Bacterial Growth and Division
535
3
2
6
1.75
3.5
3
6
1
1
Interdivision
time [min]
20
30
60
Rate of DNA
Synthesis
Cell age during division cycle
2
1
01
01
0
Amount
of DNA
Cell size at birth
1
01
01
0
Fig. 3
DNA synthesis during the division cycle. Three different
patterns are illustrated for cells growing with 20, 30, and 60 min
doubling times. For each growth rate, the time from initiation to
termination of DNA replication is 40 min. The time between
termination of replication and cell division is 20 min. The proposed
chromosome patterns at the start and ±nish of the division cycle are
illustrated above the graphs. In addition to the rate of DNA synthesis
during the division cycle, the pattern of accumulation of DNA during
the division cycle is presented. Accumulation of DNA is composed
of periods of linear synthesis. The rates during these periods are
proportional to the existing number of growing points. The graph of
the rate of synthesis is the differential of the accumulation plot. A
representation of the cell size expected for cells at the start of the
division cycle is presented below each synthetic pattern. The size of
the newborn cells is in the ratio of 1 : 2 : 4 in the three cultures
illustrated here. The number of new initiations at the start of each
divisioncycleisalsointheratioof1:2:4.
pattern of constant rates of synthesis is
derived from, and is consistent with, our
understanding of the biochemistry of DNA
synthesis.
Embedded in the patterns of Fig. 3 are
the two major rules for the determination
of the pattern of replication of DNA during
the bacterial division cycle. The ±rst rule
is that once DNA replication is initiated
at the origin of replication, the time for
a complete round of replication – the C-
period – is relatively invariant (40 min in
E. coli
at 37
C). The second rule is that the
time between termination of replication
and cell division – the D-period – is also
relatively invariant.
The constant C- and D-periods are most
clearly evident in the 60 min cells in Fig. 3.
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