Detecting Genetically Modified Food
Curious about whether your favorite food item is genetically
modified? For each group of two: Please bring a dry food item
(such as chips, muffin mix, flour) to our next lab
I. Learning objective
1. Be able to discuss which foods are genetically modified and for what
purposes.
2. Be able to discuss pros and cons of genetic modification of food.
3. Understand the techniques used to genetically modify food.
4. Be able to carry out modern molecular techniques to detect genetic
modification in food products.
5. Be able to describe how DNA extraction, DNA amplification through PCR
and DNA separation through gel electrophoresis work.
II. Before coming to lab:
-
Read the article “What are genetically modified foods?” by the Human
Genome Project available at
http://www.ornl.gov/sci/techresources/Human_Genome/elsi/gmfood.shtml.
-
Read “How do scientists create an organism that expresses foreign genes?”
below.
-
Read “How can we detect genetic modification in plant products?” Below.
-
Watch these animations on PCR (available at http://highered.mcgrawhill.com/olc/dl/120078/micro15.swf) and gel electrophoresis (available at
http://www.dnalc.org/ddnalc/resources/electrophoresis.html)
III. Background
How do scientists create an organism that expresses foreign genes?
Remember that plants and animals usually have their genetic instructions
organized in two sets of chromosomes, one set from each parent. Humans have
2 x 23 chromosomes, with a total of ~ 26,000 genes (each gene comes in two
copies). Chromosomes are made from long DNA molecules wrapped around
proteins. DNA is a double stranded molecule built from four different subunits
called nucleotides: A, T, C, and G (see figure 1). The genetic information of a
chromosome is contained in stretches of DNA called genes. Each of our cells
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has the same genome (the totality of all genes), as each cell is a descendant of
once cell: the fused egg and sperm cells.
Figure 1: DNA molecule (http://ancestrytest.com/Basic_Genetics/DNA-oflife.jpg)
Creating a genetically modified organisms involves the following steps:
1. Identify the gene of interest that you want to use, e.g., a gene expressed
in a bacterium that can kill insects or resist herbicides.
2. Isolate that gene and link it to a promoter. Promoters, like genes, are
stretches of DNA with a particular sequence of nucleotides. Each gene
has a unique promoter associated with it. The promoter acts like a switch
that can turn expression of a gene on or off, depending on proteins bound
to it. Promoters are engineered so that they bind proteins that are present
only in cells that are supposed to express the gene of interest and only at
times when the product is needed. For example: Goats are genetically
modified with a human gene called alpha trypsin that is harvested from
milk. Even though every goat cell has the gene for alpha trypsin, the
promoter binds to proteins only present in mammary glands during
lactation and thus alpha trypsin is only secreted into milk. Some
promoters are always “on” because the gene product is always needed.
3. Introduce the promoter-gene DNA sequence into the plant cell from which
you will grow a new modified plant.
How can we detect genetic modification in plant products?”
In 80% of all genetically modified plants, a particular promoter called 35S is used.
This promoter is derived from the tobacco mosaic virus and is called a
constitutive promoter, because it is permanently switched on. This makes is
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useful for traits such as herbicide resistance or pesticide expression, because
you want all cells at all times to resist herbicides or kill pests. Because 35S is so
widely used, it is a good target for detection of various genetic modifications in
plants.
Here are the steps in detecting this promoter (and thus, whether the plant is
genetically engineered or not):
1. Isolate cells from three sources
a. The plant you are investigating
b. A plant you know is genetically modified (the positive control)
c. A plant you know is not genetically modified (the negative control)
The controls serve two functions: they will show you
- what a positive and a negative result looks like
- whether your reagents and procedures work properly.
2. Isolate the DNA from the cells.
3. Chemically mark the region around the promoter (if there) and a control
region of DNA (present in all cells). Make copies of these two DNA regions
using PCR (for an animated tutorial of PCR watch
http://www.dnalc.org/ddnalc/resources/pcr.html).
4. Visualize the presence/absence of the regions using gel electrophoresis
(for an animated tutorial of gel electrophoresis watch
http://www.dnalc.org/ddnalc/resources/electrophoresis.html.)
IV. Review Questions
From the Human Genome Project Reading
1. Define genetic modification.
2. Which countries are planting the most genetically modified crops?
3. Which plants are most commonly genetically modified?
4. What traits are most commonly engineered into plants?
5. List two pros and two cons regarding genetic engineering of foods.
From the reading above and your work in lab
1. How do the terms chromosome, DNA, gene, and nucleotide relate to each
other.
2. What is a promoter?
3. What are the steps involved in detecting genetic recombination?
4. What function do the positive and negative control serve?
5. Why are we amplifying the tubulin gene?
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6. Why are we amplifying the 35S promoter?
7. What does PCR do?
8. What is the purpose of gel electrophoresis?
9. How does gel electrophoresis work?
10. Why do our DNA fragments run though a gel?
11. Why do they run through a gel at different speeds?
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