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