Bacterial Growth and Division
547
CD
E
E
′′
CD
CD
BB
CD
CDB
C
0
1a
2a
1b
2b
X
Y
Fig. 7
The
Caulobacter
cell cycle.
Caulobacter
cells divide unequally, producing a smaller
swarmerce
l
lthatismoti
leandalargersta
lked
cell that is generally attached to a surface by the
stalk. As drawn here, the development of
Caulobacter
can be followed by concentrating on
the two new poles formed by the Frst division.
The development of the poles in both cells is a
succession from bald, to flagellated, to stalked.
Because the swarmer cell is smaller the
development is slightly delayed, suggesting a
size control for both the initiation of DNA
replication and the completion of pole
development. As drawn here, the
cell-cycle-speciFc synthesis of flagella protein
(flagellin) is a result of pole development
following cell separation. The ultimate end of
pole development is a stalk, after which no
further changes in the pole occur. Thus, with
regard to the general model of cytoplasm
synthesis proposed here, the
Caulobacter
cytoplasm (excluding surface) does not exhibit
cell-cycle-speciFc patterns of gene expression.
9.3
Streptococcal Growth during the Division
Cycle
The growth of spherical bacteria such as
Streptococci reveals a different pattern of
surface growth. There is no apparent in-
tercalation of new peptidoglycan material
into the preexisting older peptidoglycan.
Rather, the old peptidoglycan is conserved
and new peptidoglycan grows at the cen-
ter of the cell to make a completely new
cell pole.
10
Mass Increase Is the Driving Force of the
Division Cycle
The driving force of the division cycle is
cytoplasm synthesis. Some part of the en-
ergy used by the cell to make cytoplasm
drives the biosynthesis of the cell surface
by causing stresses along the cell surface
and this stress leads to the breaking of
peptidoglycan bonds. The insertion of new
peptidoglycan then leads to the increase in
cell size. Cytoplasm increase also regulates
DNA synthesis. Cell size at birth is greater
at faster growth rates (Fig. 3) because the
initiation of new rounds of DNA replica-
tion occurs when the cell has a unit (or
critical) amount of cytoplasm per origin of
DNA. Cell size at initiation is proportional
to the number of replication points in the
cell, and that is why cells are larger at faster
growth rates; at faster growth rates there
are more origins per cell at initiation. The
cell ‘‘titrates’’ the amount of cytoplasm or
some speci±c molecule that is a constant
fraction of the cytoplasm. Thus, DNA syn-
thesis is initiated at all available origins
when the amount of cytoplasm per origin
reaches a particular value.
The cell cytoplasm increases as fast as
it can, given the external nutrient con-
ditions. The synthesis of DNA and cell
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