330
J. SEED
In the experiments cited above it has sometimes been assumed that
appreciable net amounts of new R N A are accumulating in the nucleus
during all parts of the interphase of rapidly dividing cells. Because of
the turnover problem, this assumption cannot readily be verified by the
use of labelled precursors only, and a method is required for the
measurement of the relative amounts of R N A at various stages of
interphase. Some relevant experiments are discussed in the following
section.
I V . T H E R E L A T I O N S B E T W E E N D N A ,
R N A
A N D
N U C L E A R P R O T E I N D U R I N G T H E I N T E R P H A S E OF R A P I D L Y
D I V I D I N G C E L L S
A. E X P E R I M E N T S W I T H C E L L C U L T U R E S
Although many qualitative observations have associated the nucleic
acids with protein synthesis (for a review, see Brachet, 1960), little
evidence of a quantitative nature has been obtained for the proposed
relations during the interphase of rapidly dividing cells of higher
organisms. One reason for this is the difficulty of carrying out the
experiments by normal large scale biochemical methods. For such
experiments one needs a synchronously dividing cell population, which
is not readily obtained with cells of higher organisms. Although some
attempts have been made in this direction with mammalian cell lines
(Newton and Wildy, 1959; Rueckert and Mueller, 1960; Terasima
and Tolmach, 1961), it is difficult to be sure that the production of
synchrony does not introduce to the cell cycle some features not normally
present in a randomly growing population. For example in synchronized
populations of HeLa cells, Newton and Wildy (1959) observed an early
increase in D N A just after telophase, which is not normally demonstrable by cytochemical techniques in randomly growing HeLa cell
populations. Another example of this pitfall is found with synchronized
thymine-deficient populations of E. coli, where D N A synthesis proceeds
as a discrete step; however, in randomly growing E. coli, D N A synthesis
appears to proceed almost continuously throughout the generation
time (Schaechter, Bentzon and Maaloe, 1959). These authors point out
the great caution required in interpreting experiments on synchronized populations in terms of the normal division cycle. The use of
regenerating liver to provide a partially synchronized cell population
is, of course, open to the additional serious objection already quoted
(Section 2) that the presence of a major fraction of non-dividing cells
may appreciably complicate any interpretation of measurements on
R N A and protein synthesis in this system.
J. SEED
In the experiments cited above it has sometimes been assumed that
appreciable net amounts of new R N A are accumulating in the nucleus
during all parts of the interphase of rapidly dividing cells. Because of
the turnover problem, this assumption cannot readily be verified by the
use of labelled precursors only, and a method is required for the
measurement of the relative amounts of R N A at various stages of
interphase. Some relevant experiments are discussed in the following
section.
I V . T H E R E L A T I O N S B E T W E E N D N A ,
R N A
A N D
N U C L E A R P R O T E I N D U R I N G T H E I N T E R P H A S E OF R A P I D L Y
D I V I D I N G C E L L S
A. E X P E R I M E N T S W I T H C E L L C U L T U R E S
Although many qualitative observations have associated the nucleic
acids with protein synthesis (for a review, see Brachet, 1960), little
evidence of a quantitative nature has been obtained for the proposed
relations during the interphase of rapidly dividing cells of higher
organisms. One reason for this is the difficulty of carrying out the
experiments by normal large scale biochemical methods. For such
experiments one needs a synchronously dividing cell population, which
is not readily obtained with cells of higher organisms. Although some
attempts have been made in this direction with mammalian cell lines
(Newton and Wildy, 1959; Rueckert and Mueller, 1960; Terasima
and Tolmach, 1961), it is difficult to be sure that the production of
synchrony does not introduce to the cell cycle some features not normally
present in a randomly growing population. For example in synchronized
populations of HeLa cells, Newton and Wildy (1959) observed an early
increase in D N A just after telophase, which is not normally demonstrable by cytochemical techniques in randomly growing HeLa cell
populations. Another example of this pitfall is found with synchronized
thymine-deficient populations of E. coli, where D N A synthesis proceeds
as a discrete step; however, in randomly growing E. coli, D N A synthesis
appears to proceed almost continuously throughout the generation
time (Schaechter, Bentzon and Maaloe, 1959). These authors point out
the great caution required in interpreting experiments on synchronized populations in terms of the normal division cycle. The use of
regenerating liver to provide a partially synchronized cell population
is, of course, open to the additional serious objection already quoted
(Section 2) that the presence of a major fraction of non-dividing cells
may appreciably complicate any interpretation of measurements on
R N A and protein synthesis in this system.