cloning
Cloning is the generation of genetically identical organisms: each group of such organisms is a clone. Ever since Aldous Huxley's Brave New World, cloning and clones have been the subject both of science fiction and of serious public concern over their possible biotechnological applications. Before taking a paranoid view, however, it is worth noting that clones occur widely and naturally. Many plant varieties are propagated as clones (for instance by grafting) and the summer aphids preying upon them are asexually produced, genetically identical individuals — clones. Identical twins are clones, and the famous Dionne quintuplets born in Canada in 1934 represent a human clone of five people.
Sexual reproduction involves a re-assortment of the genetic material from the two parents and hence the generation of new, genetically distinct individuals. In contrast to this, methods of asexual reproduction result in the production of genetically identical individuals. Bacteria, yeast, and the individual cells of multicellular organisms are able to reproduce asexually, and the products of such replication are clones. Thus, for instance, all the cells in a multicellular organism represent one clone derived from the fertilized egg. During the process of development, and indeed at later stages of life, there may be stably inherited restrictions on the use of the genetic material or new mutations which define new clonally-related groups of cells.
The cells of malignant tumours, for instance, usually carry numbers of mutations which were not originally present in the normal cells of the individual; as these cancer cells progress newer mutations may arise so that several discernibly different clones of cells may be found. One question of interest would be whether all the cells arise from one single event — is the tumour a clone? This question may be addressed in individuals where there is already more than one distinguishable clone of cells present. In women, one of the two X chromosomes will have been inactivated early in development in a random but stable manner. This results in all the tissues being a mosaic of two alternative types of cell. Tumours typically display a single type, demonstrating their clonal origin from a single precursor cell.
This illustrates another important aspect of cloning: the origin of the clone purifies it from a mixed population. For example, many cultivated plants are deliberately propagated asexually by cuttings or grafting, so that one particular variety may be maintained. In molecular biology, this property — that the isolation of a clone selects, maintains, and propagates as a single pure variant — is used directly for analysis of the genetic material itself; the DNA. Pieces of DNA are inserted into a bacterial or viral host in a form that replicates asexually. One single cell is used to start a colony — a clone — and thus large amounts of a single purified DNA fragment may be isolated.
All the cells of a multicellular organism arising from one fertilized egg are clones and, unless subsequently modified, contain the same genetic information. This was demonstrated in plants by regeneration of a whole plant from a single cell from a carrot root. In animals it was shown possible to transplant the nucleus from a gut cell of a tadpole into a fertilized egg, which had had its own nucleus destroyed, and regenerate a new tadpole which now had the genetics of the donor nucleus. Such cloning was first attempted for mammals using mice, but this did not work with any nuclei other than those from the earliest embryos. In the 1990s, however, Ian Wilmut and a team at the Roslin Research Institute in Edinburgh demonstrated a technique allowing nuclei from cells in tissue culture to be used to clone a sheep. They have now demonstrated that these tissue culture cells can be derived from an adult sheep.
The lamb (named Dolly), who was produced from a nucleus from a cell grown from the breast tissue of an adult sheep, has had major political impact as it is now clear that there is no theoretical reason why this cloning should not be possible not only with sheep but with other mammals, including humans. Cloning people is illegal in Britain, but world-wide legislation is not in place. In some quarters it is argued, however, that the technique per se might be useful to regenerate transplant tissues or organs without ever compromising the ethical, legal, and moral susceptibilities that would arise from deliberately generating whole fetuses or people.
— Martin Evans
See also biotechnology; stem cells.
