6 . C E L L D I V I S I O N
225
to the next one, probably indicating that the reservoir begins to refill
immediately after its energy starts to be spent. In this case, the energy
gathering occurs almost during the whole cycle, which is very short.
In spite of various attempts to identify it, the nature of this chemical
energy remains unknown.
No clear-cut experiment of this kind is yet possible on cells dividing
asynchronously, as they normally do in culture. However, the rather
constant duration of G 2 also suggests that a more or less fixed amount of
some substance has to be accumulated before mitosis can begin. It is also
remarkable that numerous substances and conditions affecting the
energy systems of the cell, such as cyanide (Pomerat and Willmer, 1939),
anaerobiosis (S. Chevremont and Frederic, 1955), fluoride, iodoacetate
(Hughes, 1950), etc., eventually prevent entry into division, but have
little effect on the mitotic process once begun. Many of the substances
active at this stage, affect mitochondria and we have already stressed
the importance of an intact chondriome for division to begin (see p. 217).
At the end of G 2 , in cultured vertebrate cells at least, dry mass (due
essentially to protein) reaches a maximum and some loss of water
occurs 30 min to 1 h before division (see p. 220).
Just before mitosis, the cell is certainly in a special metabolic state
which cannot yet be completely defined. It probably includes the
following: the number of deoxyribonucleoprotein strands in the
chromosomes has been doubled, R N A has accumulated especially in the
nucleus; some form of chemical energy store has been filled; protein
mass is at its maximum; a small contraction of volume, eliminating
water, has finally brought the cell content to a high concentration.
Then, and only then, can begin the chromosome ballet which has been
so fascinating to the eyes of cytologists for the last eighty years.
D. G E N E R A L C O N S I D E R A T I O N S
This description of the metabolic changes in the dividing cell in
culture is very crude and the attempt perhaps premature as most of the
information about the cycle comes from other types of cells (Prescott,
1961). That the metabolism of the cell is differently orientated at the
different stages in the cycle should not be doubted, however, and there
is a need to analyse these changes more completely.
In spite of its convenience, the notion of the cell cycle should not be
taken too absolutely. W e will mention conditions where the sequence
of events is modified (see p. 227). All the preparatory events for one
division do not necessarily occur in the period between the preceding
telophase and the prophase: some may have occurred earlier; some
may already be preparing a further division. If methionine is omitted
from the medium where Tetrahymena grow, they can divide once more,
i
225
to the next one, probably indicating that the reservoir begins to refill
immediately after its energy starts to be spent. In this case, the energy
gathering occurs almost during the whole cycle, which is very short.
In spite of various attempts to identify it, the nature of this chemical
energy remains unknown.
No clear-cut experiment of this kind is yet possible on cells dividing
asynchronously, as they normally do in culture. However, the rather
constant duration of G 2 also suggests that a more or less fixed amount of
some substance has to be accumulated before mitosis can begin. It is also
remarkable that numerous substances and conditions affecting the
energy systems of the cell, such as cyanide (Pomerat and Willmer, 1939),
anaerobiosis (S. Chevremont and Frederic, 1955), fluoride, iodoacetate
(Hughes, 1950), etc., eventually prevent entry into division, but have
little effect on the mitotic process once begun. Many of the substances
active at this stage, affect mitochondria and we have already stressed
the importance of an intact chondriome for division to begin (see p. 217).
At the end of G 2 , in cultured vertebrate cells at least, dry mass (due
essentially to protein) reaches a maximum and some loss of water
occurs 30 min to 1 h before division (see p. 220).
Just before mitosis, the cell is certainly in a special metabolic state
which cannot yet be completely defined. It probably includes the
following: the number of deoxyribonucleoprotein strands in the
chromosomes has been doubled, R N A has accumulated especially in the
nucleus; some form of chemical energy store has been filled; protein
mass is at its maximum; a small contraction of volume, eliminating
water, has finally brought the cell content to a high concentration.
Then, and only then, can begin the chromosome ballet which has been
so fascinating to the eyes of cytologists for the last eighty years.
D. G E N E R A L C O N S I D E R A T I O N S
This description of the metabolic changes in the dividing cell in
culture is very crude and the attempt perhaps premature as most of the
information about the cycle comes from other types of cells (Prescott,
1961). That the metabolism of the cell is differently orientated at the
different stages in the cycle should not be doubted, however, and there
is a need to analyse these changes more completely.
In spite of its convenience, the notion of the cell cycle should not be
taken too absolutely. W e will mention conditions where the sequence
of events is modified (see p. 227). All the preparatory events for one
division do not necessarily occur in the period between the preceding
telophase and the prophase: some may have occurred earlier; some
may already be preparing a further division. If methionine is omitted
from the medium where Tetrahymena grow, they can divide once more,
i