CELL-CYCLE REGULATION OF DNA-REPLICATION - THE ENDOREDUPLICATION PERSPECTIVE

In recent years considerable effort has been invested toward understan ding the molecular mechanisms that regulate and restrict DNA replicati on to once per each cell cycle. An important contribution came from st udying the phenomenon of endoreduplication-an endonuclear duplication of chromosomes which occurs in the absence of mitosis leading to the p roduction of chromosomes with doubling series of chromatids. Because e ndoreduplicating nuclei retain the capability of replication without p assing through mitosis, they provide a unique system for studying the molecular mechanisms that restrict DNA replication to once per cycle. Three types of endoreduplication can be identified: I, multiple initia tions within a given S phase; II, reoccurring S phase; and III, repeat ed S and Gap phases. Each of these illuminates a different control lev el acting over the onset of S phase, which coordinately restrict DNA s ynthesis to once per each cell cycle. (C) 1998 Academic Press.