Faithful DNA replication is essential for the accurate transmission of genetic information to daughter cells. To this end, cells have evolved several mechanisms to ensure that DNA is duplicated once and only once. In this thesis, we investigated the existence of a DNA replication checkpoint that relies on Rev7, a member of the trans-lesion polymerase zeta. We show that it may function similarly to the spindle attachment checkpoint that relies on the Rev7 homolog Mad2. We also discover the positions where cells replicate their DNA during mitosis when DNA replication regulation fails. We mapped mitotic DNA synthesis in several cell lines and studied the transcriptional and replication landscape of such sites. We discovered that mitotic DNA synthesis is transcription dependent and occurs in origin depleted regions.