<P> While oscillatory transcription plays a key role in the progression of the yeast cell cycle, the CDK - cyclin machinery operates independently in the early embryonic cell cycle . Before the midblastula transition, zygotic transcription does not occur and all needed proteins, such as the B - type cyclins, are translated from maternally loaded mRNA . </P> <P> Analyses of synchronized cultures of Saccharomyces cerevisiae under conditions that prevent DNA replication initiation without delaying cell cycle progression showed that origin licensing decreases the expression of genes with origins near their 3' ends, revealing that downstream origins can regulate the expression of upstream genes . This confirms previous predictions from mathematical modeling of a global causal coordination between DNA replication origin activity and mRNA expression, and shows that mathematical modeling of DNA microarray data can be used to correctly predict previously unknown biological modes of regulation . </P> <P> Cell cycle checkpoints are used by the cell to monitor and regulate the progress of the cell cycle . Checkpoints prevent cell cycle progression at specific points, allowing verification of necessary phase processes and repair of DNA damage . The cell cannot proceed to the next phase until checkpoint requirements have been met . Checkpoints typically consist of a network of regulatory proteins that monitor and dictate the progression of the cell through the different stages of the cell cycle . </P> <P> There are several checkpoints to ensure that damaged or incomplete DNA is not passed on to daughter cells . Three main checkpoints exist: the G / S checkpoint, the G / M checkpoint and the metaphase (mitotic) checkpoint . </P>

G1 s g2 phases of the cell cycle