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-
Line lar organisms, are eukaryotic-their large, complex cells
(5) have a well-formed nucleus and many organelles. On the
other hand, the true bacteria are prokaryotic cells, 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 respect-
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 ancestor
(25) 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 arganisms that are
significantly different both form 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 organisms
have produced evolutionary information about the degree to
which organisms are related, the time since they diverged
(35) from a common ancestor, and the reconstruction of ances-
tral versions of genes. These techniques have strongly
suggested that altough the true bacteria indeed from a
large coherent group, certain other bacteria, the archaebac-
teria, which are also prokaryotes and which resemble true
(40) bacteria, represent a distinct evolutionary branch that far
antedates the common ancestor of all true bacteria.
According to the passage, which of the following statements about the two-category hypothesis is likely to be true?
(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.
Q/A-e why not c
divided into two fundamental and exhaustive categories.
Multicellular plants and animals, as well as many unicellu-
Line lar organisms, are eukaryotic-their large, complex cells
(5) have a well-formed nucleus and many organelles. On the
other hand, the true bacteria are prokaryotic cells, 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 respect-
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 ancestor
(25) 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 arganisms that are
significantly different both form 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 organisms
have produced evolutionary information about the degree to
which organisms are related, the time since they diverged
(35) from a common ancestor, and the reconstruction of ances-
tral versions of genes. These techniques have strongly
suggested that altough the true bacteria indeed from a
large coherent group, certain other bacteria, the archaebac-
teria, which are also prokaryotes and which resemble true
(40) bacteria, represent a distinct evolutionary branch that far
antedates the common ancestor of all true bacteria.
According to the passage, which of the following statements about the two-category hypothesis is likely to be true?
(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.
Q/A-e why not c
