ABSTRACT
Cloning is by definition the reproduction of genetically identical organisms. In practice it involves a non-sexual mode of reproduction. Cloning is the natural mode of reproduction in unicellular organisms (bacteria and yeasts). In multicellular organisms, cloning involves a phenomenon of dedifferentiation. The cells of a tissue are mostly specialized in a particular function. To be specialized, they express a specific set of genes. These cells are said to be differentiated. As they become specialized, they lose their capacity to express other functions and they are frozen in their specialization. They are considered incapable of dedifferentiating. Dedifferentiation is thus the capacity of a cell belonging to a given tissue to divide and give rise to daughter cells that can produce all the cellular functions. In plants, cloning can be obtained by layering, cutting, or in vitro dedifferentiation of somatic cells. The cells thus dedifferentiated have the characteristics of an embryo and each can in turn result in a normal plant. No such process has ever been observed in animals.
