单选题All animals must rest, but do they really sleep as we know it? The answer to this question seems obvious. If an animal regularly stops its activities and stays quiet and unmoving—if it looks as though it is sleeping—then why not simply assume that it is in fact sleeping? But how can observers be sure that an animal is sleeping? They can watch the animal and notice whether its eyes are open or closed, whether it is active or lying quietly, and whether it responds to light or sound. These factors are important clues, but they often are not enough. Horses and cows, for example, rarely close their eyes, and fish and snakes cannot close them. Yet this does not necessarily mean that they do not sleep. Have you ever seen a cat dozing with an eye partly open? Even humans have occasionally been observed to sleep with one or both eyes partially open. Animals do not necessarily lie down to sleep either. Elephants, for example, often sleep standing up, with their tusks resting in the fork of a tree. Finally, while "sleeping" animals often seem unaware of changes in the sounds and light and other stimuli around them, that does not really prove they are sleeping either. Observations of animal behavior alone cannot fully answer the question of whether or not animals sleep. The answers come from doing experiments in "sleep laboratories" using a machine called the electroencephalograph (EEC). The machine is connected to animals and measures their brain signals, breathing, heartbeat, and muscle activity. The measurements are different when the animals appear to be sleeping than when they appear to be awake. Using the EEC, scientists have confirmed that all birds and mammals studied in laboratories do sleep. There is some evidence that reptiles, such as snakes and turtles, do not truly sleep, although they do have periods of rest each day, in which they are quiet and unmoving. They also have discovered that some animals, like chimpanzees, cats, and moles (who live underground), are good sleepers while others, like sheep, goats, and donkeys, are poor sleepers. Interestingly, the good sleepers are nearly all hunters with resting places that are safe from their enemies. Nearly all the poor sleepers are animals hunted by other animals: they must always be watching for enemies, even when they are resting.

单选题对于磷酸戊糖途径的叙述恰当的是

单选题有关tRNA的叙述，错误的是

单选题不参加脂酸β－氧化的辅因子是

单选题脂酸β-氧化的循环步骤是

单选题体内能量的储存形式是

单选题不参与DNA复制的酶是

单选题获得已知目的DNA序列最常用的方法是

单选题酶的Km值大小主要与

单选题 关于逆转录酶的叙述，错误的是 A、以单链RNA为模板 B、以单链DNA为模板 C、水解杂化双链中的RNA D、促使新合成DNA转入宿主细胞 E、能生成cDNA双链

单选题有机磷农药抑制的酶是

单选题与mRNA密码子ACG相对应的tRNA反密码子是

单选题核酸分子中的酸性基团是（ ）

单选题蛋白酶激活酶原的过程属于

单选题下列哪项不是递氢体

单选题The term "nationalism" is generally used to describe two phenomena: (1)the members of a nation care about their national identity and(2) that the members of a nation seek to achieve (or sustain) self-determination. It is traditional, therefore, to distinguish nations from states—whereas a nation often consists of an ethnic or cultural community, a state is a political entity with a high degree of sovereignty. While many states are nations in some sense, there are many nations which are not fully sovereign states. As an example, the Native American Iroquois constitute a nation but not a state, since they do not possess the requisite political authority over their internal or external affairs. If the members of the Iroquois nation were to strive to form a sovereign state in the effort to preserve their identity as a people, they would be exhibiting a state—focused nationalism. Nationalism has long been ignored as a topic in political philosophy, written off as a relic from bygone times. It has only recently come into the focus of philosophical debate. The surge of nationalism usually presents a morally ambivalent and for this reason often fascinating picture. "National awakenings" and struggles for political independence are often both heroic and inhumanly cruel; the formation of a recognizably national state often responds to deep popular sentiment, but can and does sometimes bring in its wake inhuman consequences, including violent expulsion and "cleansing" of non-nationals, all the way to organized mass murder. The moral debate on nationalism reflects a deep moral tension between solidarity with oppressed national groups on the one hand and repulsion in the face of crimes perpetrated in the name of nationalism on the other. Nationalism may manifest itself as part of official state ideology or as a popular (non-state) movement and may be expressed along civic, ethnic, cultural, religious or ideological lines. These self-definitions of the nation are used to classify types of nationalism. However, such categories are not mutually exclusive and many nationalist movements combine some or all of these elements to varying degrees. Nationalist movements can also be classified by other criteria, such as scale and location. Nationalism does not necessarily imply a belief in the superiority of one race over others, but in practice, many nationalists support racial protectionism or racial supremacy. Such racism is typically based upon preference or superiority of the indigenous race of the nation.

单选题辨认DNA复制起始点的是

单选题A．蛋白激酶C(PKC)B．鸟苷酸环化酶C．酪氨酸蛋白激酶D．腺苷酸环化酶E．蛋白激酶G(PKG)

单选题关于生长因子的叙述，错误的是

单选题下列对关键酶的叙述，不正确的是