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Het Replisome
DNA-replicatie wordt uitgevoerd door een groot complex van eiwitten die op een gecoördineerde manier werken om high-fidelity DNA-replicatie te bereike…
DNA replication is carried out by a highly coordinated multi-protein assembly known as the DNA replication machinery or the replisome, which increases the efficiency of DNA replication.
The core components of the machinery are helicase, single-strand DNA binding proteins, DNA primase, sliding clamps, a clamp loader, and multiple DNA polymerases, which are all associated with each other near the replication fork.
Replicative DNA polymerases have a processivity of around 10 nucleotides, which is the number of nucleotides it can add to the daughter strand before dissociating from the template strand.
This is too inefficient to copy entire genomes in a reasonable timeframe and this problem is solved with the help of sliding clamp proteins.
When ATP associates with the clamp loader protein, they will bind to and open the sliding clamp so that its ring-like structure can surround the primer-template DNA complex. Once bound, the clamp loader hydrolyzes the ATP to ADP, causing the clamp loader to disassociate and the clamp to close around the DNA.
Then, DNA polymerase binds to the clamp proteins and together they slide along the template DNA, tethering DNA polymerase to the strand and increasing its processivity up to 1000 nucleotides.
This increased processivity allows the DNA polymerase to carry out continuous DNA replication on the leading strand.
However, on the lagging strand template, another DNA polymerase performs discontinuous DNA replication in a manner that allows the DNA polymerase molecules to synthesize the leading and lagging strands simultaneously. This process is sometimes described as the “Trombone model.”
The lagging strand and its template strand form a loop when DNA polymerase initiates Okazaki fragment synthesis from an RNA primer.
The DNA loop grows from both directions as helicase unwinds DNA and the lagging strand is synthesized.
When the DNA polymerase encounters the next RNA primer, it detaches from the template strand. Meanwhile, primase adds another primer to the lagging strand, and the growing DNA loop is released.
The clamp and clamp loader proteins allow DNA polymerase to quickly reassociate with the primed DNA template.
The formation and subsequence collapse of the DNA loop repeats with the synthesis of each new Okazaki fragment.
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Q1: What is the replisome and why is it important for DNA replication?
The replisome is a highly coordinated multi-protein assembly that carries out DNA replication and significantly increases its efficiency. Core components include helicase, single-strand DNA binding proteins, DNA primase, sliding clamps, clamp loaders, and multiple DNA polymerases, all working together near the replication fork to enable rapid and accurate genome copying.
Q2: How do sliding clamp proteins increase DNA polymerase processivity?
Sliding clamp proteins tether DNA polymerase to the template strand, dramatically increasing processivity from approximately 10 nucleotides to 1,000 nucleotides. The clamp loader uses ATP hydrolysis to open the clamp's ring-like structure around the primer-template complex. Once closed, the polymerase-clamp complex slides along DNA, enabling continuous synthesis on the leading strand.
Q3: What is the Trombone model and how does it explain lagging strand synthesis?
The Trombone model describes how leading and lagging strands are synthesized simultaneously through DNA loop formation and collapse. When DNA polymerase initiates Okazaki fragment synthesis from an RNA primer, the lagging strand forms a loop that grows as helicase unwinds DNA. When polymerase encounters the next primer, the loop releases and the cycle repeats, allowing coordinated synthesis of both strands.
Q4: What are the key differences between prokaryotic and eukaryotic replisome components?
Prokaryotes use single hexamer helicases and single-subunit primase (DnaG), while eukaryotes employ double hexamer helicases requiring Cdc45 and GINS proteins, and multisubunit primase (DNA polymerase-α primase). Prokaryotes use one replicative polymerase (Pol III), whereas eukaryotes use two (Pol ε and Pol δ). Sliding clamps differ too: β-clamp in prokaryotes versus PCNA in eukaryotes.
Q5: How do single-strand DNA binding proteins function in the replisome?
Single-strand DNA binding proteins prevent separated DNA strands from reannealing after helicase unwinding. In prokaryotes, SSB proteins consist of a single subunit, while eukaryotic versions form a heterotrimeric complex called replication protein A (RPA). These proteins stabilize single-stranded regions, keeping template strands accessible for polymerase synthesis and primer synthesis.
Q6: What role does primase play in initiating DNA synthesis?
Primase synthesizes short RNA primers that provide the 3'-OH group required for DNA polymerase to begin synthesis. In prokaryotes, DnaG primase creates approximately 12-nucleotide primers, while eukaryotic DNA polymerase-α primase generates around 25-nucleotide RNA-DNA hybrid primers. These primers are essential because DNA polymerases cannot initiate synthesis de novo.
Q7: How do clamp loaders attach sliding clamps to DNA?
Clamp loaders are pentameric AAA+ ATPase proteins that use ATP hydrolysis to load sliding clamps onto DNA. When ATP binds, the clamp loader opens the clamp's ring structure, allowing it to encircle the primer-template complex. ATP hydrolysis causes the clamp loader to dissociate and the clamp to close, securing the polymerase to the DNA template for processive synthesis.