322
J. S E E D
now be appropriate to consider some of the contributions o f Tissue Culture to the subject. Most of the work on nucleic-acid metabolism in cell
cultures has been carried out with rapidly dividing cells rather than with
"differentiated" cells, and used in this way cell cultures have two great
advantages. The first is that cultures can be prepared in which it can
be verified that over 90 °/o of the cells under observation are in a cycle
leading to a further mitosis, a situation not readily obtained in
other living systems. For example, even in such a rapidly growing
system as regenerating liver, only a small percentage o f the total
population may be proceeding to a further division; the presence of a
major fraction of non-dividing cells can, of course, complicate appreciably the interpretation of experiments carried out on such a system.
The second advantage of cell cultures is that, in mono-layer cultures,
measurements (for example of D N A content) can readily be made on
single cells, an approach which enables one to obtain information
about the synthetic cycle not easily acquired by normal, large-scale
biochemical methods.
The relative metabolic stability of D N A in rapidly growing cell
cultures was demonstrated by Thomson and Paul (1957) and Thomson,
Paul and Davidson (1956, 1958). L-Strain mouse cells were cultured
in the presence of
1 4
C formate and then allowed to grow in nonradioactive medium for eight generations. Cell samples were removed
at intervals and the isotope contents of the D N A and R N A examined.
The results indicated that D N A was relatively stable, whereas at least
a part of the R N A , in this cell type, appeared to suffer breakdown and
to be replaced by newly synthesized products. Graham and Siminovitch
(1957), using
3 2
P as tracer, reported similar results for L-cells in the
logarithmic phase of growth: the D N A activity was found to remain
constant throughout the experiment, whereas that of the R N A -
3 2
P
decreased by 10-30% during the first 24 h.
M u c h of our knowledge of the timing of the cycle of D N A synthesis
stems from measurements on single cells. It was first demonstrated by
Swift (1950a, b ; see also Alfert, 1950) that the spread of values observed
in photometric measurements of the amounts of Feulgen stain for
individual cell nuclei in a dividing tissue could be explained in terms
of the synthesis of D N A during interphase. A plot of the distribution of
DNA-values per cell showed a peak at the post-telophase amount of
D N A , corresponding to the high probability of finding cells in the
relatively long pre-synthetic period, and a smaller peak corresponding to
the pre-prophase amount, together with a number of intermediate
values appropriate to cells actually engaged in D N A synthesis. Cells
in prophase, in common with the larger cells still in interphase, were
found to have twice the D N A content of cells in telophase. Fig. 1
J. S E E D
now be appropriate to consider some of the contributions o f Tissue Culture to the subject. Most of the work on nucleic-acid metabolism in cell
cultures has been carried out with rapidly dividing cells rather than with
"differentiated" cells, and used in this way cell cultures have two great
advantages. The first is that cultures can be prepared in which it can
be verified that over 90 °/o of the cells under observation are in a cycle
leading to a further mitosis, a situation not readily obtained in
other living systems. For example, even in such a rapidly growing
system as regenerating liver, only a small percentage o f the total
population may be proceeding to a further division; the presence of a
major fraction of non-dividing cells can, of course, complicate appreciably the interpretation of experiments carried out on such a system.
The second advantage of cell cultures is that, in mono-layer cultures,
measurements (for example of D N A content) can readily be made on
single cells, an approach which enables one to obtain information
about the synthetic cycle not easily acquired by normal, large-scale
biochemical methods.
The relative metabolic stability of D N A in rapidly growing cell
cultures was demonstrated by Thomson and Paul (1957) and Thomson,
Paul and Davidson (1956, 1958). L-Strain mouse cells were cultured
in the presence of
1 4
C formate and then allowed to grow in nonradioactive medium for eight generations. Cell samples were removed
at intervals and the isotope contents of the D N A and R N A examined.
The results indicated that D N A was relatively stable, whereas at least
a part of the R N A , in this cell type, appeared to suffer breakdown and
to be replaced by newly synthesized products. Graham and Siminovitch
(1957), using
3 2
P as tracer, reported similar results for L-cells in the
logarithmic phase of growth: the D N A activity was found to remain
constant throughout the experiment, whereas that of the R N A -
3 2
P
decreased by 10-30% during the first 24 h.
M u c h of our knowledge of the timing of the cycle of D N A synthesis
stems from measurements on single cells. It was first demonstrated by
Swift (1950a, b ; see also Alfert, 1950) that the spread of values observed
in photometric measurements of the amounts of Feulgen stain for
individual cell nuclei in a dividing tissue could be explained in terms
of the synthesis of D N A during interphase. A plot of the distribution of
DNA-values per cell showed a peak at the post-telophase amount of
D N A , corresponding to the high probability of finding cells in the
relatively long pre-synthetic period, and a smaller peak corresponding to
the pre-prophase amount, together with a number of intermediate
values appropriate to cells actually engaged in D N A synthesis. Cells
in prophase, in common with the larger cells still in interphase, were
found to have twice the D N A content of cells in telophase. Fig. 1