6. C E L L D I V I S I O N
227
irradiation is given before D N A synthesis has set in. However, this
third effect has not been found in the accurate counts of cultured human
U-12 fibroblasts (Harrington, 1960) or of HeLa cells (Painter and
Robertson, 1959). Difficulties of interpretation, differences between
strains (Stroud, Brues and Svoboda, 1961) and the effects of the
additional irradiation by the labelled precursor used in these experiments (Drew and Painter, 1959; Looney, 1961) may well account for
some discrepancies.
Similar differences in the importance of chromosome damage are
observed when cells are irradiated at different moments during the cell
cycle (Hsu, Dewey and Humphrey, 1962).
I V . M I T O G E N E S I S
When we look at a living culture there is no more puzzling problem
than to find that, from a large number of apparently identical cells, all
genetically similar and placed in a homogeneous medium, at best only
a few per cent divide at any one time. O f course, as we can imagine from
what we have just said, each individual history is different and the
various cells are at different points in their cycle. But this is not a
sufficient explanation for it is known that some go quickly through the
cycle, others more slowly or not at all.
For a long time a search has gone on for the event that leads a cell to
division, for the trigger of mitosis. The finding of such a trigger and of
specific means of action on it would enable us to promote or inhibit
division at will, perhaps without interfering with the normal physiology
of the cell. It is, in fact, a false problem. The large number of preparatory events leading to mitosis are all necessary, but none are decisive.
After none of them, can we say that the cell is irreversibly bound to
divide, for it is always possible to stop the process, even after mitosis
has actually begun.
Further, the order in which these events occur, though usually the
same, is not a necessary order: they are not linked by rigid causal
relations. W e mentioned the fairly constant duration of the G 2 period
in cultures after the end of the S period of D N A synthesis and suggested that it may be due to the fact that a certain amount of some
metabolite has to be accumulated before division can begin. However,
by changing the temperature, it is possible to modify the sequence S, G 2 .
When cultures, growing actively at 37°, are cooled to 16° for 24 h,
DNA synthesis is stopped. If these cultures are brought back to 37°,
DNA synthesis occurs rapidly and, within 3 h, a spectacular wave of
mitoses follows with a very short interval between the end of S and the
227
irradiation is given before D N A synthesis has set in. However, this
third effect has not been found in the accurate counts of cultured human
U-12 fibroblasts (Harrington, 1960) or of HeLa cells (Painter and
Robertson, 1959). Difficulties of interpretation, differences between
strains (Stroud, Brues and Svoboda, 1961) and the effects of the
additional irradiation by the labelled precursor used in these experiments (Drew and Painter, 1959; Looney, 1961) may well account for
some discrepancies.
Similar differences in the importance of chromosome damage are
observed when cells are irradiated at different moments during the cell
cycle (Hsu, Dewey and Humphrey, 1962).
I V . M I T O G E N E S I S
When we look at a living culture there is no more puzzling problem
than to find that, from a large number of apparently identical cells, all
genetically similar and placed in a homogeneous medium, at best only
a few per cent divide at any one time. O f course, as we can imagine from
what we have just said, each individual history is different and the
various cells are at different points in their cycle. But this is not a
sufficient explanation for it is known that some go quickly through the
cycle, others more slowly or not at all.
For a long time a search has gone on for the event that leads a cell to
division, for the trigger of mitosis. The finding of such a trigger and of
specific means of action on it would enable us to promote or inhibit
division at will, perhaps without interfering with the normal physiology
of the cell. It is, in fact, a false problem. The large number of preparatory events leading to mitosis are all necessary, but none are decisive.
After none of them, can we say that the cell is irreversibly bound to
divide, for it is always possible to stop the process, even after mitosis
has actually begun.
Further, the order in which these events occur, though usually the
same, is not a necessary order: they are not linked by rigid causal
relations. W e mentioned the fairly constant duration of the G 2 period
in cultures after the end of the S period of D N A synthesis and suggested that it may be due to the fact that a certain amount of some
metabolite has to be accumulated before division can begin. However,
by changing the temperature, it is possible to modify the sequence S, G 2 .
When cultures, growing actively at 37°, are cooled to 16° for 24 h,
DNA synthesis is stopped. If these cultures are brought back to 37°,
DNA synthesis occurs rapidly and, within 3 h, a spectacular wave of
mitoses follows with a very short interval between the end of S and the