8.4
The 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.
Q1: What is the replisome and what are its main components?
The replisome is a multi-protein enzyme complex that synthesizes DNA during replication. It coordinates the activities of DNA polymerase, helicase, and other accessory proteins to unwind the double helix and catalyze nucleotide incorporation. This coordinated machinery ensures efficient and accurate duplication of genetic material at the replication fork.
Q2: How does the replisome handle leading and lagging strand synthesis differently?
The replisome synthesizes the leading strand continuously in the 5' to 3' direction as the replication fork advances. The lagging strand is synthesized discontinuously as Okazaki fragments in the opposite direction. This asymmetry arises because DNA polymerase can only add nucleotides in one direction, requiring the leading strand and lagging strand synthesis to proceed through distinct mechanisms.
Q3: What role does helicase play within the replisome?
Helicase is a critical replisome component that unwinds the DNA double helix ahead of the polymerase, breaking hydrogen bonds between base pairs. This unwinding exposes single-stranded template DNA, allowing DNA polymerase to access the bases and catalyze nucleotide incorporation. Without helicase activity, the replisome cannot progress along the chromosome.
Q4: How does the replisome coordinate with proofreading mechanisms?
The replisome integrates proofreading exonuclease activity into DNA polymerase, which immediately removes mismatched nucleotides during synthesis. This real-time error correction occurs as the polymerase extends the growing strand, reducing mutation rates before the DNA is fully replicated. Proofreading ensures high fidelity during the synthesis process.
Q5: What happens when the replisome encounters DNA damage?
When the replisome encounters damaged DNA, replication forks stall and cannot proceed. This triggers checkpoint mechanisms that halt cell cycle progression and activate repair pathways. If damage is not resolved, restarting stalled replication forks becomes necessary to resume DNA synthesis and allow the cell cycle to continue.
Q6: How does the replisome differ between prokaryotes and eukaryotes?
Prokaryotic replisomes are simpler, with a single origin of replication and fewer accessory proteins. Eukaryotic replisomes are more complex, with multiple origins firing simultaneously and additional proteins managing chromatin structure and telomere replication. Both systems maintain the core function of coordinating helicase and polymerase activities to synthesize DNA accurately.
Q7: Why is the replisome considered a processively moving machine?
The replisome is processive because it remains attached to DNA and continuously synthesizes new strands without dissociating. This sustained engagement allows rapid nucleotide incorporation and efficient replication of large genomic regions. Processivity is maintained through protein-protein interactions and sliding clamp mechanisms that keep polymerase bound to the template.