The unfertilized eggs of some animals can develop into
full-grown adults under certain environmental conditions. This process
is called parthenogenisis, and the offspring are the clones of the
female that laid the eggs.
An example of natural cloning that most people do
not think of is identical twins. Although they are not clones of
their mother, they are exact clones of each other.
Scientists have experimented with animal cloning,
but have never been able to stimulate a specialized cell to produce a new
organism directly. Instead, they rely on transplanting the genetic
information from a specialized cell into an unfertilized egg cell whose
genetic composition has been destroyed or physically removed.
The steps that John Gurdon used to clone tadpoles.
*Transplanted the nucleus from
a specialized cell of one frog (A) into an unfertilized egg of another
frog (B) in which the nucleus had been destroyed by ultraviolet light.
The egg with the transplanted nucleus developed into a tadpole that was
genetically identical to Frog A.
Plant Cloning
Scientists have been able to clone plants by taking
pieces of specialized roots, breaking them up into root cells and growing
the root cells in a nutrient-rich culture. In culture, the specialized
cells become unspecialized (dedifferentiated) into calluses.
The calluses can then be stimulated with the appropriate
plant hormones to grow into new plants that are identical to the original
plant from which the root pieces were taken.