After a cell divides into two daughter cells, the total cell surface area of the daughter
cells should increase to the original size to maintain cell size homeostasis in a single cell
cycle. Previously, three models have been proposed to explain the regulation of cell size
homeostasis: sizer, timer, and adder models. Here, we precisely measured the total cell
surface area of Dictyostelium cells in a whole cell cycle by using the agar-overlay
method, which eliminated the influence of surface membrane reservoirs, such as
microvilli and membrane winkles. The total cell surface area linearly increased during
interphase, slightly decreased at the metaphase, and then increased by approximately
20% during cytokinesis. From the analysis of the added surface area, we concluded that
the cell size was regulated by the near-adder model in interphase and by the timer model
in the mitotic phase. The adder model in the interphase is not caused by a simple cell
membrane addition, but is more dynamic due to the rapid cell membrane turnover. We
propose a ‘dynamic adder model’ to explain cell size homeostasis in the interphase.