Bacterial Growth and Division
543
1
0.5
0.5
0.25
0.25
Cytoplasm
DNA
0.5
0.5
0.30
.20
p
q
Peptidoglycan
Fig. 6
Segregation of cytoplasm, surface, and DNA at division.
The cytoplasm of the cell is segregated randomly at division. Each
daughter cell shares equally in the parental cytoplasm
(ribosomes, soluble proteins, ions, etc.), which can be considered
randomly distributed throughout the dividing cell. The DNA is
segregated nonrandomly, but each daughter cell gets a complete
and equal complement of DNA from the dividing cell. The
nonrandom segregation is at the level of a single strand. Since
each DNA duplex is made up of an ‘‘older’’ strand and a new
complement made on the older strand, and each cell is composed
of an ‘‘older’’ pole and a pole just made at the previous division,
one can see whether strands of one age go preferentially toward
one or the other age pole. Experimental analysis indicates that the
DNA strands segregate as though the older DNA strand goes
preferentially toward the pole that it went to at the previous
division. This may be phrased as the newer strand goes toward
the newer pole. The degree of nonrandom segregation is not
perfect, but is stochastic, and is approximately 60/40 rather than
50/50. The surface of the cell segregates to the daughter cells at
division such that the poles are conserved and the sidewall
segregates with no apparent conservation of sidewall
peptidoglycan. It is not clear whether insertion of new wall
material over the side is perfectly uniform or whether there is
some nonuniform pattern of insertion.
5.2
Segregation of DNA
DNA segregates so that each daughter
cell receives a complete genome from
the dividing mother cell (Fig. 6). As the
DNA nucleoid is relatively compact within
a dividing cell, if the nucleoids were
apportioned completely randomly, one
might expect that at some divisions one
of the daughter cells would not receive any
DNA and the other daughter cell would
previous page 543 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online next page 545 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online Home Toggle text on/off