Bacterial Growth and Division
545
DNA synthesis, and cell-surface synthesis
are uneventful during the division cycle.
In contrast to our understanding of the
principles of cytoplasm, cell-surface, and
DNA synthesis, the biochemical mecha-
nisms of the four events occurring during
the division cycle – the initiation and ter-
mination of DNA synthesis and the initia-
tion and cessation of pole formation – are
still unknown. While a great deal is known
about the biochemistry of initiation of
DNA replication, the mechanism by which
this initiation is regulated is less well un-
derstood. In addition, very little is known
of the biochemical principles involved in
either the termination of DNA replication
or the start and cessation of pole forma-
tion. A cell-surface structure, the periseptal
annulus, has been proposed as a possible
Frst step in the formation of a new pole.
If the periseptal annulus is the start of
pole formation, the important question is
whether there are deFnable steps between
the formation of the periseptal annulus
and the start of invagination.
7
Do Checkpoints Control the Bacterial Cell
Cycle?
In eukaryotic cell-cycle studies, the concept
of ‘‘checkpoints’’ looms large. Checkpoints
are the postulated functions of a cell that
determine that one function is properly
completed before a subsequent function
occurs. Thus, in eukaryotic cells it is pos-
tulated that a checkpoint function deter-
mines that DNA has replicated completely
and properly before allowing mitosis to
occur. The checkpoint is thus a valuable
function that prevents abnormal functions
from occurring.
Studies of this type of phenomenon
in bacteria can provide a corrective lens
for understanding or more rigorously
deFning checkpoints. In bacteria, if DNA
synthesis is inhibited, cells with only one
genome (i.e. cells that have not completed
replication of a total chromosome) do not
divide. Even when DNA is inhibited or
damaged, a cell with two genomes will
divide, while a cell with one genome will
not. Thus, in bacteria, DNA damage
per se
does not activate a checkpoint to prevent
division. It is believed that the presence
of a single genome in the center of a cell
prevents division by its central presence,
not the activation of a function that checks
for completion of DNA replication.
The bacterial experience thus suggests
that a distinction should be made between
the existence of a function external to
the tested function (i.e. some activity that
checks on whether DNA replication has
been completed) and the actual function
itself acting as a brake on further activity.
The failure to complete DNA replication is
the inhibitor of division, and there is no
checking by some ‘‘checkpoint’’ activity.
A similar analysis may be applied to the
ubiquitous postulation of restriction points
in eukaryotic cells. Restriction points are
points or events postulated to exist in the
G1-phase of the division cycle. Passage of
a cell through the restriction point is a
requirement for entering the S-phase and
eventual division. The restriction point was
deFned by the effect of different starvation
regimens on DNA replication and the
resumption of DNA replication following
release from starvation.
It is interesting that a bacterial restriction
point was postulated to exist before the
proposal
of
the
eukaryotic
restriction
point, although the term
restriction point
was not used. Subsequent analysis of
the bacterial restriction point indicated
that the proposed control point was an
experimental artifact related to leakage of
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