6. C E L L D I V I S I O N
213
interest of this is that it seems to be due to inhibition of a phosphatase,
both effects appearing at the same dosage and being relieved simultaneously by an excess of magnesium (Chevremont and Firket, 1952a,
b). In spite of some uncertainty about the exact localization of this
enzyme (see Chevremont and Firket, 1953, for a discussion), there is no
doubt that its amount in cultures is proportional to the intensity of
growth (Chevremont and Firket, 1949). Continuous observations of the
living cells and films have often shown that after prolonged and frustrated attempts to separate the chromosomes, a single large nucleus is
suddenly reconstructed (Fig. 6).
Such cells may probably make further attempts to divide after
another doubling of their D N A as demonstrated by the increased
percentage of nuclei containing the tetraploid or higher amount of
DNA (Firket, 1958a). Similar progress towards tetraploidy and higher
ploidy is described by Bloch (1953) and Hsu and Kellog (1960) in
colchicine-treated cultures. It illustrates the independence of the D N A
duplication from the spindle mechanism and also that a cell, unable to
divide because of the failure of one highly specialized organelle, will try
again if the preparation for mitosis has been completed.
In studying the action of poisons on the spindle, we think a distinction should be made between absence of formation of the mitotic
apparatus and inhibition of its normal performance. In the sea urchin
the spindle arises from a reorientation of pre-existing proteins that can
be identified in the cell during interphase. It probably does not do so in
vertebrate somatic cells. When chloramphenicol, an inhibitor of protein
synthesis is added to newt cultures (E. W . Taylor, 1959), or when the
cytoplasm alone is irradiated by a u.v. beam before prophase (Bloom
et al., 1955), no spindle is formed. There is also a definite difference in
the response to colchicine. In somatic cells, only metaphase block is
observed. In cleaving eggs, the cell block may occur at any stage of the
nuclear cycle (Sentein, 1961).
Contrary to this view, Lettre and Lettre (1958) put forward the
hypothesis that spindle filaments connect the centromeres to the
centrioles not only during mitosis but permanently, throughout the
interphase. It seems difficult to reconcile such an idea with the fact that,
in several vertebrate cell types at least the spindle appears first at the
end of prophase in the cytoplasm on one side of the nucleus (see above).
If inadequate, this view of the Lettres has the advantage of putting
into light again one of the most difficult problems of spindle physiology:
how is the connection between spindle fibres and centromeres effected
so that sister chromatids will separate in an orderly manner? In spite
of many speculations over several decades, we have as yet no satisfactory
answer to this question.
213
interest of this is that it seems to be due to inhibition of a phosphatase,
both effects appearing at the same dosage and being relieved simultaneously by an excess of magnesium (Chevremont and Firket, 1952a,
b). In spite of some uncertainty about the exact localization of this
enzyme (see Chevremont and Firket, 1953, for a discussion), there is no
doubt that its amount in cultures is proportional to the intensity of
growth (Chevremont and Firket, 1949). Continuous observations of the
living cells and films have often shown that after prolonged and frustrated attempts to separate the chromosomes, a single large nucleus is
suddenly reconstructed (Fig. 6).
Such cells may probably make further attempts to divide after
another doubling of their D N A as demonstrated by the increased
percentage of nuclei containing the tetraploid or higher amount of
DNA (Firket, 1958a). Similar progress towards tetraploidy and higher
ploidy is described by Bloch (1953) and Hsu and Kellog (1960) in
colchicine-treated cultures. It illustrates the independence of the D N A
duplication from the spindle mechanism and also that a cell, unable to
divide because of the failure of one highly specialized organelle, will try
again if the preparation for mitosis has been completed.
In studying the action of poisons on the spindle, we think a distinction should be made between absence of formation of the mitotic
apparatus and inhibition of its normal performance. In the sea urchin
the spindle arises from a reorientation of pre-existing proteins that can
be identified in the cell during interphase. It probably does not do so in
vertebrate somatic cells. When chloramphenicol, an inhibitor of protein
synthesis is added to newt cultures (E. W . Taylor, 1959), or when the
cytoplasm alone is irradiated by a u.v. beam before prophase (Bloom
et al., 1955), no spindle is formed. There is also a definite difference in
the response to colchicine. In somatic cells, only metaphase block is
observed. In cleaving eggs, the cell block may occur at any stage of the
nuclear cycle (Sentein, 1961).
Contrary to this view, Lettre and Lettre (1958) put forward the
hypothesis that spindle filaments connect the centromeres to the
centrioles not only during mitosis but permanently, throughout the
interphase. It seems difficult to reconcile such an idea with the fact that,
in several vertebrate cell types at least the spindle appears first at the
end of prophase in the cytoplasm on one side of the nucleus (see above).
If inadequate, this view of the Lettres has the advantage of putting
into light again one of the most difficult problems of spindle physiology:
how is the connection between spindle fibres and centromeres effected
so that sister chromatids will separate in an orderly manner? In spite
of many speculations over several decades, we have as yet no satisfactory
answer to this question.