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发布时间：2006-06-23 17:06     点击：

　　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 unicellu- lar organisms, are eukaryotic-their large, complex cells (5) have a well-formed nucles 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 ulti- (10) mately 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 (15) the two forms are different and characteristic of the respec- tive forms. For example, the amino acid sequences of vari- ous 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 (20) that the tree of life had only two stems. Moreover, argu- ments pointing out the extent of both structural and func- tional differences between eukaryotes and true bacteria convinced many biologists that the precursors of the eukaryotes must have diverged from the common (25) ancestor before the bacteria arose.

　　1. The passage is primarily concerned with

　　(A) detailing the evidence that has led most biologists to replace the trichotomous picture of living organisms with a dichotomous one

　　(B) outlining the factors that have contributed to the current hypothesis concerning the number of basic categories of living organisms

　　(C) evaluating experiments that have resulted in proof that the prokaryotes are more ancient than had been expected.

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