544
Bacterial Growth and Division
receive two genomes. That this is a rare
event supports the notion that segregation
of the DNA is a regulated and deterministic
process that leads to very few nonnucleated
cells in a culture.
In addition to the segregation of individ-
ual nucleoids, the separate strands of the
DNA have rules that describe their segre-
gation and which are compatible with the
deterministic equipartition of DNA to the
two daughter cells. As all rod-shaped cells
are asymmetric in that one of the poles
is the new pole (produced at the last di-
vision) and one pole is necessarily older,
one can see how older DNA strands and
newer DNA strands (the newer strands
having just been made on an older, pre-
existing strand) segregate with regard to
pole age. The general rule is that the older
strands segregate slightly preferentially to
the older poles and the younger strands
segregate preferentially to the newer or
younger poles. This phenomenon is con-
sistent with other data that indicate some
attachment of the DNA to the cell sur-
face.
5.3
Segregation of the Cell Wall
The
peptidoglycan
is
a
large
macro-
molecule that grows by intercalation of
new material between old material. Since
all the peptidoglycan is covalently linked
in one macromolecule, there is no abil-
ity for the peptidoglycan components to
move with respect to each other. The seg-
regation of cell wall material is therefore
determined by the location of the cell wall
m
a
t
e
r
i
a
l
.On
c
einp
l
a
c
e
,th
ew
a
l
ls
eg
r
e
-
gates into the newborn cells so that the
old poles go to each cell, and the side-
wall is segregated to the newborn cells
(Fig. 6).
6
Are There Events during the Division Cycle?
There are only four discrete events that
occur during the division cycle of a
growing rod-shaped bacterial cell. They
are (1) the initiation of DNA replication,
(2) the termination of DNA replication,
(3) the initiation of pole formation, and
(4) the
completion
of
pole
formation.
Other aspects of cell growth are too
continuous to be considered cell-cycle-
speci┬▒c events. For example, the addition
of new nucleotides at position 43,567 on
the chain of DNA may be considered
a unique event, and one that occurs at
a particular time in the division cycle,
but because of cell-cycle variability, the
time of this synthetic occurrence can
never be precisely obtained. The time of
insertion of a particular nucleotide pair
has no meaning for the cell cycle; only
the initiation and termination of DNA
replication are important. With regard
to the division cycle, the DNA is an
amorphous material with no biochemical
speci┬▒city along the strand.
Similar considerations apply to the cell
surface and the cytoplasm. No aspect of
peptidoglycan strand insertion is unique
in time during the division cycle. A new
strand is inserted at random sites in re-
sponse to the stretching of the cell surface.
This may occur in one cell at position
0.376 of the cell length; at the same time,
in another cell, the new strand may be
inserted at position 0.549. For an analysis
of the cell cycle, we should consider pole
and sidewall growth as uneventful exten-
sions of the cell surface. Regarding the
cytoplasm, one may consider the increase
in ribosome content from 37,411 to 37,412
an event; but such an individual event is
not measurable and is without meaning in
the cell cycle. Thus, cytoplasm synthesis,
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