Читать книгу The SAGE Encyclopedia of Stem Cell Research - Группа авторов - Страница 97

In biology, the term cloning refers to the process of producing genetically identical individuals. Cloning is the practice of separating a group of cells or cell derivates so that each produces only an identical copy of its type. This process occurs naturally in organisms that reproduce asexually, such as plants, insects, and bacteria. The practice of cloning has long historical roots, as it has been a common part of the human experience for thousands of years; for example, growing a plant from a cutting is an early type of cloning. The more recent history of experimental cloning of animals that do not normally reproduce asexually dates back to the early 1900s, when scientist Hans Spemann split an early salamander embryo into two parts, which then developed into two full and distinct organisms. This proved that embryonic cells carry all the genetic information required to develop into a new organism. However, cloning of advanced animals had not been considered possible until the successful cloning of the first mammal, a sheep called Dolly, in 1997. The cloning of Dolly was a momentous scientific and technological development; it also opened the door to the possibility of human cloning, and with it, myriad related medical issues and ethical concerns. Many other mammals, from rodents to cows, have since been successfully cloned.

During the 1970s, cells of two separate embryos of different strains of mice were joined at an early stage of development, in order to produce a single chimeric embryo that later developed into adult mice, which exhibited characteristics of both strains. Progress in the experimental manipulation of mammalian embryos has led to the development of cutting-edge techniques for the production of genetically modified animals. Injection of cloned DNA into the pronuclei of fertilized one-cell eggs allows the production of a transgenic animal with a foreign gene that has been deliberately inserted into its genome. The foreign gene is generally constructed using recombinant DNA technologies. In addition to a structural gene that will code for an RNA or protein product, the recombinant DNA usually includes other sequences in order to be expressed correctly in the host cells. Other important techniques were developed later, such as retrovirus-mediated transgenesis and embryonic stem cell–mediated gene transfer, which is based on the creation of chimeric embryos.

The technology to produce transgenic and cloned animals often have overlapping goals but they produce different types of animals. Transgenic commonly refers to an animal or plant that contains one or more genes from another organism, incorporated into its own genome. It is capable of passing on the foreign genes to its offspring. A cloned animal, however, is an organism that is genetically identical to its progenitor. Cloning remains a topic of deeply complex scientific and ethical issues. The potential for cloning to cure disease, to increase and better the food supply worldwide, and to save and recover endangered species are at the forefront of issues hotly debated today. Animal cloning has application to the medical and pharmaceutical industries, to the food industries and lately, to the pet cloning industry too. The topic of cloning today continues to have profound scientific, ethical, economic, political, and religious repercussions.