Biology 11.3: PowerPoint Notes - Genetic Engineering in Agriculture
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Improving Crops:
Farmers began primitive genetic breeding years ago by selecting seeds from their best plants,
replanting them, and gradually improving the quality of their crops over time.
Today, we use genetic engineering to select and add characteristics and modify plants by manipulating
a plant’s genes.
Genetic engineering can change plants in many ways; from making plants drought resistant to making
plants that can thrive in different soils, climates or environmental conditions.
Genetic engineers have developed crop plants that are resistant to a biodegradable weed killer called
glyphosate. This enables farmers to spray their fields with glyphosate, kill all the weeds off, and
leaves the crops unharmed.
Half of the 72 million acres of soybeans planted in the U.S. in 2000 were genetically modified to resist
glyphosate.
Scientists have also developed crops that are resistant to certain insects by inserting specific genes
into plants.
This added gene makes the plants produce proteins that make the plant unacceptable to the insects
for a food source.
More Nutritious Crops:
Genetic engineering has been able, in many instances, to improve the nutritional value of many crops.
For example, in Asia, rice is a major food crop. Rice however is low in iron and beta-carotene.
Genetic engineers have modified rice in these countries by adding genes which boost the levels of iron
and beta-carotene to the rice plants.
Risks of Modified Crops:
Risks: Many people, including many scientists, have expressed concern that genetically modified crops
(GM crops) might turn out to be dangerous.
What kind of unforeseen negative affects might we experience from the new engineered crops?
Potential problems:
We have already noted that crops such as soybeans have been genetically altered to make them
resistant to the weed killer glyphosate.
Scientists are concerned that the use of glyphosate will lead to weeds that are immune to this weed
killer. Than we will need to search for a new weed killer and alter more crops to be resistant to it.
Are GM crops harmful to the environment?
Will genes introduced into crops by genetic engineering pass on to wild varieties of plants?
This type of gene flow happens all the time between related plants.
Biology 11.3: PowerPoint Notes - Genetic Engineering in Agriculture
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In most crops however, no closely related wild version of the plant is nearby to take up the gene
changes.
Some scientists fear that insect pests may become immune (by adapting) to the toxins that are
genetically engineered in some plants.
This would lead to insect strains that are harder to kill as they would be immune to the genetically
produced changes that were supposed to repel them.
Gene technology in Farm Animals:
Farmers have, for generations, improved their stock of animals through selection of the best and cross
breeding.
Now, many farmers use genetically-engineered techniques to improve their stock or their production.
Many farmers add growth hormone to the diet of their cows to increase the amount of milk their cows
produce. The cow growth hormone gene is introduced into bacteria which is than added to the
cow’s food supply.
This increases the amount of milk the cow produces.
Scientists have also boosted growth in pigs by adding growth hormone genes to the food that pigs eat.
These procedures lead to faster growth and higher profits for farmers.
Making Medically Useful Proteins:
Another way in which gene technology is used in animal farming is in the addition of human genes to
the genes of farm animals to produce human proteins in milk.
This is used for complex human proteins that cannot be made by bacteria through gene technology.
The human proteins are extracted from the animal’s milk and sold for pharmaceutical purposes. These
animals are called transgenic animals because they have human DNA in their cells.
More recently, scientists have turned to cloning animals as a way of creating identical animals that can
make medically useful proteins.
In cloning, the intact nucleus of an embryonic or fetal cell is placed into a new egg whose nucleus has
been removed.
The egg with the new nucleus is than placed into the uterus of a surrogate mother and is allowed to
develop.
Cloning from Adult Animals:
In 1997, the first successful cloning using differentiated cells from an adult animal resulted in a cloned
sheep named Dolly.
A differentiated cell is a cell that has become specialized to become a specific type of cell.
Biology 11.3: PowerPoint Notes - Genetic Engineering in Agriculture
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In Dolly’s case; a lamb was cloned from the nucleus of a mammary cell taken from an adult sheep.
Scientists thought that a differentiated cell would NOT give rise to an entire animal. The cloning of
Dolly successfully proved otherwise.
Problems with Cloning:
Since Dolly’s birth in 1996, scientists have successfully cloned several animals.
Only a few of these cloned animals survive however. Many become fatally oversized.
Others encounter problems in development. For example, three cloned calves were born in March
2001, only to die a month later from immune system failure.
The Importance of Genomic Imprinting:
Technical problems with reproductive cloning lie within a developmental process that conditions egg
and sperm so that the “right combination of genes” are turned “on” or “off” during early stages of
development.
When cloned offspring become adults, a different combination of genes is activated.
The process of conditioning the DNA during an early stage of development is called genomic
imprinting.
In genomic imprinting, chemical changes made to DNA prevent a gene’s expression without altering its
sequence.
Usually, a gene is locked into the “off” position by adding methyl groups to it’s cytosine nucleotides.
The bulky methyl groups prevent polymerase enzymes from reading the gene, so the gene cannot be
transcribed.
Later in development, the methyl groups are removed and the gene is reactivated.
Why Cloning Fails:
Normal vertebrae development depends on precise genomic imprinting.
This process, which takes place in adult reproductive tissue, takes months for sperm and years for
eggs.
Reproductive cloning fails because the reconstituted egg begins to divide within minutes. There is
simply not enough time in these few minutes for the reprogramming to process properly.
Gene keys fail to become properly methylated, and this leads to critical problems in development.
Because of these technical problems; and because of ethical problems, efforts to clone humans are
illegal in most countries.