Understanding of Basics of Cloning

A clone can be defined as an individual or group of individuals that descend, through asexual reproduction, from a single individual. In other words, a clone is an exact copy of the original individual. Humans have practiced cloning of plant species for thousands of years. A leaf, a piece of stem, or root of a certain plant placed in a pot with soil or in a petri dish with tissue culture media can regenerate a new individual, genetically identical (clone) to the plant from which the leaf, stem, or root piece was taken. Today, cloning is a common agricultural practice used in many species that can easily reproduce asexually, such as sugar cane, banana, citrus, potato, strawberry, many grasses, roses, and many tree crops.

Cloning is based on two principles:
1) All cells of any organism contain the complete genetic makeup of the species.
2) Totipotence, the ability of one cell to differentiate and regenerate a completely new individual

Although the regeneration of a complete plant from a somatic tissue (leaf, root, stem, etc.) is an ancient practice, it was only in the 1950s that biologists discovered the principles behind regeneration of whole individuals from a single cell. Unlike animal cells, most plant cells retain their potential to express any of their genes and therefore are able to repeat the developmental processes involved in regenerating complete individuals. Cloning of plants offers the possibility of developing millions of individuals exactly identical to the original source of the regenerated cells. This is a common method of reproduction in asexual plant species.

Most animal cells do not have that same capability of naturally regenerating a complete individual from a cell. In animals, this potential is lost during cell specialization. A specific class of cells called stem cells is the only cell type known to retain their totipotence. Stem cells can be found in marrow tissue, fat tissue, and developing embryos. These types of cells have been the focus of animal cloning efforts. In animals, cloning can be accomplished using the technique called nuclear transplant. The technique has been used for many years in animal cloning using embryonic cells for amphibians such as toads. Animal embryonic cells maintain their totipotence after the first few cellular divisions. As the embryo continues its development, the cells lose their ability to differentiate into other cells and, consequently, the capability for complete regeneration ceases quickly. Contrary to the relative ease of nuclear cell transfer in amphibians, this process is much more complex in mammals. Although cloning of toads was accomplished for the first time in 1952, cloning of mice using the same technique was not accomplished until 1977.

Cloning using nuclear transfer involves the manipulation of two cells. The recipient cell is usually a nonfertilized egg from a female taken soon after ovulation. Harvesting of these eggs is done by laparoscopy or by transvaginal suction. The donor cell, which is the one providing the genetic material for regenerating the clone, is collected from the individual to be copied. Any somatic cell could be used for the purpose, including cells from the skin, mammary glands, or mucous membranes. Under a microscope, the recipient cell (egg) is held, by suction, at the end of a pipette. With an extremely fine micropipette, the chromosomes are removed. At this point the nucleus from the donor cell is then fused with the recipient egg previously deprived of its chromosomes. Some of the cells, if implanted into the uterus of a surrogate mother, start developing into an embryo and eventually a fetus. The procedure involves the removal or destruction of the chromosomes from the recipient egg cell, and the subsequent introduction of the chromosomes from the donor cell. The egg, with the newly introduced genetic material, begins the developmental process in the uterus of a surrogate mother to form a complete individual, genetically identical to the donor that supplied the nucleus. This technique has been used with success for cloning sheep, cattle, mice, monkeys, and other mammals.


The first cloned mammal from somatic cells of an adult donor was the sheep Dolly, born in February 1997. Dolly was cloned using mammary cells from an adult sheep. This widely covered event occurred at the Roslin Institute in Scotland, and the lead scientist was Dr. Ian Wilmut.

Tags: Bio Technology, Bio Genetics, Cloning

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