Learner Resource 2 –
Animal cloning summary
Few animal species naturally form clones. Some invertebrates can reproduce asexually and examples
include Hydra (budding) and planaria and starfish (fragmentation). Embryo splitting occurs naturally in
many mammals when a fertilised egg divides by mitosis and the first two cells cells completely
separate, forming identical twins. Artificial methods of animal cloning are concerned with two purposes.
1 Reproductive cloning. Artificial methods are used to mass-produce genetically identical animals.
These may be elite farm animals that have been produced by selective breeding (artificial selection
6.1.2 (h)) or by genetic modification (genetic engineering 6.1.3 (f)). They may also be animals that have
been developed to model a particular disease (e.g. the Harvard onco-mouse) to test the effects of
treatments and drugs.
2 Non-reproductive cloning. Genetically identical embryos are produced from human or other animal
cells in order to research the action of genes that control development and differentiation (link to 6.1.1),
and to try to grow new tissues and organs as replacement ‘spare parts’ for people who are ill (2.1.6
(m)). Tissues produced from a patient’s own stem cells will be genetically identical to the patient and so
will not be rejected by the immune system (4.1.1), which is normally a problem when transplanting
donated organs.
Techniques in animal cloning
1 Embryo Splitting. This has been in use in mammals since the late 1970s. A zygote is created by IVF
(in vitro fertilisation) and is allowed to divide by mitosis to form a ball of cells called a blastocyst. These
are separated, allowed to grow again to form blastocysts, and these are put into the uteruses of
surrogate mothers to develop. This is a good method of obtaining genetically identical offspring,
perhaps mice to test a drug therapy on for example, but the exact genotype and phenotype depend on
the sperm and egg used to create the zygote and is not known until the cloned offspring are born.
2 Adult Cell Cloning. This was first successful in mammals with Dolly the sheep in 1996 but Gurdon’s
experiments with Xenopus toads in the 1950s were a starting point for this nuclear transplantation
approach. The usual method is called Somatic Cell Nuclear Transfer (SCNT). An egg cell is obtained
and has its nucleus sucked out with a micropipette. A normal body cell (somatic cell) from the adult to
be cloned also has its nucleus removed, and this nucleus is fused with the empty egg cell by applying
an electric shock. The shock also triggers the egg cell to start development and mitosis as though it
had just been fertilised. The young embryo completes its development in a surrogate mother.
Problems of animal cloning are similar to the problems of plant cloning, that is lack of genetic variability
meaning that all the animals are equally susceptible to a certain disease. In addition, cloned animals
may be less healthy and have shorter life spans. The success rate of adult cell cloning is very poor and
the method is much more expensive than conventional breeding. Transgenic animals can also be
reproduced by conventional mating since once an animal possesses the transgene it can be passed to
offspring subject to the normal laws of Mendelian genetics.
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Cloning and Biotechnology
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© OCR 2016