Bacterial Growth and Division
531
made linearly, exponentially, or at an
instant
during
the
division
cycle,
in
an
exponentially
growing
culture
the
amounts of all cell components increase
exponentially and in parallel. This is
referred
to
as
balanced
growth.
The
observed balanced growth is the result of
the age distribution during growth being
invariant.
Given that between the birth of a cell
and its subsequent division, all of the cell
components double, we can ask, ‘‘At what
rate is material synthesized during the
division cycle of an individual cell, and how
does the cell ensure that there is a precise
doubling of cell components by the time
the cell divides?’’ Let us Frst analyze which
components must be dealt with in terms
of the division cycle.
2.2
Aggregation Theory and the Control of
Synthesis during the Division Cycle
In economics, the aggregation problem
is how to combine various sectors of
an
economy.
Should
the
Fgures
for
the production of capital machinery be
combined with those for the production
of consumer goods? Is paper produced
for boxes in the same economic category
as stationery? It is difFcult to treat each
item in an economy individually; some
aggregation
is
necessary
in
order
to
understand the whole system.
Now, consider the aggregation problem
for the analysis of the bacterial division
cycle. How should one aggregate the dif-
ferent cell components in order to achieve
an understanding of the biochemistry of
growth and division? Is there a unique
pattern of synthesis during the division cy-
cle for each enzyme, or are there a limited
number of patterns with different enzymes
or molecules synthesized according to any
one of these patterns? Are there ways of
grouping proteins or RNA molecules so
that one can consider classes of molecules
rather than individual molecular species?
Should we consider the cell membrane as a
different category from that of peptidogly-
can? There are approximately a thousand
proteins in the growing cell, and if each
protein had a unique cell-cycle synthetic
pattern, or if there were only a few en-
zymes exhibiting any particular pattern,
we would have an insuperable task de-
scribing the biosynthesis of the cell during
the division cycle.
±ortunately, one need consider only
three categories of molecules, each of
which is synthesized with a unique pattern.
The growth pattern of the cell is the sum
of these three biosynthetic patterns. The
Frst category is the cytoplasm, which is
the entire accumulation of proteins, RNA
molecules, ribosomes, small molecules,
water, and ions that make up the bulk
of the bacterial cell. It is the material
enclosed within the cell surface that is
not the genome. The second category is
the genome, the one-dimensional linear
DNA structure. The third category is
the
cell
surface,
which
encloses
the
cytoplasm and the genome. The surface is
composed of peptidoglycan, membranes,
and membrane-associated proteins and
polysaccharides. Everything in the cell
Fts into one of the three categories, and
each category has a different pattern of
synthesis during the division cycle. These
three patterns are simple to understand
as they can be derived from our current
knowledge of the principles involved in the
biosynthesis of the cytoplasm, the genome,
and the cell surface.
The division cycle of bacteria in general
may be best described by discussing the
division cycle of the bacterium about which
most is known,
E. coli
, and then describing
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