Why do bacteria divide?

Abstract

The problem of not only how but also why cells divide can be tackled using recent ideas. One idea from the origins of life - Life as independent of its constituents - is that a living entity like a cell is a particular pattern of connectivity between its constituents. This means that if the growing cell were just to get bigger the average connectivity between its constituents per unit mass - its cellular connectivity - would decrease and the cell would lose its identity. The solution is division which restores connectivity. The corollary is that the cell senses decreasing cellular connectivity and uses this information to trigger division. A second idea from phenotypic diversity - Life on the Scales of Equilibria - is that a bacterium must find strategies that allow it to both survive and grow. This means that it has learnt to reconcile the opposing constraints that these strategies impose. The solution is that the cell cycle generates daughter cells with different phenotypes based on sufficiently complex equilibrium (E) and non-equilibrium (NE) cellular compounds and structures appropriate for survival and growth, respectively, alias 'hyperstructures.' The corollary is that the cell senses both the quantity of E material and the intensity of use of NE material and then uses this information to trigger the cell cycle. A third idea from artificial intelligence - Competitive Coherence - is that a cell selects the active subset of elements that actively determine its phenotype from a much larger set of available elements. This means that the selection of an active subset of a specific size and composition must be done so as to generate both a coherent cell state, in which the cell's contents work together harmoniously, and a coherent sequence of cell states, each coherent with respect to itself and to an unpredictable environment. The solution is the use of a range of mechanisms ranging from hyperstructure dynamics to the cell cycle itself.

Keywords: FtsZ; competitive coherence; connectivity; dualism; heterogeneity; hyperstructure; molecular assembly; neural net.

FIGURE 1

Profile of Lydia Cassatt (1880). This portrait entitled Autumn of the seriously ill Lydia by her sister, Mary Cassatt, can be interpreted as showing her losing her identity (copied from http://hoocher.com/Mary_Cassatt/Mary_Cassatt.htm original in the Musee du Petit Palais, Paris, France).

FIGURE 2

Loss of identity through growth. There are six elements in the autocatalytic set linked by two links per element. (A) The links:elements ratio (L:E) is 1 and stays 1 in the above case as the cell grows. However, this ratio per mass goes down with growth as (L:E)/M goes to (L:E)/2M. (B) After the first stage of growth (50% increase in mass), the (L:E)/M has gone down from 1 to (6:9)/1.5M, i.e., 0.4444 whilst after continuing but unbalanced growth (100% increase in mass), the (L:E)/M has gone down from 1 to (6:12)/2M, i.e., 0.25.

FIGURE 3

Strand segregation of hyperstructures creates complementary, coherent phenotypes. The hyperstructure (hexagon) associated with an old strand (continuous circle) is segregated with the old strand into the daughter cell. New hyperstructures (translucent hexagons) associated with the new strands (dotted circles) are smaller because they must compete with the parent cell’s hyperstructures (solid hexagons).