532
Bacterial Growth and Division
other bacteria and their patterns of growth
and division with respect to this archetypal
pattern. The division cycle of
E. coli
will
be described, indicating how the various
components of the cell are made and
distributed into two new daughter cells.
3
Synthesis of Cell Components during the
Division Cycle
3.1
Cytoplasm Synthesis during the Division
Cycle
3.1.1
Exponential Cytoplasm Synthesis
during the Division Cycle
Consider a unit volume of cytoplasm.
It contains enzymes required for the
breakdown of nutrients, enzymes for the
production of energy for biosynthesis,
enzymes for the biosynthesis of low molec-
ular weight precursors of macromolecules,
and enzymes for the synthesis of macro-
molecules. Each unit of cytoplasm pro-
duces a small amount of new cytoplasm
over a small interval of time. If the new cy-
toplasm is indistinguishable from the old,
and if the new cytoplasm acts to synthe-
size more cytoplasm immediately, then the
pattern of cytoplasm synthesis is exponen-
tial. This is because after each interval of
synthesis, the rate of synthesis increases
owing to the new increment of material
added to the cytoplasm. This exponential
pattern is illustrated in Fig. 2 with both the
rate of synthesis and the amount of ma-
terial increasing exponentially during the
division cycle.
The pattern for cytoplasm as a whole
is
reflected
in
the
synthesis
of
the
individual cytoplasmic components. There
are no changes in the speci±c rate of
synthesis or pattern of cytoplasm synthesis
that are related to any particular cell-
cycle event. (Actually, there is a minor
caveat
to this generalization, which is
treated below.) No particular molecule of
the cytoplasm is made differently from
any other molecule of the cytoplasm
during the division cycle. In addition,
there is no variation, during the division
cycle, in the relative concentration of any
component of the cytoplasm. All parts of
the cytoplasm accumulate exponentially
during the division cycle, and this pattern
is unchanged even when the cell is
dividing. At the instant of division, the
combined rate of cytoplasm synthesis in
the two new daughter cells is precisely
equal to the synthetic rate in the dividing
mother cell.
One minor exception to the rule that
speci±c proteins are made exponentially
during the division cycle regards the effect
of momentary sequestration of DNA upon
replication. Speci±c regions of DNA are
sequestered in the membrane at different
times during the cell cycle at the time of
their replication. During this sequestration
period, the DNA is not available for
transcription. Thus, for all genes there
Fig. 2
Cytoplasm synthesis during the
division cycle. Both the rate of
cytoplasm synthesis and the pattern of
accumulation of cytoplasm are
exponential during the division cycle.
1
2
Cell age
01
Rate of cytoplasm
synthesis [log scale]
Amount of cytoplasm
per cell
[log scale]
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