As DNA technology has advanced, it has become possible to clone entire organisms from a single cell. Cloning is of special interest because of its ability to produce an embryo from which to harvest stem cells. A stem cell is a cell that is able to reproduce indefinitely and develop into other types of specialized cells. Much research has been devoted to producing these cells because of their potential uses in treating illnesses (such as heart disease) and regenerating damaged tissues (such as in spinal cord injuries, burns, and arthritis). Human embryonic stem cells are particularly valuable because they are pluripotent, meaning they can give rise to many different cell types. Adult stem cells, on the other hand, are more limited in the cell types that they can become. Currently, the only way to obtain embryonic stem cells is to harvest them from a human embryo. However, this practice raises serious ethical concerns. Alternative methods of gaining human cell pluripotency from adult cells are being researched, with promising results.The process of cloning plants has been used for millennia, after the discovery that many mature plant cells are totipotent, meaning they can give rise to an entire organism from a single cell. For example, spores and zygotes are totipotent cells. This discovery prompted the question of whether the same was true for animal cells. Research found that animal development is more complicated, and other techniques would be necessary to make animal clones. A laboratory technique has allowed an animal to be made that has the exact same genome as its parent. Nuclear transfer is the process of placing the nucleus of a body cell into an enucleated egg cell (an egg cell whose nucleus has been removed) in order to create a clone (duplicate) of an organism. The new egg can divide and produce an embryo. If that embryo is implanted into a surrogate mother, it may develop into an adult that is genetically identical to the body cell donor.
Cloning can be useful for making several copies of an especially hardy farm animal. It is also practical for creating optimal "control" animals for research experiments. Taking cloning a step further, the potential exists for producing a human organ tailored to the specific patient who needs it. Cloning has its disadvantages, too; mammals are extremely difficult and inefficient to clone. Animal clones are often weaker and more susceptible to illness. Many attempts have been made, but the animal cloning technique often produces unsuccessful embryos or animals that become sick and die before reaching adulthood. Interestingly, some clones, such as the first cloned cat, have observable traits that are different from their parents as a result of the effects of the environment interacting with the expression of genes, despite the genetic similarity.