Morphogenes of Escherichia coli

Figure 1 represents the cell cycle of Escherichia coli (central circle), together with the phenotypes that result when mutational blocks are present at various stages (radiating spokes). Starting at the top of the circle is a rod-shaped cell with a single nucleoid (which is the visible body consisting of the cell’s DNA). The cycle comprises a continuous increase in cell length, accompanied by DNA replication and nucleoid segregation and followed by septation and cell division. The duration of the cycle depends on the growth medium (at constant temperature), and cell dimensions in turn depend on growth rate. The approximate relationship between growth rate, R (doublings per hr) and average cell volume (V R ) in an asynchronous population is given byV¯R=V¯u·2R where V u is a constant, equal to average cell volume in a population when R = 0 (Donachie 1981). In consequence, mean cell volume will be approximately eight times greater in cells growing with a generation time of 20 minutes in a very rich medium than in genetically identical cells, which have been growing very slowly (Schaechter et al. 1958). Despite the very large changes in cell volume under different growth conditions, the shape of cells (i.e., ratio of mean length to width) always remains much the same (Zaritsky 1975). This follows from the observation (Donachie et al. 1976; Grover et al. 1977) that average cell length (L R ) varies with growth rate to a much smaller extent than does cell volume, such that, to a first approximation,L¯R=L¯u·2(R/3) where L u ...