Bacterial Growth and Division
533
is a time during the division cycle at
which
gene
expression
is
prohibited.
This momentary cessation of transcription
should not obscure the more fundamental
point that there does not appear to be
any speciFc control of gene expression for
any gene during the division cycle that is
related to the regulation of that gene during
the division cycle. One should imagine this
cessation of gene expression as merely a
necessary accommodation of transcription
to the needs of the cell for DNA replication.
It could be imagined that if the cell
were able to replicate DNA without any
cessation of transcription, there would be
no reason to expect that the cell would
alter speciFc gene expression at any point
during the division cycle.
The evidence for a smooth, continuous,
and exponential increase in cytoplasm
comes from experiments that use the
membrane-elution method, colloquially
referred to as the
baby
machine
(see
below). The pattern of DNA synthesis
during the division cycle, which was
subsequently shown to be correct by flow-
cytometric and other analyses, was actually
discovered using the membrane-elution
method. Thus, the results from a method
capable of producing an accurate analysis
of biosynthesis during the division cycle
have conFrmed the exponential synthesis
of cytoplasm.
In addition to this experimental support,
an evolutionary argument can be made
for an invariant rate of accumulation of
cytoplasm during the division cycle. If
the synthetic rate of an enzyme changed
abruptly during the division cycle, there
would be a relative excess or deFciency
of that enzyme at some point during the
division cycle; this would be an inefFcient
use of resources. The optimal allocation of
resources is for each component to be at a
constant concentration during the division
cycle. Cells would not evolve controls that
make them less efFcient in cell production.
The ideal pattern of cytoplasm synthesis
is an invariant cytoplasm composition
during the division cycle.
A third argument supporting exponen-
tial cytoplasm synthesis is that only this
pattern is explained in known biochemical
terms. The cycle-independent exponential
synthesis of cytoplasm can be derived
from our current understanding of the
biochemistry of macromolecule synthesis.
Enzymes make RNA and protein, which
make ribosomes, which then lead to more
protein synthesis. More proteins mean
more RNA polymerases, more catabolic
and anabolic enzymes, and the continu-
ously increasing ability of the cell to make
more and more cytoplasm. In contrast, if
newly synthesized material were
not
active
or available for synthesis when made, but
instead was recruited for biosynthesis only
at the instant of division or at some spe-
ciFc cell-cycle age, one would have linear
cytoplasm synthesis during the division
cycle. There are no known mechanisms
enabling the cell to distinguish between
newly synthesized cytoplasm and old cyto-
plasm, or which lead to cell-cycle-speciFc
variation in the synthetic rate of any par-
ticular molecule during the division cycle.
We therefore conclude that in theory as
well as in practice, cytoplasm increases
uneventfully and exponentially during the
division cycle.
3.1.2
Variation in the Rate of Cytoplasm
Increase
The continuous variation in allowable
growth rates may be understood by parti-
tioning the cytoplasm into those molecules
involved in cytoplasm synthesis and that
are absolutely necessary for growth (ribo-
somes, RNA polymerases, etc.) and those
molecules that are involved in dispensable
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