element (host) such as a bacterial plasmid. Molecular biologists use DNA cloning to create many identical copies of a DNA molecule or to isolate a particular stretch DNA (which involves making identical copies of the DNA of interest).

In 1997, scientists used a somatic cell (a cell that is not an egg or sperm cell) from an adult sheep to produce a reproductive clone via a process called somatic cell nuclear transfer (SCNT). With this technique, scientists transferred the nucleus from a somatic cell of an adult sheep into an egg from which the nucleus had been removed. This type of cloning, called reproductive cloning, still is very rare and difficult to achieve for vertebrate animals.

SCNT also is used in therapeutic cloning to produce many copies of stem cells. Stem cells are undifferentiated cells that can be used as replacement cells to treat a variety of diseases and disorders. The purpose of this type of cloning is not to produce another organism, but to generate copies of cells in sufficient quantities for research and medical treatments.

To clone DNA, scientists use restriction enzymes to cut out the specific DNA segment to be replicated (copied). The segment then is inserted into a bacterial plasmid for replication. Bacterial plasmids are circular DNA molecules distinct from the normal bacterial genome and are capable of replicating separately. Once inserted, the recombinant DNA is replicated, along with the host cell's DNA. Plasmids can carry up to 20,000 base pairs of foreign DNA.

Human insulin often is produced by recombinant DNA technology. The human insulin gene is inserted into a bacterial plasmid and can be induced to produce vast quantities of insulin for the treatment of diabetes. Other specific applications of recombinant DNA technology include the production of human growth hormone, erythropoietin for kidney dialysis patients, clotting factor for hemophiliacs, and hepatitis B vaccine. Although viruses, bacterial artificial chromosomes (BACs), and yeast artificial chromosomes (YACs) also may be used for replicating DNA, bacterial plasmids are most commonly used in this technology.

The concept of vertebrate animal cloning was introduced seriously in 1938. Early stages of cloning research incorporated a process called "twinning." Twinning takes a fertilized egg (after a sperm has naturally fertilized the egg) and waits until it divides into two identical cells. These cells then are separated and each is implanted in a mother. The resulting offspring are genetic twins or identical clones.

Early attempts at animal cloning always used embryonic cells. Remember, as an organism develops, cells differentiate and specialize (nerve, kidney, etc.). Once cells specialize, some of their DNA  "turns off" and becomes inaccessible. Using an embryonic cell for cloning bypasses this hurdle, since all of the DNA still is accessible.
 
In the 1980s, researchers selected embryos at early stages of development and used individual cells to create clones. For example, scientists might have taken a four-cell embryo, broken it up and isolated each of the four cells. Each of these cells then could produce a whole organism on its own, and each new organism would be identical to (or a clone of) the others. However, it was not until 1997 that researchers were able to take a somatic cell from an adult organism and use it to produce a cloned embryo that developed into an adult organism, genetically identical to the donor organism.