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  3. Cancer arises when the descendants of just one of our more than ten thousand billion cells proliferate out of control, eventuall

Cancer arises when the descendants of just one of our more than ten thousand billion cells proliferate out of control, eventuall

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问题     Cancer arises when the descendants of just one of our more than ten thousand billion cells proliferate out of control, eventually interfering with normal body functions. Since so many cells are at risk, the most amazing thing about cancer to me is how many years it usually takes to develop the disease. One major obstruction to the proliferation of cancerous cells is the phenomenon of apoptosis, which causes nearly all of our cells to kill themselves whenever they start to behave aberrantly.
    A complicated cellular signaling network determines the balance between the pro-apoptotic and anti-apoptotic proteins inside animal cells. Each of our many cells is constantly sensing its external and internal environment and will sacrifice itself(for our own good)if it is either not correctly located or not behaving normally. Without mechanisms of this type, the evolution of large complex organisms such as ourselves would probably not have been possible, because the tumors caused by cancerlike diseases would have overtaken us early in life.
    Tumors arise after a long process of random mutation followed by multiple rounds of selection for those cells able to proliferate best. One change selected for is in apoptotic mechanisms, which will be altered in different ways in different tumors. Imagine that we could determine why the cells in an individual’s tumor incorrectly compute that they need not kill themselves, as normal cells would do in their condition. If we understood the fundamental mechanisms by which cells make these decisions, we would stand an excellent chance of creating a tailored mixture of drugs that causes the tumor cells to compute differently, so that they commit suicide without harming normal cells.
    Another promising strategy takes advantage of the fact that essentially all cancer cells have acquired a defect in some aspect of their "DNA metabolism" , often some aspect of DNA repair that causes them to become highly mutable. This genetic instability of cancer cells is selected for early in tumor development, because only such cells can evolve the multiple additional changes, including defects in apoptosis, that are necessary for most cell types to become malignant.
    Cells that are too genetically unstable will die. Therefore, a treatment that blocks a particular DNA repair process can be lethal for a cancer cell, while sparing normal cells. If we could determine why the cells in a particular individual tumor are genetically unstable, we might be able to design drugs that kill the cells in that cancer highly selectively, with little harm to normal cells.
    These examples of rational approaches to cancer therapy were only a dream until recently. But by targeting these types of alterations in cancer cells, researchers have made impressive progress and are thus much closer to being able to design highly selective therapies based on the critical molecular defects in an individual tumor. But for most tumors, this type of approach is still hit or miss, because oncologists are severely hampered by an inadequate understanding of the fundamental processes that are altered in a particular tumor.  
The function of the cellular signaling network is to
选项 A、maintain a balance between the internal and the external environment.
B、help enhance the protein within our body.
C、would overtake human beings early in life.
D、kill those ill-located or mal-functioned cells.
答案D
解析 事实细节题。根据题干中的cellular signaling network可定位至第二段第二句。文章谈到cellular signaling network可以促使细胞“在其位置出错或表现异常的情况下牺牲自己”，即[D]。而[A]中的“保持内外环境之间的平衡”与文中的“决定动物体内促凋亡蛋白和抗凋亡蛋白的平衡”不符；[C]是对文中“如果没有这种机制的话，像人类这样庞大复杂的有机体就不会进化，因为癌症治疗的疾病所产生的肿瘤会使我们过早地夭折”的曲解；而[B]文章没有谈到，故均排除。
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