Regulation of Pre-replication Complex Components in the Mitotic and Meiotic Cell Cycles

Strict controls operate to limit DNA replication to a single round during each cell cycle. The Mcm2-7 proteins playa vital role in this process. These proteins are generally thought to function as a complex. However, subcomplexes have been discovered, and in Xenopus, individual Mcm2-7 subunits have been shown to bind to chromatin in a stepwise manner which may suggest another level of replication control. In this study, the chromatin association of individual Mcm2-7 subunits was analysed in the mitotic cell cycle. The assembly of Mcm2, 4 and 7 onto chromatin was shown to occur at a similar time in anaphase with no evidence for sequential binding. The concentration of Mcm2-7 subunits was estimated to be approximately 104 molecules/cell, with Mcm4 being present at the highest level, Mcm6 the lowest and Mcm2, 3 and 7 present at intermediate levels. The levels of Mcm2-7 proteins were also compared to that of other essential proteins, showing a ratio of approximately 1 : 2 : 15 : 15 : 6 for Cdc23 : Orc6 : Mcm2 : Mcm7 : Cdc45. Additionally, upon exit from the cell cycle to a quiescent GO-like state, Mcm2-7 proteins are displaced from chromatin and this displacement occurs in a step independent of DNA replication. Strict control of DNA replication is also vital for cells undergoing meiosis. It is crucial that cells only replicate their DNA once during premeiotic S phase and prevent any replication from occurring between the two meiotic divisions. The pre-replication components Cdc18 and Cdt1 were shown to be absent during the time of nuclear divisions whilst other replication factors analysed in this study were present during this interval. Furthermore, the phenotype of ectopic expression of both Cdc18 and Cdt1 in late meiosis suggests that the absence of these two proteins may be a contributing factor in preventing any replication from occurring between the two meiotic divisions.EThOS - Electronic Theses Online ServiceGBUnited Kingdo