324
J. SEED
was true also in human bone-marrow cells in culture (see Vol. 2,
Chapter 3). A relatively long delay was found to occur after telophase
before the start of D N A synthesis, and there was a shorter delay after
completion of synthesis and before the onset of prophase.
Subsequently, Richards and collaborators (Richards, 1955; Richards,
Walker and Deeley, 1956) made photometric measurements of Feulgenpositive material in individual cells of both embryo-mouse fibroblasts
in vitro and mouse ascites tumour cells in vivo, and from the distributions
of values obtained for the cell populations they deduced that in both
cell types D N A synthesis occurred late in interphase, thus confirming
the earlier work of Swift (1950a, b ) . Moreover, the accurate measurements of Richards et al. (1956) showed in the ascites tumour cells there
was an approximately linear relation between the number of chromosomes per cell and the amount of Feulgen stain measured photometrically. Later, a distribution diagram of Feulgen stain values, again
indicating the synthesis of D N A during interphase was obtained for
HeLa cells in culture by Smith, Newton and Wildy (1959).
In more recent autoradiographic studies, tritiated thymidine has
been used as a precursor specific for D N A synthesis; incorporation into
D N A is almost instantaneous and the pool size of free precursor within
the cell is very small (Taylor, Woods and Hughes, 1957; Hughes, Bond,
Brecher, Cronkite, Painter, Quastler and Sherman, 1958; Feinendegen,
1961). Because of the short range of the tritium /2-particle ( ~ l / x ) , a
high resolution is obtainable in tritium autoradiographs, and as would
be expected from our knowledge of the localization of cellular DNA,
the thymidine incorporation is found to be entirely in the nucleus,
within the limits of accuracy of the autoradiographic observations.
Firket and Verly (1958) incubated hanging-drop cultures of chick
fibroblasts in
3
H-thymidine and examined the frequency of labelled
mitotic figures after various exposure times. From the results, they
concluded that D N A synthesis occurred during only a part of interphase, and that there was a gap between the end of synthesis and the
onset of mitosis, in agreement with the work already quoted (see
also Chapter 6 and Firket, 1958). Observations of a similar nature on
the D N A cycle have been made by Painter and Drew (1959) for HeLa
cells; by Harris (1959a) for rat heart-fibroblasts; and by Stanners
and Till (1960) for L-strain mouse cells in culture. It has also been
shown, by using time-lapse photography of cell cultures to provide
cells of known ages in interphase, that in monkey kidney cells, embryo
human fibroblasts, embryo mouse fibroblasts, HeLa cells and L-strain
cells, the Feulgen-stain photometric and thymidine-autoradiographic
methods have given equivalent results on the timing of the D N A cycle
(Seed, 1962).
J. SEED
was true also in human bone-marrow cells in culture (see Vol. 2,
Chapter 3). A relatively long delay was found to occur after telophase
before the start of D N A synthesis, and there was a shorter delay after
completion of synthesis and before the onset of prophase.
Subsequently, Richards and collaborators (Richards, 1955; Richards,
Walker and Deeley, 1956) made photometric measurements of Feulgenpositive material in individual cells of both embryo-mouse fibroblasts
in vitro and mouse ascites tumour cells in vivo, and from the distributions
of values obtained for the cell populations they deduced that in both
cell types D N A synthesis occurred late in interphase, thus confirming
the earlier work of Swift (1950a, b ) . Moreover, the accurate measurements of Richards et al. (1956) showed in the ascites tumour cells there
was an approximately linear relation between the number of chromosomes per cell and the amount of Feulgen stain measured photometrically. Later, a distribution diagram of Feulgen stain values, again
indicating the synthesis of D N A during interphase was obtained for
HeLa cells in culture by Smith, Newton and Wildy (1959).
In more recent autoradiographic studies, tritiated thymidine has
been used as a precursor specific for D N A synthesis; incorporation into
D N A is almost instantaneous and the pool size of free precursor within
the cell is very small (Taylor, Woods and Hughes, 1957; Hughes, Bond,
Brecher, Cronkite, Painter, Quastler and Sherman, 1958; Feinendegen,
1961). Because of the short range of the tritium /2-particle ( ~ l / x ) , a
high resolution is obtainable in tritium autoradiographs, and as would
be expected from our knowledge of the localization of cellular DNA,
the thymidine incorporation is found to be entirely in the nucleus,
within the limits of accuracy of the autoradiographic observations.
Firket and Verly (1958) incubated hanging-drop cultures of chick
fibroblasts in
3
H-thymidine and examined the frequency of labelled
mitotic figures after various exposure times. From the results, they
concluded that D N A synthesis occurred during only a part of interphase, and that there was a gap between the end of synthesis and the
onset of mitosis, in agreement with the work already quoted (see
also Chapter 6 and Firket, 1958). Observations of a similar nature on
the D N A cycle have been made by Painter and Drew (1959) for HeLa
cells; by Harris (1959a) for rat heart-fibroblasts; and by Stanners
and Till (1960) for L-strain mouse cells in culture. It has also been
shown, by using time-lapse photography of cell cultures to provide
cells of known ages in interphase, that in monkey kidney cells, embryo
human fibroblasts, embryo mouse fibroblasts, HeLa cells and L-strain
cells, the Feulgen-stain photometric and thymidine-autoradiographic
methods have given equivalent results on the timing of the D N A cycle
(Seed, 1962).