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  3. It was once assumed that all living things could be divided into two fundamental and exhaustive categories. Multicellular plants

It was once assumed that all living things could be divided into two fundamental and exhaustive categories. Multicellular plants

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问题     It was once assumed that all living things could be divided into two fundamental and exhaustive categories. Multicellular plants and animals, as well as many unicellular organisms, are eukary-otic—their large, complex cells have a well-formed nucleus and many organelles. On the other hand, the true bacteria are prokaryotic cell, which are simple and lack a nucleus. The distinction between eukaryotes and bacteria, initially defined in terms of subcellular structures visible with a microscope, was ultimately carried to the molecular level. Here prokaryotic and eukaryotic cells have many features in common. For instance, they translate genetic information into proteins according to the same type of genetic coding. But even where the molecular processes are the same, the details in the two forms are different and characteristic of the respective forms. For example, the amino acid sequences of various enzymes tend to be typically prokaryotic or eukaryotic. The differences between the groups and the similarities within each group made it seem certain to most biologists that the tree of life had only two stems. Moreover, arguments pointing out the extent of both structural and functional differences between eukaryotes and true bacteria convinced many biologists that the precursors of the eukaryotes must have diverged from the common ancestor before the bacteria arose.
    Although much of this picture has been sustained by more recent research, it seems fundamentally wrong in one re- spect. Among the bacteria, there are organisms that are significantly different both from the cells of eukaryotes and from the true bacteria, and it now appears that there are three stems in the tree of life. New techniques for determining the molecular sequence of the RNA of organisms have produced evolutionary information about the degree to which organisms are related, the time since they diverged from a common ancestor, and the reconstruction of ancestral versions of genes. These techniques have strongly suggested that although the true bacteria indeed form a large co- herent group, certain other bacteria, the archaebacteria, which are also prokaryotes and which resemble true bacteria, represent a distinct evolutionary branch that far antedates the com- mon ancestor of all true bacteria.
According to the passage, investigations of eukaryotic and prokaryotic cells at the molecular level supported the conclusion that
选项 A、most eukaryotic organisms are unicellular.
B、complex cells have well-formed nucles.
C、prokaryotes and eukaryotes form two fundamental categories.
D、subcellular structures are visible with a microscope.
E、prokaryotic and eukaryotic cells have similar enzymes.
答案C
解析 在分子级别对真核、原核细胞的研究支持以下哪个结论?A．大部分真核生物单细胞。荒谬。B．复杂细胞有完整细胞核。只是真核细胞有。complex cell和eukaryotic这两个概念不能随便互换。C．正确。原核、真核组成两个基本种类。见原文L10—15，这种二分法最终要以在分子层面得以证实。D．此选项和本文内容无关。E．原核。真核有相似的酶。说反了。L23～26，酶的序列可分为原核型和真核型。
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