546
Bacterial Growth and Division
synthesis under starvation conditions. The
eukaryotic restriction point is subject to
thesamecritique.
8
Temporal Variation of the Bacterial Division
Cycle
Division cycles in a culture are not pre-
cisely timed. There is a great deal of
variability in division cycles, with a culture
having a 60-min doubling time proba-
bly having cells with interdivision times
between 40 and 100 min. The variability
of the division cycle can be visualized
as due to the variability of the separate
components that make up the division
cycle. Thus, for two identical newborn
cells one can have the subsequent divi-
sion occurring at different times because
of (1) variability in mass synthesis in the
two cells leading to variation in the time
of initiation of DNA replication, (2) varia-
tion in the rate of DNA replication in the
two cells, (3) variation in the time between
termination and division, (4) and variation
in the equality of division (which leads to
variation in the next division cycle).
9
Alternative Bacterial Division Cycles
9.1
Caulobacter crescentus
Division Cycle
At Frst glance, the division cycle of the
stalked bacterium
Caulobacter crescentus
appears
quite
different
from
that
of
E. coli
. At division, two different-sized
and different looking cells are produced
(division is not in the middle of the
mother cell and one cell is stalked and
the other is flagellated), the DNA patterns
during the division cycle differ in each
of the daughter cells, and there are
cell-cycle-speciFc syntheses of flagellum-
related materials. The most important
difference is that the poles of the daughter
cells are different in that one cell has
a flagellum and the other has a stalk.
Because of the production of two different
appearing cells, the growth and division of
Caulobacter
has been described as a model
of cell differentiation.
The division cycle of
Caulobacter
can
be shown to correspond to that of the
standard
E. coli
model by considering that
the different DNA patterns are due to
the different sizes of the two daughter
cells arising by division not occurring in
the middle of the dividing mother cell,
and that the cell-cycle-speciFc pattern of
expression of flagellum-related genes is
related to the completion of poles after the
act of cell division. ±lagellum synthesis is
thus considered a part of surface synthesis.
The cycle-speciFc pattern observed in cells
is related more to the window of time in
which poles are made rather than synthesis
at a particular time during the division
cycle (±ig. 7).
9.2
Bacillus subtilis
Division Cycle
The division cycle of the gram-positive
bacterium
Bacillus
subtilis
is relatively
similar to that
of
E. coli
.
The
major
difference between the organisms lies in
thefactthatthe
B. subtilis
cell wall is much
thicker (giving rise to its gram-positive
character), and this requires an inside-to-
outside pattern of peptidoglycan growth.
The cell wall is made adjacent to the
cytoplasmic membrane, and this wall layer
moves out as subsequent layers are made
between it and the cytoplasmic membrane.
This leads to the multilayered
B. subtilis
peptidoglycan.
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