Cloning

The media often uses the term “clone,” but fails to properly define it. This leads to confusion and misinformed conceptions. In reality, there are three distinct types of genetically identical beings of which only one, a copy made from an adult, is a true clone.

The first type of genetically identical beings are twins, and they are naturally abundant. Identical twins appear the same because they have identical nuclear DNA--that is, they have all the same genes. Twins also have identical mitochondrial DNA, though, which set them apart from all other identical beings. (Mitochondria are the “powerhouses” in each cell, converting food into energy. Mitochondrial DNA has approximately 60 base pairs). Not only are twins found in nature, but they can be produced in laboratories, too. Scientists simply have to chemically divide the zygote (fertilized egg cell) of a female animal before it begins to undergo mitosis (natural cell division). This is called twinning or “embryo cloning”. Human zygotes have been twinned in laboratories, although the embryos were never born.

The second method of yielding two genetically identical animals relies on nuclear transfer technology (which simply means taking the nucleus with the DNA from one cell and putting it into another). In this process, the nucleus from a daughter cell of a fertilized egg cell is placed in a single unfertilized egg cell of the same species. This must occur before the original daughter cells have differentiated (Differentiated cells are cells in which some of the DNA shuts off so that the cells can be individualized to make liver cells, bone cells, etc. Undifferentiated cells are all alike.). After the two organisms have matured, they are genetically identical (although again, the mitochondrial DNA differs). The two organisms are rather like brothers.

Recently, two Rhesus Monkeys were “cloned” this way, although, again, they aren’t true clones as the media proclaimed.

Dolly with her surrogate mother. By permission of Roslin Institute

The final method is true cloning. Before, we’ve mentioned twinning and nuclear transfer technology, but both yield “false clones” because both begin by using undifferentiated zygotes. True cloning, however, begins with an adult animal--an animal that has differentiated, or somatic, cells. Until Dolly arrived, this was always thought to be impossible because in a somatic cell, some of the DNA has been shut off (in other words, skin cells have only skin DNA turned on, heart cells have only heart DNA turned on, and so forth). Scientists had to discover how to turn on all the DNA in a somatic cell so that they could clone it. Once they figured this out, they were able to transfer the somatic cell’s DNA (again, an adult’s DNA) to an unfertilized egg cell of the same species. The product is two genetically identical animals: true clones.