534
Bacterial Growth and Division
syntheses (e.g. enzymes involved in amino
acid biosynthesis). In a minimal medium,
in which many dispensable enzymes are
being synthesized, the ribosomes may be
considered to be involved in the synthesis
of ribosomal protein and a large number of
dispensable proteins. Since only a fraction
of the ribosomes are making ribosomal
protein at any moment, the time for a dou-
bling in the ribosomal content is relatively
long. As one increases the number of nu-
trients in the medium, thus relieving the
cell of having to make the dispensable en-
zymes, a larger fraction of ribosomes are
involved in ribosomal protein synthesis.
Thus, in rich medium the time for ribo-
some number to double is shorter than
in poor medium. The variation in growth
rates is thus a reflection of the contin-
uous ability of the cell to partition the
ribosomes (or the RNA polymerases) be-
tween making ribosomal components or
nonribosomal components.
Considering the time required for a
ribosome to make another ribosome (i.e.
using 15 amino acids incorporated in
ap
r
o
t
e
inch
a
ininas
e
c
on
d
)
,on
ec
an
calculate that it would take eight minutes
for one ribosome to synthesize a single
ribosome. If a cell were in some way
supplied with all proteins other than
ribosomes, one could consider that the
fastest rate of cell doubling would be eight
minutes, the time for ribosomes to double
their number.
3.2
DNA Synthesis during the Division Cycle
One of the most important generaliza-
tions regarding the regulation of linear
macromolecule synthesis is that the rate
of synthesis is regulated at the point of ini-
tiation of polymerization. This principle is
exhibited most clearly in DNA replication,
where the process of initiation of replica-
tion can be clearly dissociated from the
continued replication of DNA that has
already initiated replication. If one in-
hibits the initiation of replication, then the
rounds of DNA replication in progress will
continue until their normal termination.
3.2.1
Pattern of DNA Synthesis during the
Division Cycle
The DNA synthesis pattern during the
bacterial division cycle (speciFcally for
E. coli
and related bacteria) is composed
of one or more periods of constant rates of
DNA accumulation (±ig. 3). In cells with
a 60 min interdivision time, there is a
constant rate of DNA synthesis for the
Frst 40 min, with one bidirectional pair of
replication forks, followed by a zero rate of
synthesis during the last 20 min. In cells
with a 30 min interdivision time, there is a
constant rate of DNA synthesis for the Frst
10 min, which falls to a constant rate that
is two-thirds of the initial rate for the last
20 min. In cells with a 20 min interdivision
time, there is a constant rate of DNA
synthesis throughout the division cycle.
Other growth rates have similar particular
patterns of DNA synthesis.
The constant rates of DNA synthesis
can
be
understood
in
terms
of
the
molecular
aspects
of
DNA
synthesis.
DNA is synthesized by the movement
of a replication point along the parental
double helix, leaving two double helices
in its wake. The rate of movement of
a growing point appears to be invariant
and independent of its location in the
genome. This means that the rate of DNA
synthesis is proportional to the extant
number of replication points. The number
of replication points is constant for any
period of the division cycle. Thus, the rate
of DNA synthesis during any period of
the division cycle is also constant. The
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