6. C E L L D I V I S I O N
207
filaments change into short, thick and more dense chromosomes at
metaphase and return to the elongated form at telophase.
The state of the cytoplasm conditions that of the chromosomes. It is
a common observation to see two nuclei included in the same cytoplasm going through mitosis at the same pace. If one or several chromosomes are separated from the main group, they usually undergo
anaphasic and telophasic changes and get surrounded by a new membrane at the same time as the rest, forming a small satellite nucleus.
The nature of this cytoplasmic influence is unknown.
FIG. 1. Part of a chick fibroblast metaphase stained by the alkaline phosphatase test.
Longitudinal splitting and banding (or coiling) of some chromosomes are barely visible,
x 4740. (Chevremont and Firket, 1949.)
Other cyclic changes affect the chromosomes. Fibroblast chromosomes contain a not inconsiderable amount of ribonucleic acid (RNA)
from prophase to anaphase. The observations of Jacobson and Webb
(1952) and Boss (1955) on Giemsa-stained chromosomes of fibroblasts
in culture are entirely confirmed by the measurements of u.v. absorption
(Davies, 1952). During anaphase, R N A seems to be released between
the two groups of chromosomes. This R N A is not synthesized during
division (Feinendegen, Bond, Shreeve and Painter, 1960; Prescott
and Bender, 1962); neither does it come from the nucleoli, as it
appears on the chromosomes before the nucleoli start to fade (see
also p. 217). It is probably a condensation of chromatin R N A .
One can only speculate about its function. When ribonuclease is
added to the same type of cells, a large percentage of the cells is
eventually blocked in a "pseudoprophasic" stage (Fig. 2) (Chevremont,
Chevremont and Firket, 1956). A temperature of 44°C for 5 h,
increases R N A catabolism and has a similar effect (Firket, 1958a).
These pseudoprophasic nuclei have the same amount of D N A as that
found normally in prophase (i.e. twice the usual quantity). They are
apparently unable to proceed to division. These results, and others, on
the effect of kinetin on plant cells (Guttman, 1956) are not unfavourable
to the hypothesis, first put forward by Serra (1947) that the R N A
207
filaments change into short, thick and more dense chromosomes at
metaphase and return to the elongated form at telophase.
The state of the cytoplasm conditions that of the chromosomes. It is
a common observation to see two nuclei included in the same cytoplasm going through mitosis at the same pace. If one or several chromosomes are separated from the main group, they usually undergo
anaphasic and telophasic changes and get surrounded by a new membrane at the same time as the rest, forming a small satellite nucleus.
The nature of this cytoplasmic influence is unknown.
FIG. 1. Part of a chick fibroblast metaphase stained by the alkaline phosphatase test.
Longitudinal splitting and banding (or coiling) of some chromosomes are barely visible,
x 4740. (Chevremont and Firket, 1949.)
Other cyclic changes affect the chromosomes. Fibroblast chromosomes contain a not inconsiderable amount of ribonucleic acid (RNA)
from prophase to anaphase. The observations of Jacobson and Webb
(1952) and Boss (1955) on Giemsa-stained chromosomes of fibroblasts
in culture are entirely confirmed by the measurements of u.v. absorption
(Davies, 1952). During anaphase, R N A seems to be released between
the two groups of chromosomes. This R N A is not synthesized during
division (Feinendegen, Bond, Shreeve and Painter, 1960; Prescott
and Bender, 1962); neither does it come from the nucleoli, as it
appears on the chromosomes before the nucleoli start to fade (see
also p. 217). It is probably a condensation of chromatin R N A .
One can only speculate about its function. When ribonuclease is
added to the same type of cells, a large percentage of the cells is
eventually blocked in a "pseudoprophasic" stage (Fig. 2) (Chevremont,
Chevremont and Firket, 1956). A temperature of 44°C for 5 h,
increases R N A catabolism and has a similar effect (Firket, 1958a).
These pseudoprophasic nuclei have the same amount of D N A as that
found normally in prophase (i.e. twice the usual quantity). They are
apparently unable to proceed to division. These results, and others, on
the effect of kinetin on plant cells (Guttman, 1956) are not unfavourable
to the hypothesis, first put forward by Serra (1947) that the R N A