Cloning refers to the ability to make a genetic replica of a cell (or even an entire organism, also called reproductive cloning). Interest in cloning is due primarily to its potential to create stem cells. A stem cell is a cell that has not yet differentiated and retains the ability to turn into many different tissues. Stem cells have enormous potential in medical applications because they can be used to replace diseased or damaged tissues and aid in organ repair or replacement.
In order for reproductive cloning to work, a differentiated cell must “dedifferentiate,” allowing it to access the genes for any given cell type. This cell can then give rise to all the specialized cell types of an organism. One source of undifferentiated stem cells is from early-stage embryos. Considering the ethical implications of embryonic stem (ES) cells, research has begun to focus on adult stem cells. Adult stem cells are found in a number of tissues, including dental pulp, bone marrow, and the brain. These cells, however, are not able to differentiate into all cell types and thus are not as beneficial as embryonic stem cells.
In 2007, researchers announced their successful reprogramming of fully differentiated adult cells, thus inducing an ES-cell state. These induced pluripotent stem (iPS) cells can do everything an ES cell can do without the ethical implications associated with embryonic cells.