550
Bacterial Growth and Division
such as the start of DNA synthesis and
the start of septation. A large number of
mutations affecting these processes have
been isolated, and their study has revealed
that a number of enzymes and proteins
are required for cell-surface synthesis and
DNA synthesis.
14
Die Einheit der Zellbiologie
Are the interrelationships of cell-cycle
regulation, proposed here for
E. coli
, appli-
cable to other organisms? Other regulatory
mechanisms have been proposed in other
cell types. For example, a circular regula-
tory system was proposed for
Caulobacter
and a stochastic mechanism as well as
a rate-controlled model for animal cells.
Is the mechanism of cell-cycle regula-
tion subject to historical accident? Or is
there a unity of mechanisms regulating
the division cycle of all types of cells? I
conjecture that the
E. coli
model of cell-
cycle regulation is generally applicable to
other organisms, and that the optimal de-
sign of the cell cycle is exhibited by the
E. coli
cell cycle.
In the second decade of this century,
Kluyver and his colleagues explicitly pro-
posed the principle of the Unity of Bio-
chemistry, or as originally proposed, Die
Einheit der Biochemie (the oneness of
biochemistry). Although the original pro-
posal dealt primarily with the nature of
oxidation–reduction reactions, the basic
principle has evolved to encompass even
more unifying principles. The nature of
the gene, the genetic code, the mechanism
of protein synthesis, the use of enzymes
for biochemical changes, the patterns of
enzyme regulation, and all of the fun-
damental aspects of biochemistry appear
similar throughout the different organ-
isms on earth. While there are exceptions
and unique aspects to many organisms,
the essential principle that there is a uni-
±ed biochemistry has been supported.
Here we must now add, with regard
to
the
cell
cycle,
the
Unity
of
Cell
Biology –
Die Einheit der Zellbiologie
–that
is analogous to the Unity of Biochemistry.
Phenomena suggesting the existence of
other modes of cell-cycle regulation in
animal cells have been shown to be
consistent with the model for the bacterial
division cycle proposed here. For example,
the constancy of C- and D-periods is
mimicked
by
the
constancy
of
their
analogues in animal cells, the S and
G2 periods. The variation in cell size
with growth rate and the rules regarding
size determination also appear to hold
for all organisms. The rules of the cell-
division cycle – that is, the logic and
design principles of the division cycle – are
t
h
es
am
ef
o
r
Escherichia
and escargot,
Salmonella
and salmon. Although there
are many detailed differences between
eukaryotic and prokaryotic division cycles,
at the deepest level the principle of the
Unity of Cell Biology proposes that the
division cycle is ultimately regulated by
the continuous accumulation of some
molecule that is titrated against a ±xed
amount of another cell component. From
this viewpoint, cell-cycle-speci±c events
identi±ed
in
eukaryotic
cells
are
the
result or symptom of a deeper regulatory
principle that is produced in a cell-cycle-
independent manner. The question for
future analysis is whether there is some
deeper rule that ensures this common
pattern in all cells. I conjecture that this is
indeed the case and hope that a search for
the underlying meaning of size control in
a
l
lce
l
lsisnowthenewgoa
lo
fce
l
l
-cyc
le
research.
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