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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  42
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, which of the following statements about the two-category hypothesis is likely to be true?
选项 A、It is promising because it explains the presence of true bacteria-like organisms such as organelles in eukaryotic cells.
B、It is promising because it explains why eukaryotic cells, unlike prokaryotic cells, tend to form multicellular organisms.
C、It is flawed because it fails to account for the great variety among eukaryotic organisms.
D、It is flawed because it fails to account for the similarity between prokaryotes and eukaryotes.
E、It is flawed because it fails to recognize an important distinction among prokaryotes.
答案 E
解析 哪一个关于二分法的说法正确?A、B有前景，和原文第二段的认识相反。C．没能解释真核生物的多样性。无。D．没能解释原核、真核生物的相似性。无。E．正确。此理论有错，因为不能认识到原核生物中的重要区别。第二段指出：原核生物中还有原细菌、真细菌的区别。
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