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S-Cdk 启动 DNA 复制
细胞周期是导致 DNA 复制、随后细胞内容物分裂形成两个子细胞的一系列事件。 细胞周期分四个阶段进行 — 细胞增大（间隙 1 或 G1 期）、复制其 DNA（合成或 S 期）、准备分裂（间隙 2 或 G2 期）和分裂（有丝分裂或 M相）。
复制起点的两种状态
在真核生物中，复制起始发生在染色体上的许…
在真核生物中，DNA复制发生在细胞周期的S期。由一系列调控蛋白组成的细胞周期控制系统，调控着细胞周期的进程。
S期周期蛋白依赖性激酶（S-Cdks）是参与S期细胞周期调控的酶复合物。该调控系统可确保基因组中的每个核苷酸在复制过程中仅被复制一次。
DNA复制起始于染色体上多个位置存在的复制起点。在复制启动之前，称为复制起点识别复合物的多蛋白复合体与DNA结合，并作为其他蛋白质的停靠位点。
在细胞周期的G1期早期，调控蛋白Cdc6和Cdt1会结合到ORC上，并协助将一组称为MCM蛋白的蛋白质组装成非活性的环状复合物，结合在相邻的DNA上，从而形成称为前复制复合物（pre-RCs）的大型多蛋白复合物。MCM蛋白在DNA复制中发挥DNA解旋酶的功能。
S期开始时，S-Cdk被激活，并通过磷酸化特定的起始蛋白触发复制起点的激活，即DNA复制的起始。被磷酸化的起始蛋白促进解旋酶激活复合物的招募。这些复合物激活DNA解旋酶并招募DNA聚合酶，从而启动复制过程。
S期周期蛋白依赖性激酶不仅启动DNA复制，还防止同一复制起点发生再次复制。
S期CDKs磷酸化Cdc6和Cdt1蛋白，促进它们从ORC上解离，导致这些蛋白降解以及前复制复合物（pre-RCs）的解体。
DNA复制完成后，当解旋酶从DNA链上解离时，S期周期蛋白依赖性激酶（S-Cdks）会磷酸化解旋酶，从而触发其从细胞核中输出。
这种涉及S-Cdk的细胞周期调控机制可防止复制的再次起始，从而确保每个细胞周期中DNA复制仅发生一次。
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Q1: What role does S-Cdk play in initiating DNA replication?
S-Cdk, a cyclin-dependent kinase, is the primary molecular trigger for DNA replication initiation. It phosphorylates key licensing factors and replication proteins, converting cells from G1 phase into S phase. This activation marks the commitment point where the cell begins duplicating its entire genome, a critical step within the cell cycle.
Q2: How does S-Cdk differ from other cyclin-dependent kinases?
S-Cdk is specifically active during S phase and targets replication machinery proteins, whereas other Cdks regulate different cell cycle transitions. S-Cdk's substrate specificity and timing distinguish it as the primary replication initiator, making it essential for coordinating DNA synthesis with other cell cycle events.
Q3: What happens to licensing factors when S-Cdk becomes active?
S-Cdk phosphorylates licensing factors like Cdt1 and Cdc6, preventing them from rebinding to chromatin. This phosphorylation ensures DNA replicates only once per cell cycle by blocking re-licensing of origins that have already fired, maintaining genomic stability and preventing gene amplification.
Q4: Why is S-Cdk activity restricted to S phase?
S-Cdk activity is tightly regulated by cyclin S levels, which rise only during S phase. This temporal restriction ensures DNA replication initiates at the appropriate time and prevents inappropriate re-replication. Once S phase ends, cyclin S degradation inactivates S-Cdk until the next cell cycle.
Q5: What proteins does S-Cdk phosphorylate to activate replication origins?
S-Cdk phosphorylates origin recognition complex proteins and helicase loader proteins, facilitating the assembly of replication machinery at origins of replication. These phosphorylation events promote the loading of MCM2-7 helicases and other initiation factors required to unwind DNA and begin synthesis.
Q6: How does S-Cdk prevent re-replication of DNA?
S-Cdk phosphorylates and inactivates licensing factors, preventing them from re-loading onto replicated DNA. This mechanism ensures each chromosomal region replicates exactly once per cell cycle. The inhibition persists until licensing factors are dephosphorylated in the next G1 phase, maintaining genomic integrity.
Q7: What is the relationship between S-Cdk activation and G1/S transition?
S-Cdk activation marks the G1/S checkpoint transition, committing cells to DNA replication. Rising S-cyclin levels drive S-Cdk activity, which phosphorylates substrates that both activate replication origins and prevent re-licensing. This dual action ensures orderly progression through the cell cycle and prevents genomic instability.