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  3. Tobacco’s public image couldn’t be more stained these days, given the war on cigarettes in the courts, federal and state governm

Tobacco’s public image couldn’t be more stained these days, given the war on cigarettes in the courts, federal and state governm

admin2012-01-21  64
问题     Tobacco’s public image couldn’t be more stained these days, given the war on cigarettes in the courts, federal and state governments, the medical establishment — even in Doonesbury. But the tobacco plant is not an irredeemable never-do-well. Because its genetic makeup is fairly straightforward and well understood, scientists believe tobacco could turn out to be the perfect blotch factory for protein-based drags. By splicing human genes — a technique developed in the early 1990s — researchers have enabled tobacco plants to produce a number of drugs and vaccines and even human blood components. Within 10 years, researchers are hopeful that tobacco farmers might be raising millions of acres of biofactories rather than "the killer weed".
    The latest breakthrough in tobacco "pharming" may bring such a vision one step closer to reality. Scientists at Monsanto Co. (MTC) reported in the March issue of Nature Biotechnology that they were able to genetically engineer tobacco plants to produce human growth hormone, otherwise known as somatotropin — an extremely costly drug used to treat dwarfism. But the Monsanto discovery was more important for the press than the product. The scientists were able to induce the tobacco plants to manufacture the drug in an abundant piece of the plant cell known as a chloroplast, which is responsible for converting light into food. Other efforts to create drug-producing plants have altered plant nuclei instead — far less numerous cell components.
    DNA Dilemma. "We were able to produce 300-fold more human somatotropin in the chloroplasts than in the nucleus," says Jeffrey M. Staub, manager of the Monsanto team that developed the technique. "As an optimistic scientist, I think it is very reasonable to believe, we will be able to produce commercial quantities of human proteins in the plants in the next couple of years."
    There has been notable progress with gene-spliced plants in the past two years. In 1998, British researchers reported that a human antibody grown in tobacco plants prevented one of the bacterial infections that led to tooth decay in human volunteers. And last August, the Energy Dept.’s Pacific Northwest National Laboratory announced that its scientists had successfully transplanted the necessary human genes into tobacco plants to produce human blood components.
    Even with plants, though, volume remain a problem. All plant cells contain subunits called organelles, two of which are the nucleus and the chloroplast. Gene splicing to date has focused on altering the nucleus — essentially the brain of the cell, because it controls all of the plant’s activities. But drug production was limited, since there is only one nucleus per cell. Moreover, the genetic alteration is transmitted to the plant’s pollen, making it difficult to control the spread of the reengineered plants. Chloroplasts, however, are abundant, and they do not spread their genetic changes to any other cells in the plant.
Based on the information of the last paragraph, which of the following is true?
选项 A、The number of chloroplasts is limited.
B、Chloroplasts do not spread their genetic changes to other cells in the plant.
C、Nucleus and chloroplast are all the subunits that plant cells contain.
D、Gene splicing date has focused on altering all the organelles.
答案B
解析 A中表达的意思与最后一句中的chloroplasts(叶绿体)are abundant相反。B 中的意思与最后一句意思一致，是该题的答案。C表达的意思是nucleus(细胞核)和 chloroplast是植物细胞中所包含的全部organelles(细胞器官)，而原文的意思是叶绿体和细 胞核是植物细胞中众多细胞器官中的两个，这两句话意义不完全相等。D表达的意思与该 段的第三句意思不符合。
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