Day 52

Passage 52

Transgenic Plants

①

The natural movement of genes between species can occur through gene transfer mediated by natural processes. This natural gene movement has been widely detected during genetic investigations of various mobile genetic elements that naturally translocate to new sites in a genome, such as transposons and retrotransposons. These various mobile genes play a major role in dynamic changes to chromosomes during evolution, yet only happen in a haphazard fashion.

②

Since about 1930 on, new plant varieties containing the genes of two species were created through hybridization or cross breeding by classical plant breeders in order to develop crop varieties with desired properties. They deliberately forced wide crosses by using a number of in vitro techniques such as protoplast fusion, embryo rescue or mutagenesis to generate diversity to produce plants that would not exist in nature. These artificial ways of gene transfer serve important roles in securing a sustainable future for agriculture by protecting crops from diseases and pests and helping land and water to be used more efficiently. Hope, one wheat variety bred by E. S. McFadden with a gene from a wild grass, saved American wheat growers from devastating stem rust outbreaks in the 1930s.

③

However, such breeding was only carried out between two related species. From the mid-70s on, another intentional way of creating transgenic plants, the laboratory-based recombinant DNA method is introduced. Recombinant DNA technology is a technique of creating transgenic organisms by injecting the genetic material or the DNA of an organism into the nucleus of the reproductive cell of another organism of the same or different species, with an aim to produce or transfer a desirable trait. This can be achieved by either infecting plant cells with plasmids as vectors carrying the desired gene, or by shooting microscopic pellets containing the gene directly into the cell. If all goes well, the transgene will be incorporated into the pollen and passed on to the next generation. Such kind of modern biotechnology and genetic engineering incorporate genetic material from not only related species, but also from unrelated species, in order to create transgenic and genetically modified plants.

④

Even now the production of transgenic plants and the introduction of foreign genetic material is still a highly debatable issue. A number of benefits as well as risks of generating such plants have been highlighted by scientific studies. One of the main benefits of such plants and crops is the incorporation of some highly desirable traits like resistance to certain diseases, pests or herbicides, which can increase crop yield to meet the rising demand for food. Another advantage is that, with the help of genetic engineering, it is very much possible to produce plants that can tolerate harsh environmental conditions like drought and cold. Even soil laden with high amounts of salt can be made cultivable by producing genetically modified plants that can grow in saline soil. In addition, transgenic plants resistant to herbicides and pesticides can play an important role in reducing the use of these chemicals, thereby improving the environmental quality. Apart from these, some transgenic plants can produce higher level of certain crucial nutrients, which can improve the nutritional quality of the foods.

⑤

Despite all these advantages, the development of genetically modified or altered plants is largely criticized, mainly due to the fact that these plants can have an adverse impact on the natural environment, ecosystem and biodiversity. Many have expressed a fear that the genetic material of such plants can escape to other native or non-target plant species. The same quality that is desirable in transgenic plants can be harmful to other species. Transgenes that are resistant to herbicides, for example, can make weed control quite hard if they are transferred to the weeds. Besides, transgenic plants can release antibiotic resistant genes in the soil, which can make the soil microorganisms resistant to antibiotics.

⑥

Another issue raised by the critics is the consequences of changing the occurrence and density of prey for natural enemies. Most insect-resistant plants are aimed at reducing the densities of certain phytophagous insects. If the density of prey is reduced, a direct flow-on effect could be a reduced density of their natural enemies. A documented decrease of ground beetle probably has something to do with the transgenic potatoes controlling the Colorado potato beetle.

——2012年2月11日北美考试机经

The word “they” in Paragraph 5 refers to____.

A. plants

B. transgenes

C. herbicides

D. weeds

核心词汇：

词汇练习：

阅读下列句子，用所给单词的正确形式填空。

secure relate inject harsh laden adverse release density yield tolerate reproductive trait deliberately generate mediate incorporate dynamic

1. Sociologists view primary groups as bridges between individuals and the larger society because they transmit, _____, and interpret a society's cultural patterns and provide the sense of oneness so critical for social solidarity.（TPO-13:Types of Social Groups）

2. Most people consider the landscape to be unchanging, but Earth is a _____ body, and its surface is continually altering-slowly on the human time scale, but relatively rapidly when compared to the great age of Earth(about 4,500 billion years).（OG:Geology and Landscape）

3. In 1950, myxoma virus, a parasite that affects rabbits, was _____ introduced into Australia to control the rabbit population.（TPO-17:Symbiotic Relationships）

4. In the 1990s enough electricity to meet about half the needs of San Francisco was being_____there.（TPO-21:Geothermal Energy）

5. This development, coming as it did when the bottom had fallen out of the European economy, provided an impetus to a long-held desire to _____ direct relations with the East by establishing a sea trade.（TPO-17:Europe's Early Sea Trade with Asia）

6. The rate at which the flowing water overcomes this resistance is _____ to the permeabil-ity of rock.（TPO-28:Groundwater）

7. Somewhat more promising have been recent experiments for releasing capillary water (water in the soil) above the water table by _____ compressed air into the ground.（TPO-28:Depletion of the Ogallala Aquifer）

8. The ancestors of plants were photosynthetic single-celled organisms that gave rise to plants presumably lacked true roots, stems, leaves, and complex _____ structures such as flowers.（TPO-25:The Evolutionary Origin of Plants）

9. Therefore, these social _____ helped to produce the nomadic and daring settlers who kept pushing westward beyond the fringes of settlement.（TPO-20: Westward Migration）

10. Areas rapidly_____into the empire were not long affected by the military.（TPO-25:The Roman Army's Impact on Britain）

11. The increased _____ and price of land justified reclaiming and draining even more land.（TPO-23:Seventeenth-Century Dutch Agriculture）

12. Perhaps dinosaurs could not _____ these extreme temperature changes and became extinct.（TPO-8:Extinction of the Dinosaurs）

13. The _____ conditions in deserts are intolerable for most plants and animals.（TPO-26:Survival of Plants and Animals in Desert Conditions）

14. In some industrial regions, heavily _____ wagons, with flanged wheels, were being hauled by horses along metal rails; and the stationary steam engine was puffing in the factory and mine.（TPO-6:Powering the Industrial Revolution）

15. Advancing technology and strict laws, however, are helping control some of these _____environmental effects.（TPO-4:Petroleum Resources）

16. The coming together of the oppositely charged particles neutralizes the electrical tension and _____ a tremendous amount of energy, which we see as lightning.（TPO-18:Lightning）

17. The actual level of intensity depends on such factors as the physical layout, popula-tion_____and productive activities of a metropolis.（TPO-23:Urban Climates）

参考答案：

1. mediate 2. dynamic 3. deliberately 4. generated 5. secure 6. related 7. injecting 8. reproductive 9. traits 10. incorporated 11. yield 12. tolerate 13. harsh 14. laden 15. adverse 16. releases 17. density