Effect of repair on specific growth rate. (A) Following asymmetrically dividing single cells over consecutive cell divisions, indicated by numbers, in which they repeatedly inherited all damage (old-pole cells), shows that the specific growth rate of a cell without repair (red) starts higher but decreases faster than that of a cell with optimal repair (magenta, β = 0.07). Specific growth rates of symmetrically dividing cells do not change at division giving horizontal lines: lower without repair (blue) than with optimal repair (cyan, β = 0.07). (B) Specific growth rate distribution in steady-state populations of asymmetrically dividing cells. Only new-pole cells grow faster without repair (red) than with optimal repair (magenta). (C, D) Snapshots of age and size distributions in the population without repair (C) or with optimal repair (D). Each dot represents a cell with a certain mass and age. Age is constant, i.e. in a steady state, in symmetrically dividing cells, and reduced with optimal repair. In asymmetrically dividing cells, young cells grow older during the cell cycle while the damage that older cells have inherited can become diluted by growth, which decreases age during the cell cycle. Cells are younger with optimal repair. (A-D) The environment was constant and damage toxic, accumulating at a rate of 0.1 h−1.