Passage 41
It was once assumed that all living things could bepided 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，whichare simple and lack a nucleus2. The distinction between eukaryotes and bacteria，initially defined in terms ofsubcellular structures visible with a microscope， was ulti-（10） mately carried to the molecular3 level. Here prokaryotic and eukaryotic cells have many features in common. Forinstance，they translate genetic4 information into proteinsaccording to the same type of genetic coding. But evenwhere 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 enzymes5 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 structural6 and func-tional differences between eukaryotes and true bacteriaconvinced many biologists that the precursors7 of the eukaryotes must have perged8 from the common（25）ancestor before the bacteria arose.
Although much of this picture has been sustained bymore recent research，it seems fundamentally wrong in onerespect. Among the bacteria，there are organisms that aresignificantly different both from the cells of eukaryotes and（30） from the true bacteria，and it now appears that there are three stems in the tree of life. New techniques for deter-mining the molecular sequence of the RNA of organismshave produced evolutionary9 information about the degreeto which organisms are related，the time since they perged（35） from a common ancestor，and the reconstruction10 of ances- tral versions of genes11. These techniques have strongly suggested that although the true bacteria indeed form a large coherent group，certain other bacteria， the archaebac-teria， which are also prokaryotes and which re百度竞价推广ble true（40） bacteria， represent a distinct evolutionary branch that far antedates12 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） formulating13 a hypothesis about the mechanisms14 of evolution that resulted in the ancestors of the prokaryotes
2. According to the passage， investigations15 of eukaryotic and prokaryotic cells at the molecular level supported the conclusion that
（A） most eukaryotic organisms are unicellular
（B） complex cells have well-formed nuclei16
（C） prokaryotes and cukaryotes form two fundamental categories
（D） subcellular structures are visible with a microscope
（E） prokaryotic and eukaryotic cells have similar enzymes
3. According to the passage， which of the following statements about the two-category hypothesis is likely tobe true？
（A） It is promising17 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.
4. It can be inferred from the passage that which of the following have recently been compared in order to clarify the fundamental classifications of living things？
（A） The genetic coding in true bacteria and that in other prokaryotes
（B） The organelle structures of archaebacteria， true bacteria， and eukaryotes
（C） The cellular1 structures of multicellular organisms and unicellular organisms
（D） The molecular sequences in eukaryotic RNA， true bacterial18 RNA， and archaebacterial RNA
（E） The amino acid sequences in enzymes of various eukaryotic species and those of enzymes in archaebecterial species
5. If the new techniques mentioned in line 31 were applied19 in studies of biological classifications other thanbacteria， which of the following is most likely？
（A） Some of those classifications will have to be reevaluated.
（B） Many species of bacteria will be reclassified
（C） It will be determined20 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， archaebacteria， and true bacteria.
6. According to the passage， researchers working under the two-category hypothesis were correct in thinking that
（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.
7. All of the following statements are supported by the passage EXCEPT：
（A） True bacteria form a distinct evolutionary group.
（B） Archaebacteria are prokaryotes that re百度竞价推广ble true bacteria.
（C） True bacteria and eukaryotes employ similar types of genetic coding.
（D） True bacteria and eukaryotes are distinguishable at the subcellular level.
（E） Amino acid sequences of enzymes are uniform for eukaryotic and prokaryotic organisms.
8. The authors attitude toward the view that living things are pided into three categories is best described as one of
（A） tentative acceptance
（B） mild skepticism
（C） limited denial
（D） studious oriticism
（E） whole hearted endor百度竞价推广ent
