224
H . F I R K E T
(c) The G 2 period. It has been also variously named preprophase or
antephase. The cell is now preparing for division. The duration of this
phase has been found to be fairly constant for a given type of cell
though there may be considerable variation of the intensity of growth
(Firket, 1958a; Sisken and Kinosita, 1961b). However, Painter and
Drew (1959) mention individual deviations. It may be much shorter in
vivo than in vitro: Cronkite, Fliedner, Bond, Rubini, Brecher and
Quastler (1958) mention delays as short as 30 min after injection of
thymidine for obtaining labelled mitoses in mouse haemopoietic or
intestine cells. This gives a minimum time for G 2 .
Most of the information on the processes characteristic of this period
is obtained by indirect means. A number of physical or chemical
agents bring about an immediate or rapid prevention of entry into
mitosis. Hypotheses as to the nature of their interference can only be
deduced from their known biochemical effects on isolated systems.
There is some danger in relying too much on this type of reasoning, but
it can sometimes be correlated with the evidence obtained on materials
where the G 2 period is more readily identifiable than in vertebrate cells.
In root tips of Viciafaba (Woodard, Rasch and Swift, 1961), in yeast
(Williamson and Scopes, 1960), in Protozoa (Prescott, 1960; Woodard,
Gelber and Swift, 1961) and in other organisms, G 2 is characterized by
an increase in R N A synthesis, especially in the nucleus. Experiments
showing an increase in the incorporation of radio-active precursors
could be the sign of increased turnover only. In their very complete
study of Vicia faba, Woodard et al. have shown by cytophotometric
measurements of R N A (coloured by azure B) that there is also a net
increase of this substance. Little direct evidence of this kind is available
for vertebrate cells, but it is reasonable to assume a somewhat similar
situation. W e have mentioned the accumulation of R N A on chromosomes during prophase and the block somewhere near this stage when
ribonuclease is added. A pre-prophasic synthesis of R N A is probably
of general occurrence in the preparation for division. But this should be
controlled more directly.
Another process occurring probably during G 2 is the accumulation of
chemical energy for mitosis. That such accumulation is necessary has
been demonstrated in sea urchin eggs by Swann (1954) by the effect of
carbon monoxide intoxication: the next cleavage is delayed for exactly
the same length of time as the poison has been administered. Swann
interpreted this result as meaning that some "energy reservoir"
has to be filled, at a constant rate, before the onset of division. Once
filled, the reservoir empties and mitosis proceeds. If C O is applied after
a definite point in the cycle (probably the beginning of prophase), the
cleavage following immediately is not affected, but the delay is related
H . F I R K E T
(c) The G 2 period. It has been also variously named preprophase or
antephase. The cell is now preparing for division. The duration of this
phase has been found to be fairly constant for a given type of cell
though there may be considerable variation of the intensity of growth
(Firket, 1958a; Sisken and Kinosita, 1961b). However, Painter and
Drew (1959) mention individual deviations. It may be much shorter in
vivo than in vitro: Cronkite, Fliedner, Bond, Rubini, Brecher and
Quastler (1958) mention delays as short as 30 min after injection of
thymidine for obtaining labelled mitoses in mouse haemopoietic or
intestine cells. This gives a minimum time for G 2 .
Most of the information on the processes characteristic of this period
is obtained by indirect means. A number of physical or chemical
agents bring about an immediate or rapid prevention of entry into
mitosis. Hypotheses as to the nature of their interference can only be
deduced from their known biochemical effects on isolated systems.
There is some danger in relying too much on this type of reasoning, but
it can sometimes be correlated with the evidence obtained on materials
where the G 2 period is more readily identifiable than in vertebrate cells.
In root tips of Viciafaba (Woodard, Rasch and Swift, 1961), in yeast
(Williamson and Scopes, 1960), in Protozoa (Prescott, 1960; Woodard,
Gelber and Swift, 1961) and in other organisms, G 2 is characterized by
an increase in R N A synthesis, especially in the nucleus. Experiments
showing an increase in the incorporation of radio-active precursors
could be the sign of increased turnover only. In their very complete
study of Vicia faba, Woodard et al. have shown by cytophotometric
measurements of R N A (coloured by azure B) that there is also a net
increase of this substance. Little direct evidence of this kind is available
for vertebrate cells, but it is reasonable to assume a somewhat similar
situation. W e have mentioned the accumulation of R N A on chromosomes during prophase and the block somewhere near this stage when
ribonuclease is added. A pre-prophasic synthesis of R N A is probably
of general occurrence in the preparation for division. But this should be
controlled more directly.
Another process occurring probably during G 2 is the accumulation of
chemical energy for mitosis. That such accumulation is necessary has
been demonstrated in sea urchin eggs by Swann (1954) by the effect of
carbon monoxide intoxication: the next cleavage is delayed for exactly
the same length of time as the poison has been administered. Swann
interpreted this result as meaning that some "energy reservoir"
has to be filled, at a constant rate, before the onset of division. Once
filled, the reservoir empties and mitosis proceeds. If C O is applied after
a definite point in the cycle (probably the beginning of prophase), the
cleavage following immediately is not affected, but the delay is related