|
Passage 44 (44/63)
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 eukaryotic—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 respect. 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 coherent group, certain other
bacteria, the archaebacteria, which are also prokaryotes and which resemble true bacteria,
represent a distinct evolutionary branch that far antedates the common ancestor of all true
bacteria. 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
(D) summarizing the differences in structure and function found among true
bacteria, archaebacteria, and eukaryotes
(E) formulating a hypothesis about the mechanisms of evolution that resulted in
the ancestors of the prokaryotes 175. The best answer is B. The first paragraph reviews inquires leading to the hypothesis that two categories of organism exist; the second explains how “more recent research” (line 27) supports a three-category hypothesis. Thus, the passage is primarily concerned with outlying factors contributing to the current hypothesis about the number of such categories. Choice A is wrong because the passage describes the replacement of a dichotomous with a trichotomous model, not the reverse. C is wrong because the passage mentions no experimental proof that the prokaryotes were older than expected; D is wrong because the passage only briefly discusses the structure and function of eukaryotes and prokaryotes, never mentioning those of archaebateria. E is wrong because the passage mentions no particular “mechanisms of evolution” that created the ancestors of the prokaryotes. 这道题我看不懂B选项的意思@@ 有谁可以帮我翻译一下麽，什么叫做factors that have contributed to the current hypothesis？  | 
京公网安备11010202008513号 京ICP证101109号 京ICP备12012021号
发表于 2007-9-29 11:54:00
楼主