Recalling that seasons are not variables that determine weather, but merely intervals during which there occur changes in the state of the weather. Thus the major players in the cell cycle are kinases, cyclins and proteolysis. A good recent example from plants is the identification of the D-type cyclin CycD3 in a mutant of Arabidopsis with a high level of cytokinin. It is also suggested that this regulation is primarily mediated at transcription. These are asymmetric cell divisions and in some cases morphological distinctions may not be evident at the time of division.
Cell walls prevent rearrangement of cells and thus asymmetric divisions are an obvious fact.
Approximately correct tissue patterns, however, can emerge in plants mutated in their ability to divide in the carrect plane. This means that there is much still to be understood. In yeast and certain animal species gene products that meet the criteria of intrinsic factors capable of mediating asymmetric cell divisions are identified and some common themes emerge. These include localized factors that initiate polarization of the cytoskeleton, the identification of factors that go to one end of the cell, the orientation of the spindle apparatus so that at division, factors end up in one daughter cell and not both daughter cells. In yeast the ASH 1 gene with its mRNA and protein have been identified, and this gene and its products acts as a cell fate determinant by repressing transcription of the HO gene whose product is a site-specific endonuclease.