Introduction to Cloning
Cloning, any of various techniques used to reproduce genetically identical organisms from an individual organism; the organisms so produced are called clones. The term "cloning" also refers to the technique used in genetic engineering to produce identical segments of DNA (deoxyribonucleic acid), and to the asexual reproduction of organisms in nature.
Cloning is used commercially to reproduce individual plants and animals that have desirable traits. In horticulture, cloning makes it possible to produce a large number of plants with specific characteristics, typically in much less time than it takes to grow the plants from seed. In animal husbandry, cloning makes it possible to produce a group of farm animals with valuable traits in less time and with more certainty than with conventional breeding. Cloning is also used to produce laboratory animals with specific traits.
Cloning of Plants
Plants can be cloned using such techniques as cutting and tissue culture. Cutting is a simple technique, but it is not effective with all varieties of plants. Many plants that cannot be readily cloned by cutting (such as orchids) can be cloned through tissue culture.
CuttingIn this technique a stem, root, or leaf is cut from a plant and placed in soil, sand, or water, where it develops into a new plant.
Tissue CultureIn tissue culture a small piece of tissue from a mature plant is removed and placed in a laboratory culture dish containing a growth medium. The piece of tissue is left in the culture dish until it has grown large enough to be transplanted into soil.
Cloning of Animals
Animals can be cloned using either embryonic or adult cells. Cloning from embryonic cells involves simpler procedures and yields a higher percentage of surviving offspring, but cloning from adult cells allows the desirable traits to be more easily identified and chosen.
Embryonic CellsIn the most common technique for cloning from embryonic cells, the nucleus of a body cell is extracted from a very young embryo. The nucleus is inserted into a newly fertilized egg cell whose own nucleus has been removed. The egg is then placed in a culture dish until it develops into an embryo. The embryo is inserted into the womb of a female of the same species. This surrogate mother eventually bears the offspring developed from the embryo. It is possible to produce a large group of identical offspring by using this technique with a number of cells from the same embryo and using a number of surrogate mothers.
In another technique, known as embryo splitting, a very young embryo is split into two or more parts. The parts are placed in culture dishes and allowed to develop into individual, identical embryos. These embryos are then placed into a surrogate mother's womb to continue developing. Although this technique produces genetically identical offspring, some scientists do not consider it to be true cloning.
Adult CellsIn the first technique developed for cloning adult cells, a cell taken from an adult is inserted into an unfertilized egg cell whose nucleus has been removed. The two cells are then fused with an electrical pulse. The fusion causes the cells to develop into an embryo. Each embryo is then placed into a surrogate mother's womb to continue development.
In another technique, a nucleus is removed from an adult cell and injected into an egg cell whose nucleus has been removed. The genetic material of the injected nucleus is allowed to develop for up to six hours. The cell with its new nucleus is then given a chemical bath that causes the nucleus and the cell to fuse and develop into an embryo. The embryo is then placed into a surrogate mother's womb to continue development.
History of Cloning
Although the origins of cutting are uncertain, there are records of its use dating from the Middle Ages. Tissue culture came into commercial use in the 1950's, primarily to reproduce orchids, and in the 1970's its use with other plants became widespread.
Frogs were the first animals to be cloned using embryonic cells, in the early 1950's. By the early 1980's embryonic cell techniques were being used to clone laboratory animals, including mice; and livestock, including cattle and sheep. By the late 1990's researchers had developed cloning methods that used adult animal cells. In 1996 a lamb (called Dolly) was the first vertebrate cloned from an adult body cell. In 1998 researchers cloned dozens of mice from adult body cells. The resulting mouse clones were then themselves cloned, producing multiple generations of cloned mice. In 2000, researchers cloned a pig using an adult body cell, creating a litter of five clones. The techniques eventually developed in such experiments could potentially be used to engineer pig organs for transplants into humans.
The advances in cloning techniques in the late 1990's increased the likelihood that human beings could be cloned, raising concerns about the ethics of doing so. In response to such concerns, several nations banned government-funded research into cloning humans. In 2000, Great Britain granted a patent to a United States company for a specific cloning process and for the human embryos resulting from the process for their first few days of development. In 2004, a few scientists began to use nuclear-transfer techniques to produce human embryos that were clones of adults. Stem cells (unspecialized cells that can develop into various types of tissue) were then extracted from these embryos for medical research.