7.2
真核生物でのレプリケーション
真核細胞では、DNA 複製は高度に保存され、厳密に調節されています。 細胞分裂の前に複数の直鎖状染色体を高い忠実度で複製する必要があるため、複製プロセスで特殊な役割を果たすタンパク質が多数存在します。 複製は開始、伸長、および終了の 3 つの期で起こり、核内の 2 つの完全な染色体をセットして終了し…
真核生物の染色体におけるDNA複製は、複数の複製起点で開始され、それらは起源認識複合体(ORC)によって同定され、結合されます。
次に、ORCはヘリカーゼをリクルートしてDNAを巻き戻し、2つの複製フォークで複製バブルを生成します。
2つのフォークは反対方向に動き、前方のヌクレオソームを破壊します。その後、これらのヌクレオソームは娘鎖上で再集合し、クロマチン構造を維持します。
各フォークでは、RNAプライマーがDNAポリメラーゼの部位を提供し、遅延鎖の先頭鎖と岡崎断片を伸長させます。
次に、RNase酵素がこれらのプライマーを除去し、DNAポリメラーゼがギャップを埋めます。最後に、DNAリガーゼがフラグメントを一緒にシールします。
しかし、最後のプライマーが線状染色体の末端にある遅行鎖から取り除かれると、テンプレートDNAの張り出したストレッチが生成されます。
テロメラーゼと呼ばれる酵素は、この張り出したストレッチをノンコーディングDNAで拡張し、その後の複製サイクルでコーディングDNAが失われるのを防ぎます。
複製は、隣接する複製バブルが結合し、染色体全体が複製されるまで続きます。
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Q1: What is DNA replication in eukaryotes and why is it essential?
DNA replication in eukaryotes is the process of copying the entire genome before cell division. It ensures each daughter cell receives an identical set of genetic instructions. This process is fundamental to maintaining genetic continuity across generations and is tightly regulated by the cell cycle control system to prevent errors.
Q2: How does the replication process begin at the DNA level?
DNA replication begins at specific sites called origins of replication where the double helix unwinds. Enzymes called helicases break the hydrogen bonds between base pairs, creating a replication fork. DNA polymerase then synthesizes new strands by adding complementary nucleotides to each template strand.
Q3: What are the key differences between leading and lagging strand synthesis?
The leading strand is synthesized continuously in the 5' to 3' direction as the replication fork advances. The lagging strand is synthesized discontinuously in short fragments called Okazaki fragments, also in the 5' to 3' direction but opposite to fork movement. Both strands require DNA polymerase but follow different synthesis patterns.
Q4: How do eukaryotes manage replication of their large genomes?
Eukaryotes have multiple origins of replication distributed across their chromosomes, allowing simultaneous replication at many sites. This parallel replication significantly reduces the time needed to copy the entire genome. Molecular factors affecting cell division coordinate the timing and regulation of these multiple replication events.
Q5: What mechanisms ensure accuracy during eukaryotic DNA replication?
DNA polymerase possesses proofreading ability, removing incorrectly paired nucleotides immediately after incorporation. Mismatch repair systems scan newly replicated DNA for errors and correct them post-replication. These quality control mechanisms maintain replication fidelity at approximately one error per billion nucleotides, ensuring genetic stability.
Q6: How is DNA replication regulated in the eukaryotic cell cycle?
Replication occurs exclusively during S phase of the cell cycle, controlled by checkpoint mechanisms and regulatory proteins. Once replication completes, re-replication is prevented until the next cell cycle begins. This regulation ensures DNA replicates exactly once per cell division cycle.
Q7: What role do telomeres play in eukaryotic DNA replication?
Telomeres are repetitive DNA sequences at chromosome ends that shorten with each replication cycle due to the end-replication problem. Telomerase, an enzyme with reverse transcriptase activity, extends telomeres in certain cell types. This mechanism protects chromosome integrity and limits cellular replication potential.