6. C E L L D I V I S I O N
217
material of the nucleus and the cytoplasm, the nucleolus remains the
most mysterious of the cell organelles. Its behaviour at mitosis does not
throw much light on its functions. Where does its substance go when it
fades at the end of prophase? Is this fading essential to the division
mechanism? Is its R N A a precursor ofthe chromosomal R N A , or is the
reverse true? These questions can only be partly answered (see note
added in proof, p. 238).
When cobalt salts in low concentration are added to fibroblasts in
culture, this anion accumulates on the nucleolus. This organelle does
not then disappear at prophase, but is retained for several hours in one
or several pieces in the cytoplasm of one or both daughter cells. In
spite of this, division proceeds without hindrance, as it does in several
plant tissues where nucleolar retention is occurring normally. In
vertebrate tissue cultures, however, these nucleolar fragments do not
seem to be functional, as new nucleoli are formed among the chromosomes ofthe daughter fibroblasts (Heath, 1954).
In contrast to these experiments, other observations suggest that the
reappearance of nucleoli at the nucleolar organizer sites of some
chromosomes at telophase is not a synthesis of entirely new material.
Recently, in fibroblast cultures, Guttes and Guttes (1961) stained by
silver a nucleolar component (already described elsewhere by Estable
and Sotelo, 1954) dispersed among the chromosomes at the end of
prophase and reaggregated at telophase. Labelling with radioactive
amino acids suggests that such material is a protein (Harris, 1959, 1961).
Destruction of the nucleoli of HeLa cells by u.v. microbeam reduces
the amount of nucleolar R N A (Montgomery and Hundley, 1961) and
also the incorporation of cytidine in the cytoplasm (Perry, 1960), but no
information on mitosis was obtained in these experiments, probably
because no cell so attacked could recover sufficiently to undertake a
division. A similar irradiation in interphase stops mitosis in grasshopper
neuroblasts. During prophase, it is less effective (Gaulden and Perry,
1958). The evidence points towards a function of this organelle in cell
division or, at least, in the preparation for division. Whatever it is, it
must be over as such by the end of prophase, when the nucleolus
normally disappears.
5 . Mitochondria
Many experiments show that, without intact mitochondria, mitosis
does not begin. When fibroblasts are maintained at 16-20°C, the
mitochondria become globular. If the cells are brought back to 37°C,
mitosis takes place only after they have recovered their normal filamentous shape (Chevremont, Firket, Chevremont and Frederic, 1957).
When they are modified by various other abnormal conditions (ATP,
217
material of the nucleus and the cytoplasm, the nucleolus remains the
most mysterious of the cell organelles. Its behaviour at mitosis does not
throw much light on its functions. Where does its substance go when it
fades at the end of prophase? Is this fading essential to the division
mechanism? Is its R N A a precursor ofthe chromosomal R N A , or is the
reverse true? These questions can only be partly answered (see note
added in proof, p. 238).
When cobalt salts in low concentration are added to fibroblasts in
culture, this anion accumulates on the nucleolus. This organelle does
not then disappear at prophase, but is retained for several hours in one
or several pieces in the cytoplasm of one or both daughter cells. In
spite of this, division proceeds without hindrance, as it does in several
plant tissues where nucleolar retention is occurring normally. In
vertebrate tissue cultures, however, these nucleolar fragments do not
seem to be functional, as new nucleoli are formed among the chromosomes ofthe daughter fibroblasts (Heath, 1954).
In contrast to these experiments, other observations suggest that the
reappearance of nucleoli at the nucleolar organizer sites of some
chromosomes at telophase is not a synthesis of entirely new material.
Recently, in fibroblast cultures, Guttes and Guttes (1961) stained by
silver a nucleolar component (already described elsewhere by Estable
and Sotelo, 1954) dispersed among the chromosomes at the end of
prophase and reaggregated at telophase. Labelling with radioactive
amino acids suggests that such material is a protein (Harris, 1959, 1961).
Destruction of the nucleoli of HeLa cells by u.v. microbeam reduces
the amount of nucleolar R N A (Montgomery and Hundley, 1961) and
also the incorporation of cytidine in the cytoplasm (Perry, 1960), but no
information on mitosis was obtained in these experiments, probably
because no cell so attacked could recover sufficiently to undertake a
division. A similar irradiation in interphase stops mitosis in grasshopper
neuroblasts. During prophase, it is less effective (Gaulden and Perry,
1958). The evidence points towards a function of this organelle in cell
division or, at least, in the preparation for division. Whatever it is, it
must be over as such by the end of prophase, when the nucleolus
normally disappears.
5 . Mitochondria
Many experiments show that, without intact mitochondria, mitosis
does not begin. When fibroblasts are maintained at 16-20°C, the
mitochondria become globular. If the cells are brought back to 37°C,
mitosis takes place only after they have recovered their normal filamentous shape (Chevremont, Firket, Chevremont and Frederic, 1957).
When they are modified by various other abnormal conditions (ATP,