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How are transgenic animals produced?
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Since the discovery of the molecular structure of DNA by Watson
and Crick in 1953, molecular biology research has gained momentum.
Molecular biology technology combines techniques and expertise
from biochemistry, genetics, cell biology, developmental biology,
and microbiology.
Scientists can now produce transgenic animals because, since
Watson and Crick's discovery, there have been breakthroughs in:
-
recombinant
DNA (artificially-produced DNA)
-
genetic
cloning
-
analysis of
gene expression (the process by which a gene gives rise to a
protein)
-
genomic mapping
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The
underlying principle in the production of transgenic animals is the
introduction of a foreign gene or genes into an animal (the inserted
genes are called transgenes). The foreign genes "must be transmitted
through the germ line, so that every cell, including germ cells, of
the animal contain the same modified genetic material."(Germ cells
are cells whose function is to transmit genes to an organism's
offspring.)
To
date, there are three basic methods of producing transgenic animals
-
DNA
microinjection
-
Retrovirus-mediated gene transfer
-
Embryonic stem cell-mediated gene transfer
Gene
transfer by microinjection is the predominant method used to produce
transgenic farm animals. Since the insertion of DNA results in a
random process, transgenic animals are mated to ensure that their
offspring acquire the desired transgene. However, the success rate of
producing transgenic animals individually by these methods is very low
and it may be more efficient to use cloning techniques to increase
their numbers. For example, gene transfer studies revealed that only
0.6% of transgenic pigs were born with a desired gene after 7,000 eggs
were injected with a specific transgene.
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1.
DNA Microinjection
The
mouse was the first animal to undergo successful gene transfer using
DNA microinjection. This method involves:
-
transfer of a
desired gene construct (of a single gene or a combination of genes
that are recombined and then cloned) from another member of the same
species or from a different species into the pronucleus of a
reproductive cell19
-
the manipulated cell, which first must be
cultured in vitro (in a lab, not in a live animal) to develop
to a specific embryonic phase, is then transferred to the recipient
female
3.
Embryonic Stem Cell-Mediated Gene Transfer
This
method involves:7,19,26
-
isolation of
totipotent stem cells (stem cells that can develop into any type of
specialized cell) from embryos
-
the desired gene
is inserted into these cells
-
cells containing
the desired DNA are incorporated into the host's embryo, resulting
in a chimeric animal
Unlike
the other two methods, which require live transgenic offspring to test
for the presence of the desired transgene, this method allows testing
for transgenes at the cell stage.
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2.
Retrovirus-Mediated Gene Transfer
A
retrovirus is a virus that carries its genetic material in the form of
RNA rather than DNA. This method involves:26
-
retroviruses used
as vectors to transfer genetic material into the host cell,
resulting in a chimera, an organism consisting of tissues or
parts of diverse genetic constitution
-
chimeras are
inbred for as many as 20 generations until homozygous (carrying the
desired transgene in every cell) transgenic offspring are born
The
method was successfully used in 1974 when a simian virus was inserted
into mice embryos, resulting in mice carrying this DNA.
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