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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  51
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] If the "new techniques" mentioned in line 45 were applied in studies of biological classifications other than bacteria, which of the following is most likely?
选项 A、Some of those classifications will have to be reevaluated.
B、Many species of bacteria will be reclassified.
C、It will be determined that there are four main categories of living things rather than three.
D、It will be found that true bacteria are much older than eukaryotes.
E、It will be found that there is a common ancestor of the eukaryotes, ar-chaebacteria, and true bacteria.
答案 A
解析 L45提到的新技术如果用于细菌以外的生物分类，会引起哪项结果?L45的新技术，即作者用来证明三分理论，论证二分理论为非的技术。A．正确。一些分类要重新评价。恰如其分，没说全部，符合原文语气。B、D在“细菌”上纠缠，原题已讲解，是细菌以外的生物分类。C．four main categories，从原文无法推出。E．完全违反原文结论“common ancestor”。
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