424
T .
G .
H S U
was presented by Bender and Gooch (1962) who studied mitosis
of human leukocytes in culture. These authors X-irradiated the cells
before cultivation. The resulting chromosome abnormalities were all of
the "chromosome type" instead of the "chromatid type", suggesting
that the chromosomes of the leukocytes in the peripheral blood are not
replicated, or that they are in the G x phase. Recently, however, Gelfant
(1962) found that in the epidermal tissue of the mouse, even though a
large proportion of the cells are in the G x phase, some are in the G 2
phase ready to divide whenever a stimulus, such as wounding, is
applied.
Some investigators use the terms "presynthetic rest" and "postsynthetic rest" for the G 1 and the G 2 phases respectively. This type of
terminology falls into the same category as the one of "resting cell"
for interphase, still used by some biologists and classical cytologists.
DNA synthesis is not the only synthetic process a cell performs. In fact,
biological scientists may have sharpened their focus too acutely upon
DNA as if it were the only important component of the protoplasm.
Even in chromosomes, where D N A resides, there are proteins and
inorganic salts. Nevertheless, to differentiate with accuracy interphase
into pre-DNA-synthetic (GO, DNA-synthetic (S), and post-DNA-synthetic (G 2 ) stages marks a major step toward understanding cell physiology. A number of biologists have turned their attention to other cellular
activities in an attempt to answer some more difficult questions. What
does a cell do during the G x and the G 2 phases? What determines the
onset of D N A synthesis? What triggers mitosis? When are the chromosomal proteins made—prior to, simultaneous with, or later than D N A
synthesis? Studies on the protozoan Euplotes, a material first employed
by Gall (1959) but glorified by Prescott and Kimball (1961), have
yielded some significant conclusions. The ribbonlike macronucleus of
this species has a morphologically distinguishable feature (a lightly
stained band) for the area undergoing D N A synthesis. This becomes a
handy marker for studying cellular activities in relation to D N A
synthesis because H
3 - T D R is not needed to determine the time and
area of D N A synthesis, so that precursors of other macromolecules
(such as protein or RNA) can be made radioactive for autoradiography.
It was demonstrated by Prescott and Kimball (1961) that, as expected,
R N A and D N A syntheses are mutually exclusive, but protein synthesis
and D N A synthesis take place concurrently. In test systems, such as
mammalian cells, where no morphological marker is available for any
activity, investigations of this type would probably resort to parallel
experiments, one for D N A and the other for the activity in question,
thereby drawing indirect conclusions, or to double autoradiography
with two different isotopes.
T .
G .
H S U
was presented by Bender and Gooch (1962) who studied mitosis
of human leukocytes in culture. These authors X-irradiated the cells
before cultivation. The resulting chromosome abnormalities were all of
the "chromosome type" instead of the "chromatid type", suggesting
that the chromosomes of the leukocytes in the peripheral blood are not
replicated, or that they are in the G x phase. Recently, however, Gelfant
(1962) found that in the epidermal tissue of the mouse, even though a
large proportion of the cells are in the G x phase, some are in the G 2
phase ready to divide whenever a stimulus, such as wounding, is
applied.
Some investigators use the terms "presynthetic rest" and "postsynthetic rest" for the G 1 and the G 2 phases respectively. This type of
terminology falls into the same category as the one of "resting cell"
for interphase, still used by some biologists and classical cytologists.
DNA synthesis is not the only synthetic process a cell performs. In fact,
biological scientists may have sharpened their focus too acutely upon
DNA as if it were the only important component of the protoplasm.
Even in chromosomes, where D N A resides, there are proteins and
inorganic salts. Nevertheless, to differentiate with accuracy interphase
into pre-DNA-synthetic (GO, DNA-synthetic (S), and post-DNA-synthetic (G 2 ) stages marks a major step toward understanding cell physiology. A number of biologists have turned their attention to other cellular
activities in an attempt to answer some more difficult questions. What
does a cell do during the G x and the G 2 phases? What determines the
onset of D N A synthesis? What triggers mitosis? When are the chromosomal proteins made—prior to, simultaneous with, or later than D N A
synthesis? Studies on the protozoan Euplotes, a material first employed
by Gall (1959) but glorified by Prescott and Kimball (1961), have
yielded some significant conclusions. The ribbonlike macronucleus of
this species has a morphologically distinguishable feature (a lightly
stained band) for the area undergoing D N A synthesis. This becomes a
handy marker for studying cellular activities in relation to D N A
synthesis because H
3 - T D R is not needed to determine the time and
area of D N A synthesis, so that precursors of other macromolecules
(such as protein or RNA) can be made radioactive for autoradiography.
It was demonstrated by Prescott and Kimball (1961) that, as expected,
R N A and D N A syntheses are mutually exclusive, but protein synthesis
and D N A synthesis take place concurrently. In test systems, such as
mammalian cells, where no morphological marker is available for any
activity, investigations of this type would probably resort to parallel
experiments, one for D N A and the other for the activity in question,
thereby drawing indirect conclusions, or to double autoradiography
with two different isotopes.