6. C E L L D I V I S I O N
221
fresh bone marrow cells (Lajtha, 1957), Ehrlich ascites tumour cells
(Edwards, Koch, Youcis, Freese, Laite and Donalson, 1960) and
others, the cell cycle divides easily into four periods determined by the
behaviour of the D N A . T w o different methods have led to the establishment of these important facts: by cytophotometry (mainly on
Feulgen-stained nuclei), it can be shown that in dividing tissues, some
interphase nuclei always appear with twice the normal (usually diploid)
amount of D N A . The population is thus distributed into two groups
near these two values (diploid and tetraploid) with few intermediates.
The relative percentages vary with the type of cell and the intensity of
growth but show that relatively few cells are actually in the synthetic
phase at any one time; for instances in cultures, see the histograms of
Deeley et al. (1954) and Firket (1958a). A second and more direct approach
is made possible by the use of specific radioactive precursors. D N A has
practically no turnover so that incorporation of its precursors can be
considered as a sign of synthesis for this molecule. Thymidine is an
almost specific precursor of DNA. Incorporation of tritiated thymidine
occurs only during a short period of the interphase (Taylor et al., 1957;
Firket, 1958a, etc.). By a further analysis of the quantitative results
obtained by the two methods, they are found to correlate exactly. In
cultures of chick fibroblasts, both techniques indicate that half the D N A
synthesis is accomplished about 7 h before division (Firket, 1958a).
With Lajtha (1957) (see also p. 415) we will define the four periods of
the cycle as:
G 1 (first gap) = post telophasic stage, D N A stable;
S (synthesis) = during which the D N A doubles its amount and the
nuclei incorporate tritiated thymidine;
G 2 (2nd gap) = preprophasic stage, D N A stable;
M = mitosis, during which D N A is quantitatively and qualitatively
halved and distributed between daughter cells.
Some estimations of the durations of the phases (excluding mitosis
itself) are given in Table II. They are usually based on percentages of
labelled mitoses after
3
H-thymidine has been added to the medium for
short periods of time. When cells are fixed several hours later, only
those cells are labelled that were engaged in D N A duplication when
the tritiated thymidine was present. By varying the delay between
fixation and the period of radioactive precursor availability, the
duration of the D N A synthesis and the duration of the gap ( G 2 ) between
it and mitosis can be established. These figures are, of course, mean
values and are valid only for the conditions of culture used in each case.
221
fresh bone marrow cells (Lajtha, 1957), Ehrlich ascites tumour cells
(Edwards, Koch, Youcis, Freese, Laite and Donalson, 1960) and
others, the cell cycle divides easily into four periods determined by the
behaviour of the D N A . T w o different methods have led to the establishment of these important facts: by cytophotometry (mainly on
Feulgen-stained nuclei), it can be shown that in dividing tissues, some
interphase nuclei always appear with twice the normal (usually diploid)
amount of D N A . The population is thus distributed into two groups
near these two values (diploid and tetraploid) with few intermediates.
The relative percentages vary with the type of cell and the intensity of
growth but show that relatively few cells are actually in the synthetic
phase at any one time; for instances in cultures, see the histograms of
Deeley et al. (1954) and Firket (1958a). A second and more direct approach
is made possible by the use of specific radioactive precursors. D N A has
practically no turnover so that incorporation of its precursors can be
considered as a sign of synthesis for this molecule. Thymidine is an
almost specific precursor of DNA. Incorporation of tritiated thymidine
occurs only during a short period of the interphase (Taylor et al., 1957;
Firket, 1958a, etc.). By a further analysis of the quantitative results
obtained by the two methods, they are found to correlate exactly. In
cultures of chick fibroblasts, both techniques indicate that half the D N A
synthesis is accomplished about 7 h before division (Firket, 1958a).
With Lajtha (1957) (see also p. 415) we will define the four periods of
the cycle as:
G 1 (first gap) = post telophasic stage, D N A stable;
S (synthesis) = during which the D N A doubles its amount and the
nuclei incorporate tritiated thymidine;
G 2 (2nd gap) = preprophasic stage, D N A stable;
M = mitosis, during which D N A is quantitatively and qualitatively
halved and distributed between daughter cells.
Some estimations of the durations of the phases (excluding mitosis
itself) are given in Table II. They are usually based on percentages of
labelled mitoses after
3
H-thymidine has been added to the medium for
short periods of time. When cells are fixed several hours later, only
those cells are labelled that were engaged in D N A duplication when
the tritiated thymidine was present. By varying the delay between
fixation and the period of radioactive precursor availability, the
duration of the D N A synthesis and the duration of the gap ( G 2 ) between
it and mitosis can be established. These figures are, of course, mean
values and are valid only for the conditions of culture used in each case.