Chromatin fractionation analysis of licensing factors in mammalian cells.

ORC, Cdc6, Cdt1, and MCM2-7 are replication-licensing factors, which play a central role in the once-per-cell cycle control of DNA replication. ORC, Cdc6, and Cdt1 collaborate to load MCM2-7 onto replication origins in order to license them for replication. MCM2-7 is a DNA helicase directly involved in DNA replication and dissociates from DNA as S phase progresses and each replicon is replicated. In the cell cycle, the loading of MCM2-7 is restricted during the end of mitosis and the G1 phase. Thus, the levels of chromatin-bound MCM2-7 and its loaders oscillate during the cell cycle. Chromatin association of these factors can be analyzed by separating a cell lysate into soluble and chromatin-enriched insoluble fractions in mammalian cells.