542
Bacterial Growth and Division
formation owing to the inappropriate loca-
tion of invagination sites.
4
Plasmid Replication during the Division
Cycle
4.1
High-copy Plasmids
If a cell has a large number (20–100)
of plasmids per newborn cell, then one
can consider that these plasmids replicate
during the division cycle in proportion
to those present (i.e. exponentially) and
then assort themselves randomly into
the newborn cells. Because the plasmid
number is high, there is little chance of
one of the daughter cells ending up with
no plasmids and with all of the plasmids
segregating to the other daughter cell. This
pattern appears to be the case for naturally
occurring, high-copy number plasmids.
One possible exception is the artiFcial
plasmid made by cloning the origin (oricC)
region of the bacterial chromosome. This
appears to have a random assortment
mechanism, but it is synthesized at a
precise time during the division cycle that
coincides with the normal time at which
the chromosome initiates replication.
4.2
Low-copy plasmids
Low-copy plasmids are usually larger than
high-copy plasmids and are present on
the order of 1 to 2 per cell. Examples
of such plasmids are the ±-factor and
the P1 plasmid. These replicate in a
precise way at a particular time during
the division cycle. The rules of their
replication are similar to that regulating
chromosome replication, with initiation of
plasmid replication occurring when a Fxed
amount of cell mass is present per plasmid
origin.
4.3
Minichromosome Replication during the
Division Cycle
Minichromosomes replicate at the same
time as normal chromosome initiation.
The behavior of minichromosomes is
paradoxical in that in a cell with numerous
additional origins of replication there
would be a competition for the ‘‘initiator’’
of replication. Such a competition would
lead to abnormal cell sizes or some
alteration in cell growth pattern. No such
alteration is observed.
5
Segregation of Cell Components at Division
Producing the right amount of material
so that the cell can divide is not enough
for a successful division cycle. At division,
the material in the mother cell must also
be properly apportioned to the newborn
cells. In a cell that divides to produce two
equivalent daughter cells, the segregation
problem is to ensure that both cells have
the same amount of each portion of
the cell material. Let us see how the
three categories of material in the mother
cell are segregated properly to the new
daughter cells.
5.1
Segregation of Cytoplasm
The cytoplasmic components of the cell
appear to segregate randomly at division.
There does not appear to be any compart-
mentalization or hindrance to segregation
of cytoplasm (±ig. 6).
previous page 542 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online next page 544 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online Home Toggle text on/off