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Eukaryotic DNA replication is a conserved mechanism that restricts DNA replication to only once per cell cycle. Eukaryotic DNA replication of chromosomal DNA is central for the duplication of a cell and is necessary for the maintenance of the eukaryotic genome. DNA replication is the action of DNA polymerases synthesizing a DNA strand complementary to the original template strand. To synthesize DNA, the double-stranded DNA is unwound by DNA helicases ahead of polymerases, forming a replication fork containing two single-stranded templates. Replication processes permit the copying of a single DNA double helix into two DNA helices, which are divided into the daughter cells at mitosis. The major enzymatic functions carried out at the replication fork are well conserved from prokaryotes to eukaryotes, but the replication machinery in eukaryotic DNA replication is a much larger complex, coordinating many proteins at the site of replication, forming the replisome.〔 The replisome is responsible for copying the entirety of genomic DNA in each proliferative cell. This process allows for the high-fidelity passage of hereditary/genetic information from parental cell to daughter cell and is thus essential to all organisms. Much of the cell cycle is built around ensuring that DNA replication occurs without errors.〔 In G1 phase of the cell cycle, many of the DNA replication regulatory processes are initiated. In eukaryotes, the vast majority of DNA synthesis occurs during S phase of the cell cycle, and the entire genome must be unwound and duplicated to form two daughter copies. During G2, any damaged DNA or replication errors are corrected. Finally, one copy of the genomes is segregated to each daughter cell at mitosis or M phase.〔 These daughter copies each contain one strand from the parental duplex DNA and one nascent antiparallel strand. This mechanism is conserved from prokaryotes to eukaryotes and is known as semiconservative DNA replication. The process of semiconservative replication for the site of DNA replication is a fork-like DNA structure, the replication fork, where the DNA helix is open, or unwound, exposing unpaired DNA nucleotides for recognition and base pairing for the incorporation of free nucleotides into double-stranded DNA.〔 ==Initiation== Initiation of eukaryotic DNA replication is the first stage of DNA synthesis where the DNA double helix is unwound and an initial priming event by DNA polymerase α occurs on the leading strand. The priming event on the lagging strand establishes a replication fork. Priming of the DNA helix consists of synthesis of an RNA primer to allow DNA synthesis by DNA polymerase α. Priming occurs once at the origin on the leading strand and at the start of each Okazaki fragment on the lagging strand. DNA replication is initiated from specific sequences called origins of replication, and eukaryotic cells have multiple replication origins. To initiate DNA replication, multiple replicative proteins assemble on and dissociate from these replicative origins. The individual factors described below work together to direct the formation of the pre-replication complex (pre-RC), a key intermediate in the replication initiation process. Association of the origin recognition complex (ORC) with a replication origin is required to recruit both cell division cycle 6 protein (Cdc6) and chromatin licensing and DNA replication factor 1 protein (Cdt1), which initiate the assembly of the pre-RC. Both Cdc6 and Cdt1 proteins associate with the already bound ORC independently from each other. The ORC, Cdc6, and Cdt1 together are required for the stable association of the minichromosome maintenance (Mcm 2-7) complex proteins with replicative origins during G1 phase of the cell cycle. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Eukaryotic DNA replication」の詳細全文を読む スポンサード リンク
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