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DNA re-replication
DNA re-replication (or simply rereplication) is an undesirable and possibly fatal occurrence in eukaryotic cells in which the genome is replicated more than once per cell cycle. Rereplication is believed to lead to genomic instability and has been implicated in the pathologies of a variety of human cancers. To prevent rereplication, eukaryotic cells have evolved multiple, overlapping mechanisms to inhibit chromosomal DNA from being partially or fully rereplicated in a given cell cycle. These control mechanisms rely on cyclin-dependent kinase (CDK) activity.〔 DNA replication control mechanisms cooperate to prevent the relicensing of replication origins and to activate cell cycle and DNA damage checkpoints.〔 DNA rereplication must be strictly regulated to ensure that genomic information is faithfully transmitted through successive generations.
==Origin licensing==
All known mechanisms that prevent DNA rereplication in eukaryotic organisms inhibit origin licensing.〔 Origin licensing is the preliminary step for normal replication initiation during late G1 and early S phase and involves the recruitment of the pre-replicative complex (pre-RC) to the replication origins. Licensing begins with the binding of the multi-subunit ATPase, the origin recognition complex (ORC), to the DNA at the replication origins. Once bound to chromatin the ORC recruits the AAA+ ATPase Cdc6 and the coiled-coil domain protein Cdt1. Cdt1 binding and the ATPase activity of ORC and Cdc6 facilitate the loading of the minichromosome maintenance (MCM) proteins 2-7 onto the chromatin.〔 The MCM complex is the DNA helicase that opens the helix at the replication origin and unwinds the two strands as the replication forks travel along the DNA. Elevated CDK activity at the end of G1 triggers the firing of the origins and the dismantling of the pre-RCs. High CDK levels, which are maintained until the end of mitosis, inhibit or destroy pre-RC components and prevent the origin from relicensing. A new MCM complex cannot be loaded onto the origin until the pre-RC subunits are reactivated with the decline of CDK activity at the end of mitosis. Thus, CDKs serve a dual role in the regulation of eukaryotic DNA replication: elevated CDK activity initiates replication at the origins and prevents rereplication by inhibiting origin re-licensing. This ensures that no replication origin fires twice in the same cell cycle.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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