The regulatory mechanism which ensures that eukaryotic chromosomes replicate precisely once per cell cycle is a basic and essential cellular property of eukaryotes. This fundamental aspect of DNA replication is still poorly understood, but recent advances encourage the view that we may soon have a clearer picture of how this regulation is achieved. This review will discuss in particular the role of proteins in the minichromosome maintenance (MCM) family, which may hold the key to understanding how DNA is replicated once, (...) and only once, per cell cycle. (shrink)

Mos, a protein kinase, is specifically expressed and functions during meiotic maturation (or G2/M progression) of vertebrate oocytes. When expressed ectopically, however, it can also readily induce oncogenic transformation (or uncontrolled G1/S transitions) in somatic cells. In both of these cell types, Mos activates mitogen‐activated protein kinase (MAPK), which seems largely to mediate its different functions in both oocyte maturation and cellular transformation. In oocyte maturation, the Mos‐MAPK pathway probably serves to activate and stabilize M‐phase promoting factor (MPF) (possibly by (...) inhibiting some negative regulator(s) of this factor), while in cellular transformation, it seems to stabilize and activate the nuclear oncoprotein c‐Fos as well as to induce transcription of its gene. Thus, the different functions of Mos in oocytes and somatic cells may arise chiefly from its different MAPK‐mediated targets in the respective cell types. This review discusses the cellular basis that may enable Mos to act differently in oocytes and somatic cells. (shrink)

During embryogenesis, cell division must be spatially and temporally regulated with respect to other developmental processes. Leech embryos undergo a series of unequal and asynchronous cleavages to produce individually recognizable cells whose lineages, developmental fates and cell cycle properties have been characterized. Thus, leech embryos provide an opportunity to examine the regulation of cell division at the level of individual well‐characterized cells within a community of different types of cells. Isolation of leech homologues of some of the highly conserved regulators (...) of the cell division cycle, and characterization of their patterns of maternal and zygotic expression, indicate that the cell divisions of early leech embryos are regulated by cell type‐specific mechanisms. These studies with leech embryos contribute to the emerging appreciation of the diverse mechanisms by which animals regulate cell division during early development. (shrink)

Cyclin‐dependent kinases and their regulatory subunits, the cyclins, are known to regulate progression through the cell cycle. Yet these same proteins are often expressed in non‐cycling, differentiated cells. This review surveys the available information about cyclins and cyclin‐dependent kinases in differentiated cells and explores the possibility that these proteins may have important functions that are independent of cell cycle regulation.