Cell cycle types

Each one of these four models can be thought of as defining the response of a black-box to a tape of symbols that is fed to it, cyclically, one cell at a time. During each cycle, the black-box reads the symbol at the appropriate cell, responds to that symbol according to a set of model-specific rules, and then moves the tape forward by one cell. What type of computational model the black-box represents depends on its general response. Although a more detailed discussion of each of these types will be given in chapter 6, we mention here that the four models make up what is conventionally called the Chomsky hierarchy Each type of computational... 

Observations from various systems, including yeast, suggest that phosphorylation and dephosphorylation of proteins play important roles in the mitotic and meiotic cell cycles and the differentiation of germ cells. Extracts from mitotic HeLa cells contained phosphoproteins also present in other mitotic and meiotic cell types, but not in interphase cells (Davis et al., 1983). Exposure of Xenopus oocytes to progesterone results in a burst of protein phosphorylation shortly before GVBD (Mailer et al.,... 

Bueno, A Richardson, H., Reed, S. I., and Russell, P. (1991). A fission yeast B-type cyclin functioning early in the cell cycle. Cell 66 149-159. 

Clarke, P. R., Hoffman, I., Draetta, G., and Karsenti, E. (1993). Dephosphorylation of cdc25-C by a type-2A protein phosphatase specific regulation during the cell cycle in Xenopus egg extracts. Mol. Biol. Cell 4 397—411. 

In summary, transcription of the Cdc25-type phosphatase encoded by string limits the progression of many embryonic cell cycles, which are rapid (1—3 h) and require little cell growth. The massive ofi-regulatory region of string acts as a sophisticated pattern sensor that is influenced by a wide variety of cell type-specific transcription factors (Fig. 2A). 

This suggests that cyclin A2 is not essential for the early embryonic cell cycles. Also D-type cyclins seem to be dispensable for the early mouse embryo cell cycle progression since embryonic stem (ES) cells do not express them at all before differentiation (Savatier et al 1996). We do not know, however, whether the D-type cyclins are also absent in the early embryo. These observations suggest that not only could the first cell cycles of the mouse embryo have specific modifications, but also further embryonic cell cycles are specifically modified as well. Mammalian embryonic cell cycles are probably modified often during development. Such studies could allow us to determine a profile of a minimal cell cycle in mammals which must, however, be much more complex than a simple S M phase embryonic cell cycle of amphibians or insects. 

As far as we are aware, OPCs are the only normal mammalian cells, other than eggs and blastomeres, that have been shown to survive, proliferate and differentiate in serum- and extract-free culture in the absence of other cell types. Indeed, a single OPC can survive and proliferate in these conditions in the absence of any other cells, suggesting that diffusible autocrine factors are not required (Y. Tokumoto, unpublished work). These properties make OPCs especially attractive for studying how intracellular programmes and extracellular signals can combine to control when the cells exit the cell cycle and differentiate. It seems likely that similar mechanisms operate in other cell lineages. 

---