Genetic enhancement is a controversial
application of molecular genetics that aims to create larger and healthier
plants and animals, mainly for human consumption. To take one example of
many, through genetic enhancement, it is possible to produce larger poultry
birds such as chicken that possess a greater amount of meat. One method by
which genetically enhanced chickens can be produced is demonstrated here. The
extent to which domestic chickens grow is partially determined by the number
of growth hormone receptors found on the surface of their cells during the
stages of early development. As shown, these receptors bind to growth hormone
molecules, depicted here as yellow pieces filling the gaps in the membrane of
this fertilized chicken egg cell. By artificially increasing the number of
growth hormone receptors found on embryonic chicken cells, researchers can
produce larger adult chickens. To accomplish this, an ordinary chicken egg is
first opened, exposing the embryo within. Germ cells, which develop into
sperm and egg cells later in life, are then removed from the embryo and
cultured under special laboratory conditions. Many copies of growth hormone
receptor genes are inserted into the DNA of each germ cell by homologous
recombination. The embryo is then exposed to radioactivity to kill any
naturally occurring germ cells that remain, and the recombinant germ cells
that contain multiple copies of the growth hormone receptor gene are implanted.
The chick that hatches from this egg will grow to the same adult size as an
ordinary chicken, but because its egg or sperm cells will have developed from
enhanced germ cells, they will contain more copies of the growth hormone
receptor gene than usual. Therefore, when the chicken reproduces, it will
pass on these extra copies of the gene on to its offspring, which as a result
will produce more growth hormone receptors on their cells than ordinary
chickens. The increased number of receptors allow
more growth hormone molecules to bind to the cells than usual, as shown,
which promotes a higher rate of growth among the young chickens than would
occur normally. The adult birds that develop from such enhancement techniques
are therefore larger than usual, and contain a greater amount of meat for
market sale. Although debate continues to arise regarding different aspects
of genetic enhancement, this technology may prove very useful, especially for
increasing the food supply in third world nations.
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