GMO_Supplemental_Reading_2

GMO_Supplemental_Reading_2 - 170 Chapter7 Genetic...

Info iconThis preview shows pages 1–8. Sign up to view the full content.

View Full Document Right Arrow Icon
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 2
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 4
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 6
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 8
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: 170 Chapter7 Genetic Engineering studying the genes in the single—celled eukaryote Saccharomyces cerevisiae (bak- ers’ yeast) they probably had no idea that most of the genes present in this yeast were also present in humans. Today, scientists manipulate the environmental conditions of yeast to better understand how genes are regulated in humans. Likewise, scientists interested in studying the diversity of tropical plants (Chapter 3) assisted in the development of many pharmaceutical agents, in cluding some of the anticancer agents described in Chapter 5. Scientists in industry typically seek to answer questions that will have an immediate and profitable application, like the production of rBGH. This more applied research is important for scientists in industry because new products and improvements to existing ones increase profitability, which in turn deter- mines the success or failure of the business. One application of applied re- search that has proven to be very lucrative has been the genetic engineering of crop plants. Genetic Engineers Can Modify Foods " Whether you realize it or not, you have been eating genetically modified foods for some time now. This may lead you to wonder why and how plants are ge- netically modified, Whether eating them is bad for your health, or whether growing them is bad for the environment. E E E E Why Are Crop Plants Genetically Modified? Crop plants are genetically modified to increase their shelf life, yield, and nu- tritive value. The first genetically engineered fresh produce, tomatoes, were available in American grocery stores in 1994. These tomatoes were engineered to soften and ripen more slowly. The longer ripening time meant that tomatoes would stay on the Vine longer, thus making them taste better. The slower ripen- ing also increased the amount of time the tomatoes could be left on grocery store shelves without becoming overripe and mushy. An enzyme called pectinase mediates the ripening process in some produce, including tomatoes. This en- zyme breaks down pectin, a naturally occurring substance found in plant cells. When the enzyme pectinase is active, it helps break down the pectin and the produce softens. In tomatoes, genetic engineers inserted a gene that produces an mRNA transcript complementary to the mRNA produced by transcription of a pecti- nase gene. In double-stranded DNA, the strand that codes for a protein is called the sense strand, and its complement is called the antisense strand. When the antisense version of the pectinase gene is transcribed, it produces an mRNA that is complementary to the mRNA from the normally transcribed (sense strand) of the pectinase gene. When the mRNA from the genetically engineered antisense gene base pairs with its naturally occurring pectinase complement, ripening is slowed (Figure 7.10). Binding the antisense and sense mRNAs leaves less of the sense pectinase mRNA available for trans- lation. Thus, less of the pectinase enzyme is produced and ripening occurs more slowly. Improving the yield of crop plants has been the driving force behind the vast majority of genetic engineering. Yield can be increased when plants are engineered to be resistant to pesticides and herbicides, drought, and freezing. For example, a gene from an Arctic fish has been transferred into a strawberry to help prevent frost damage. Many people believe that improving farmers’ yields may help decrease world hunger problems. Others argue that, since there is already enough food mummimn'mvmmmmniiMm!“WWW7wmmmum“w"WVmillmnwwwwflllui" wimwmnmmmmmivlmm 'Ilit'IIYV"IWtm7nVVvrrmiWIKINWmmmmmmmlv'mmu xmmwmmmWW"ummWm-mmmu w mm H n "mui"Him"winyimiwiwnmvmu! mt tumultmva'inw‘fl‘flmmii1v'iiiumuunmnmilurim nwmuHimmwuyiw"[XWW’V‘UWIM g E E g E Normal tomato GM tomato Contains normal pectinase gene and engineered anti-pectinase gene: C ‘G T G C A C Sense strand ,C' CAA C HT G ' Antisense strand Normal mRNA , Engineered mRNA Normal mRNA Ribosome Engineered mRNA Translation proceeds normally. When complementary mRNAs bind to each other, ribosome cannot ' , r bind and Pectmase translation is enzyme prevented. Normal rate Slowed rate of ripening of ripening Figure 7.10 Genetically modified tomato. Genetically modified tomatoes produce mRNA that decreases the effects of the pectinase gene. When the normal, sense version of the pectinase gene is transcribed and translated, ripening occurs. When the pectinase mRNA is bound to the engineered antisense version, trans- lation does not occur, and ripening is slowed. 172 Golden Rice. Golden Rice Figure 7.11 has been genetically engineered to pro- duce more B—carotene. The increased con— centration of [3-carotene makes the rice look more gold in color than nonmodi- fied rice. Figure 7.12 Genetically modifying plants. (a) Plants infected by Agrobacterium tumefaciens in nature show evidence of the infection by producing tumors called galls. (b) The Ti plasmid from A. tumefa— ciens serves as a shuttle for incorporating genes into plant cells. It can be engineered to carry a gene from a different organism and not produce galls. The recombinant plasmid is then used to infect developing plant cells, producing a genetically modi— fied plant. When the plant cell reproduces, it may pass on the engineered gene to its offspring. (a) Gall caused by A tumefaciens Chapter 7 Genetic Engineering being produced to feed the entire population, it might make more sense to use less technological approaches to feeding the hungry. Significant numbers of people around the world are malnourished, hungry, or starving, not due to a shortage of food but because access to food is tied to access to money or land. However, as the population increases, it may some day be imperative to in- crease the yield of crop plants in order to feed all the world’s people. Genetic engineers may also be able to increase the nutritive value of crops. Some genetic engineers have increased the amount of B-carotene in rice, a staple food for many of the world's people. Scientists hope the engineered rice will help decrease the number of people who become blind in underde- veloped nations because cells require B-carotene in order to synthesize vita- min A, a vitamin required for Vision. Therefore, eating this genetically modified rice, called Golden Rice, increases a person’s ability to synthesize vi- tamin A (Figure 7 .11). How Are Crops Genetically Modified? To modify crop plants, the gene must be able to gain access to the plant cell, which means it must be able to move through the plant’s rigid, outer cell wall. The "ferry" for moving genes into flowering plants is a naturally occurring plasmid of the bacterium Agrobacterz’um tumefizciens. In nature, this bacterium in- fects plants and causes tumors called galls (Figure 7.12a). The tumors are in- duced by a plasmid, called Ti plasmid (for Tumor inducing). Genes from different organisms can be inserted into the Ti plasmid by using the same restriction enzyme to cut the Ti plasmid and the gene, and then con- necting the plasmid and the gene together and reinserting it into the bacterium. A. tumefaciens, with the recombinant Ti plasmid, is then used to infect plant cells. During infection the recombinant plasmid is transferred into the host plant cell (Figure 7.12b). For genetic engineering purposes, scientists use only the portion of a plasmid that does not cause tumor formation. Moving genes into other agricultural crops such as corn, barley, and rice can also be accomplished by using a device called a gene gun. A gene gun shoots tungsten-coated pellets covered with foreign DNA into plant cells (Figure 7.13). A small percentage of these DNA genes may be incorporated into the plant’s genome. The gene gun is often used by companies that do not want to pay licensing fees to Monsanto, holder of the A. tumefaciens patent. When a gene from one organism is incorporated into the genome of anoth- er organism, a transgenic organism is produced. A transgenic organism is com- monly referred to as a genetically modified organism or GMO. Many people have raised concerns about genetically modified (GM) crop plants. One concern is that large corporations that own many farms, called agribusiness corporations, profiting from GM crop production will put owners of family farms out of business. Other concerns focus on the impact of GMOs on human health and the environment (Figure 7.14). (b) Using the Ti plasmid 3. Cut the plasmid with a restriction enzyme. 2. Isolate the Ti plasmid from the bacterium cytoplasm. 1. Isolate the bacteria from a plant gall. Genetic Engineers Can Modify Foods 173 (a) Gene gun (b) How the gene gun works Gun Shockwaves .: a x >- > '3 > a ,. "Bullet" Microscopic particles coated with gene of interest are "shot" into plant cells. Plant cells in culture Figure 7.13 Gene gun. (a) A gene gun shoots a plastic bullet loaded with tiny metal pellets coated with DNA into a plant cell. The bullet shells are prevented from leaving the gun, but the DNA—covered pellets penetrate the cell wall, cell membrane, and nuclear membrane of some cells (b). Figure 7.14 Protesters at a World Trade Organization meeting in Seattle. These people are concerned about how GMOs may affect humans and the environment. Genetically-modified plant contains new gene (and new characteristic) 4- Use the same 5. Allow the gene 6. Expose plasmids to enzyme to cut the to attach to the young plant cells in gene of interest. plasmid. culture. 174 "No GMO" labelling. Many food manufacturers and consumers con— sider the use of unmodified foods to be a selling point for their products. Figure 7.15 Chapter 7 Genetic Engineering GMOs and Health Concerns about the potential negative health effects of consuming GM crops have led some citizens to fight for legislation requiring that modified foods be labeled so consumers can make informed decisions about what foods they choose to eat. The manufacturers of GM crops argue that labeling foods is ex- pensive and will be viewed by consumers as a warning, even in the absence of any proven risk. They believe that this will decrease sales and curtail fur- ther innovation. While the labeling controversy rages, the rate at which genetically modi- fied foods floods the market increases. Most of the corn and soy used in cattle feed is genetically engineered, as is much of the canola oil, squash, and pota- toes that humans consume. Even processed foods such as bread and pasta con- tain grains that have been engineered. Products that do not contain GMOs are often labeled to promote that fact (Figure 7.15). Genetically modified crop plants must be approved by the Environmen- tal Protection Agency (EPA) prior to their release into the environment. The FDA becomes involved in testing the GM crop only when the food the gene comes from has never been tested, or when there is’reason to be concerned that the newly inserted gene may encode a protein that will prove to be a toxin or allergen. If the gene being shuffled from one organism to another is not known to be toxic or cause an allergic reaction, the FDA considers it to be substantial- ly equivalent to the foods from which they were derived, that is, GRAS. If a modified crop contains a gene derived from a food that has been shown to cause a toxic or allergic reaction in humans, it must undergo testing prior to being marketed. This method of determining potential hazard worked well in the case of a modified soybean that carried a gene from the Brazil nut. This engineering was done in an effort to increase the protein content of soybeans. Since Brazil nuts were known to cause allergic reactions in some people, the modified beans were tested and did indeed cause an allergic reaction in susceptible people. The product was withdrawn and no one was harmed. Proponents of genetic engineering cite this as an example of the efficacy of the FDA rules. Opponents of genetically modifying foods wonder whether it will always be possible to predict which foods to test. They point out that it is possible for a protein encoded by a gene with no history of toxicity or allergenicity, to interact with substances in its new environment in unpre— dictable ways. In terms of toxicity, scientists focus on the protein produced by the modified plant and not the actual gene that is inserted. This is because the gene itself will be digested and broken down into its component nucleotides when it is eaten, and therefore will not be transcribed and translated inside human cells. Allergy is a serious problem for the close to 8% of Americans that experience allergic reactions to foods. Symptoms of food allergy range from mild upset to sudden death. Genetic engineers must be vigilant about testing foods with known allergens; a person who knows to avoid peanuts may not know to avoid a food that has been genetically modified to contain a peanut gene. Concern about GM foods is not limited to their consumption. Many people are also concerned about the effects of GM crop plants on the environment. GM Crops and the Environment Aside from Golden Rice, most crops that have been genetically modified have been modified to increase their yield. For centuries, farmers have tried to in- crease yields by killing the pests that damage crops and by controlling the growth of weeds that compete for nutrients, rain, and sunlight. In the United States, farmers typically spray chemical pesticides and herbicides directly on E g g E E E g E E g E E g g g E i E E E g g llllllmmmflliIn"MilWWIIW11WNWIIllHllmmm!lll1IIlullnrllmmmmnmwllmIil1lll!WmWmrimIl1illmummnnmmimINin!llmriiiiiIimIrrimriiiiinmmHmmmmlrmmvimvlInrIIuxiIrIIfiiiIiiiavwmnmmmmvmmmmnmmnwmr!riiummvmmmummnmmmlmmmmmwrn riimmmmmmmmmmmmmmmmmmmmmmmmmmm Genetic Engineers Can Modify Foods to their fields (Figure 7.16). This practice concerns people worried about the health effects of eating foods that have been chemically treated. In addition, both pesticides and herbicides leach through the soil and contaminate the groundwater. To help decrease farmer’s reliance on pesticides, agribusiness companies have engineered plants that are genetically resistant to pests. For example, corn plants have been engineered to kill the European corn borer (Figure 7.17a). To do this, scientists transferred a gene from the soil bacterium Bacillus thurz'ngz'en— sis (Bt) into corn. The Bt gene encodes proteins that are lethal to corn borers but not to humans (Figure 7.17b). The idea of using this bacterium for pest control (a) Corn plants have been engineered to kill the insects that eat them. (b) How it works: ‘ '7Bacillus:thruringiensisf reduces a . rot in that kills orn borer " ' Scientist inserts the protein- encoding Bt gene into corn Corn borer Corn cells produce protein that kills corn borers. 175 Figure 7.16 Crop dusting. Pesticides and herbicides are sprayed on crops to improve yields and are found on the sur- face of many crop plants. The chemicals also leach into the soil, where they can contaminate the groundwater. (c) Pollen from Bt corn might unintentionally kill butterfly larvae. Monarch Milkweed butterfly (common on edges catterpillar of corn fields) Figure 7.17 The European corn borer. (a) The European corn borer damages corn and decreases yields. (b) A gene present in the bacterium Bacillus thuringiensis produces a protein that is toxic to the corn borer. When this gene is inserted into corn DNA, the plant produces the protein that kills the corn borer, thereby providing resistance to the pest. (c) Some researchers have shown that Bt corn may be harmful to other organisms. 176 Chapter7 Genetic Engineering actually came from organic farmers, who have sprayed unengineered B. thuringiensis on crop plants for many years. Genetically modified Bt corn has proven to be so successful at resisting the corn borer that close to one-half of all E corn currently grown in the United States is engineered with this gene. E Shortly after the arrival of Bt corn, concern arose about its impact on or- ganisms in the surrounding areas. One laboratory study showed that milk— % weed, a plant commonly found on the edges of cornfields that had been dusted i with pollen from Bt corn, was lethal to Monarch butterfly caterpillars, for which milkweed is the only source of food (Figure 7.17c). This research was performed in a laboratory and has been difficult to duplicate on farmers’ fields, but there may still be cause for concern about how GM crops will affect other organisms. Modified corn also caused controversy in 1996 when Bt corn was found in Kraft Taco BellTM taco shells. Since corn with high levels of Bt had not yet been approved for human consumption, there was a massive recall of the product. Critics of Bt corn point out that it is only a matter of time before corn bor- ers develop resistance to Bt corn, which will require the development of new varieties of genetically engineered corn. This is true of pesticides applied to crops as well—pests develop resistance because application of a pesticide does not always kill all of the targeted organisms. The few that have preexisting re- sistance genes and are not susceptible survive and produce resistant offspring. g Eventually, widespread resistance develops and a new pesticide must be de- veloped and applied. This problem is particularly vexing for the organic farmers who were the first to use B. thuringiensis for controlling the corn borer, but who did so in a tar- geted way. When a farmer’s chemical overspray drifts to the farms of nearby organic farmers, the organic farmer has lost a powerful tool when the bacteri- E um is killed, and must find another method of controlling this pest. E The continued need for the development of new pesticides in farming is paralleled by farmers’ reliance on herbicides. Herbicide-resistant crop plants, such as Round-Up ReadyTM soybeans, have been engineered to be resistant to Round-UpTM herbicide, used to control weeds in soybean fields. Farmers can now spray their fields of genetically engineered soybeans with herbicides that will kill everything but the crop plant. Some people worry that this resistance gene will allow farmers to spray more herbicide on their crops, since there is no chance of killing the GM plant, thereby exposing consumers to even more herbicide. There is also concern that GM crop plants may transfer engineered genes E from modified crop plants to their wild or weedy relatives. Wind, rain, birds, g and bees carry genetically modified pollen to related plants near fields con- E taining GM crops (or even to farms where no GM crops are being grown). Many cultivated crops have retained the ability to interbreed with their wild relatives; in these cases, genes from farm crops can mix with genes from the wild crops. While this is unlikely to happen with corn or soybeans, which do not have weedy relatives, it has already been seen with canola and is likely to happen with squash and rice. Thus, the herbicide is rendered ineffective since both the crop plant and its weedy relative share the same resistance gene. It may become impossible to determine whether weed plants surrounding fields of engineered crops have been pollinated with pollen containing the modified gene, and there could be unintended consequences for the ecology of the sur- rounding environment. Also, if pollen from GM crop plants drifts to farms that are not growing modified crops, it becomes impossible to determine whether a crop plant has engineered genes or not. This would be disastrous in the event of a recall. Genetic manipulation could lead to decreasing variation within a species, and this too can have evolutionary consequences. Most GM corn, in addition to car- rying the Bt resistance gene, has also been selectively bred to mature all at once, produce uniform ears, and have a particular nutrient profile. If an unforeseen E Genetic Engineers Can Modify Humans 177 disease or pest were to sweep through the area containing this corn variety, the disease would probably devastate a large portion of the crop. Most, but not all, of the genetic engineering that occurs to produce crop plants resistant to pesticides and herbicides is performed by private compa- nies and is designed to maximize profits. For example, Round-Up Ready soy— beans are purchased by farmers who then apply Round-Up herbicide; both the GM soybean and herbicide are sold by Monsanto. Some day the techniques pi— oneered by agribusiness firms may be used to help solve the problem of world hunger, but this has not been the case to date. Genetic Engineers Can Modify Humans Some genetic engineers are attempting to modify humans. These modifications may one day include replacing defective or nonfunctional alleles of a gene with a functional copy of the gene. If this happens, it might be possible for physicians to diagnose genetic defects in early embryos and fix them, allowing the em- bryo to develop into a disease-free adult. Recent developments that have led to a much better understanding of the human genome may make this scenario more likely. The Human Genome Project The Human Genome Project involves sequencing, or determining the nu— cleotide—base sequence (A,C,G, or T), of the entire human genome and the lo- cation of each of the 30,000—60,000 human genes. In 1990, the Office of Health and Environmental Research of the United States Department of Energy (DOE), along with the NIH and scientists from around the world, undertook this proj- ect. At the time, scientists involved in the project proposed to have a complete accounting of all the genes present in humans by the year 2005. However, the race to complete the sequencing of the human genome sped up drastically due to technological advances and the involvement of a private company named Celera. At stake were the rights to patent the gene sequences (see Essay 7.1). Initially, Celera wanted to retain the rights to the DNA sequences, but gov- ernment scientists were making sequences available to the public. Eventual- ly, the two groups worked together to publish a working draft of the human genome in 2001. The scientists involved in this multinational effort also sequenced the genomes of the mouse, the fruit fly, a roundworm, bakers’ yeast, and a common intestinal bacterium. Scientists thought it was important to sequence the genomes of organisms other than humans because these model organisms are easy to manipulate in genetic studies, and because important genes are often conserved from one organism to another. In fact 90% of human genes are also present in mice, 50% in fruit flies, and 31% in bakers' yeast. Therefore, under- standing how a certain gene functions in a model organism helps us under- stand how the same gene functions in humans. To sequence the human genome, scientists isolated DNA from white blood cells. They then cleaved the chromosomes into more manageable sizes using re- striction enzymes, cloned them into plasmids, and determined the base se- quence using automated DNA sequencers (Figure 7.18). These machines distinguish between nucleotides on the basis of structural differences in the ni- trogenous bases. Sequence information was then uploaded to the Internet and scientists working on this, or any other project, could search for regions of se- quence information that overlapped with known sequences. Using overlap- ping regions, scientists in laboratories all over the world worked together to patch together DNA-sequence information. In this manner, scientists sequenced Figure 7.18 DNA sequencing. These machines determine the sequence of nucleotides present in a DNA sample. ...
View Full Document

This note was uploaded on 12/26/2009 for the course LSP T07.5005.0 taught by Professor Caseyking during the Fall '09 term at NYU.

Page1 / 8

GMO_Supplemental_Reading_2 - 170 Chapter7 Genetic...

This preview shows document pages 1 - 8. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online