Mitotic control in the absence of cdc25 mitotic inducer in fission yeast

Fission yeast cells tolerate the total absence of the cdc25 mitotic inducer in two cases, either in cdc2-3w or in wee1 genetic backgrounds, In the cdc 2-3w cdc25 Delta double mutant, the rate-limiting step leading to mitosis i s reaching a critical size. However, the size control of this mutant operat es in late G(2), which is different from wild-type (WT) cells. This fact su ggests that in WT the rate-limiting molecular process during the G(2) timer is the Tyr15 dephosphorylation of cdc2, for which the cdc25 phosphatase (t ogether with its back-up, pyp3) is dependent. In the wee1-50 cdc25 Delta mu tant, the population splits into different clusters, all lacking mitotic si ze control. This strain maintains size homeostasis by a novel method, which is random movement of the cells from one cluster to another in the success ive generations. These cells should normally have a 'minimal cycle', a 'tim er' with short G(1) and G(2) phases, However, very often the cells abort mi tosis, possibly at an early event and return back to early Gz, thus lengthe ning their cycles. The inability of these cells to start anaphase might be caused by the absence of the main mitotic regulators (wee1 and cdc25) and t he improper regulation of their back-up copies (mik1 and pyp3, respectively ).