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  3. It was once assumed that all living things could be divided into two fundame

It was once assumed that all living things could be divided into two fundame

游客2024-01-13  71
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. [br] According to the passage, researchers working under the two-category hypothesis were correct in thinking that
选项 A、prokaryotes form a coherent group.
B、the common ancestor of all living things had complex properties.
C、eukaryotes are fundamentally different from true bacteria.
D、true bacteria are just as complex as eukaryotes.
E、ancestral versions of eukaryotic genes functioned differently from their modern counterparts.
答案 C
解析 二分法主张的人哪一个说法是正确的?A．原核生物形成一致的一组。这点已被后来研究否定。B．“common ancestor of all living things”原文无。C．正确。真核生物和真细菌有本质上的不同。这一点三分法也未加以否认。D．真细菌和真核生物一样复杂。无。E．基因的作用。本文未正式提出。
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