【OG24-456题】The primary purpose of the passage is to_mofa留学圈mofaGRE在线题库系统_GRE在线搜题_GRE题库解析_GRE真题机经_GRE课程_GRE培训机构

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Scientists long believed that two nerve clusters in the human hypothalamus, called suprachiasmatic nuclei (SCNs), were what controlled our circadian rhythms. Those rhythms are the biological cycles that recur approximately every 24 hours in synchronization with the cycle of sunlight and darkness caused by Earth’s rotation. Studies have demonstrated that in some animals, the SCNs control daily fluctuations in blood pressure, body temperature, activity level, and alertness, as well as the nighttime release of the sleep-promoting agent melatonin. Furthermore, cells in the human retina dedicated to transmitting information about light levels to the SCNs have recently been discovered.

Four critical genes governing circadian cycles have been found to be active in every tissue, however, not just the SCNs, of flies, mice, and humans. In addition, when laboratory rats that usually ate at will were fed only once a day, peak activity of a clock gene in their livers shifted by 12 hours, whereas the same clock gene in the SCNs remained synchronized with light cycles. While scientists do not dispute the role of the SCNs in controlling core functions such as the regulation of body temperature and blood pressure, scientists now believe that circadian clocks in other organs and tissues may respond to external cues other than light—including temperature changes—that recur regularly every 24 hours.

The primary purpose of the passage is to

challenge recent findings that appear to contradict earlier findings present two sides of an ongoing scientific debate report answers to several questions that have long puzzled researchers discuss evidence that has caused a long-standing belief to be revised attempt to explain a commonly misunderstood biological phenomenon

正确答案： D

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文章大意逻辑
文章开篇指出科学家长期以来认为人类下丘脑中的视交叉上核（SCNs）控制着昼夜节律，接着说明SCNs在一些动物中对血压、体温等的控制作用以及人类视网膜中向SCNs传输光信息的细胞被发现。然后话锋一转，提出调控昼夜节律的关键基因在多种组织中都活跃，且实验显示肝脏中时钟基因的峰值活动会变化而SCNs中的保持同步。最后得出结论，虽然SCNs在控制核心功能上的作用不容置疑，但科学家现在认为其他器官和组织中的生物钟可能对除光之外的外部线索有反应。 文章类型及逻辑简图
文章类型：新旧观点对比 逻辑简图： Long believed: SCNs controlled our circadian rhythms. Studies: SCNs control various functions in animals; light transmitting cells to SCNs found. New findings: Critical genes active in many tissues; clock gene activity differences in liver and SCNs. Now believe: Circadian clocks in other organs may respond to non light cues. 做题方法及问题类型
本题为主旨题，做题方法是梳理文章整体结构和核心观点，把握从旧观点到新观点的转变。 选项分析
A选项：文章并非挑战近期发现，而是依据新发现修正旧观点，A错误。 B选项：文章不是呈现科学辩论的两方，没有在辩论，B错误。 C选项：文章没有回答长期困扰研究者的几个问题，C错误。 D选项：文章确实讨论了让长期以来的信念（SCNs单独控制昼夜节律）被修正的证据，符合文章主旨，D正确。 E选项：文章不是在解释一个常被误解的生物现象，E错误。

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