is an organism that is descended from and genetically
identical to a single common ancestor. Animals can be cloned by
embryo splitting or nuclear transfer. Embryo splitting involves
bisecting the multicellular embryo at an early stage of
development to generate "twins". This type of cloning occurs
naturally and has also been performed in the laboratory with a
number of animal species.
Cloning can also be achieved by nuclear transfer where the
genetic material from one cell is placed into a "recipient"
unfertilized egg that has had its genetic material removed by a
process called enucleation. The first mammals were cloned via
nuclear transfer during the early1980s, almost 30 years after the
initial successful experiments with frogs . Numerous mammalian
species have been cloned from cells of preimplantation embryos:
namely mice, rats, rabbits, pigs, goats, sheep, cattle and even
two rhesus monkeys, NETI and DETTO .
DOLLY, the sheep, was the first animal that was cloned via
nuclear transfer from a cultured adult cell . A diverse range of
adult tissues have now been successfully cloned in a variety of
species including cattle , mice , pigs , cats , rabbits , goats ,
and zebrafish .
The proportion of adult cell nuclei to develop into live
offspring after transfer into an enucleated egg is very low . High
rates of abortion have been observed at various stages of
pregnancy after placement of the eggs containing the adult cell
nuclei into recipient animals . Various abnormalities have been
observed in cloned cows and mice after birth and this has been
found to be somewhat dependent on the type of tissue that
originated the nuclei used to make the clone . The reasons for the
low efficiency of cloning by nuclear transfer are currently under
investigation but it is thought that it may be related to
insufficient nuclear reprogramming as the cloned nuclei goes from
directing the production of an adult somatic cell to directing the
production of a whole new embryo.
Mammals Cloned From Adult Cells (Table from De Berardino, 2001)
Cloning offers the opportunity to make transgenic animals from
cultured cells that have been genetically engineered . The first
genetically engineered or transgenic mammalian clones were sheep
born in 1997 carrying the coding sequences for human clotting
factor IX, which is an important therapeutic for hemophiliacs. One
of these transgenic sheep, POLLY, expressed this protein in her
milk . Cloning may also be useful for the preservation of rare and
endangered species , and in human therapeutics where patients may
be able to clone their own nuclei to make healthy tissue that
could be used to replace diseased tissue without the risk of
Making Genetically Engineered Clones (Data from Schnieke et
al., 1997; Figure from De Berardino, 2001). Fetal cells in culture
were transfected with a DNA sequence containing a selectable
marker (neomycin resistance), the human gene for clotting factor
IX, and a regulatory sequence to direct the gene to function only
in the mammary gland. Following selection for neomycin resistance,
nucleus from surviving cells were each transferred to an
enucleated egg. Of the three transgenic clones born, one named
POLLY survived and later secreted human clotting factor in her
milk. Polly is the first transgenic mammalian clone.
Companies Using Cloning Technology
Advanced Cell Technology, Inc
University of Idaho
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