8.17
Restarting Stalled Replication Forks
DNA replication is initiated at sites containing predefined DNA sequences known as origins of replication. DNA is unwound at these sites by the minich…
While halting a replication fork, the DNA polymerase stops synthesizing nascent DNA, but the helicase continues to unwind the double-stranded DNA for a short way before dissociating. Next, replication protein A, or RPA, binds and protects this excess single-stranded DNA at the stalled fork.
The RPA-coated single-stranded DNA then recruits the Rad9-Rad1-Hus1, or “9-1-1” complex, which in turn enables the binding of ATR. The ATR binding triggers the phosphorylation of Chk1 and Chk1 in turn phosphorylates the phosphatase Cdc25. The phosphorylation means that Cdc25 cannot accept further phosphates from the cell cycle regulator protein Cdk1 - so Cdk1 remains inactive, and the cell cycle is paused.
Next, before repair starts, a recombination protein called Rad51 replaces RPA on the single-stranded DNA. Then, to initiate fork reversal, Rad51 loads an enzyme called SMARCAL1 on the DNA, which acts like an annealing helicase to displace and stick the two newly synthesized strands together and form a four-way junction that resembles a chicken foot. This process is called the fork regression.
There are two ways to resolve a fork regression. In the first, BRCA2 stabilizes the Rad51 nucleofilament between the toes of the chicken foot and protects the remodeled fork from degradation by nucleases. Now the nascent lagging strand can serve as a template for extending the leading strand, thus bypassing the lesions on the parental strand.
Finally, SMARCAL1 reverses the regression fork by reannealing the parental strands. Here the lesion remains in the parent strand but the template switch allows the replicated DNA to be intact.
The second way of resolving the fork regression occurs in the absence of BRCA2, and here the four-way ‘chicken foot’ junction is cleaved by the structure-specific endonuclease Mus81, complexed with a junction endonuclease, Mms4. The cleavage generates double-stranded breaks, which may be repaired by homologous recombination.
Q1: What causes a replication fork to stall during DNA synthesis?
Replication forks stall when DNA polymerase encounters obstacles that block its progression along the template strand. These obstacles include DNA lesions, secondary structures, protein-DNA complexes, or insufficient nucleotide pools. When stalled, the fork cannot continue synthesizing DNA until the obstruction is removed or bypassed through specialized cellular mechanisms.
Q2: How does DNA damage stall the cell cycle and replication?
DNA damage triggers checkpoint mechanisms that halt cell cycle progression and replication fork movement. Sensor proteins detect damage and activate signaling cascades that pause DNA synthesis, allowing time for repair before replication resumes. This protective response prevents propagation of mutations and maintains genomic stability during the replication process.
Q3: What mechanisms restart a stalled replication fork?
Cells employ multiple restart pathways to resume stalled forks. These include lesion bypass mechanisms that allow polymerase to skip over damage, recombination-based restart using homologous recombination and strand invasion, and helicase-mediated fork stabilization. The specific pathway depends on the type of obstruction and fork configuration.
Q4: What role does the replisome play in fork stability?
The replisome is the multi-protein machine that synthesizes DNA and maintains fork stability. Its components coordinate leading and lagging strand synthesis while protecting the fork from collapse. When the replisome encounters obstacles, its structural integrity helps prevent fork degradation and facilitates restart through coordinated protein interactions.
Q5: How do cells distinguish between stalled and collapsed replication forks?
Stalled forks retain their protein machinery and can be reactivated, while collapsed forks have lost their structure and require more extensive repair. Collapsed forks often result from unresolved stalls and can generate double-strand breaks. Cellular surveillance systems recognize these differences and activate appropriate restart or repair pathways accordingly.
Q6: What happens if a replication fork cannot be restarted?
Unresolved stalled forks can collapse into double-strand breaks, triggering cell cycle arrest or apoptosis. Persistent fork stalling may lead to genomic instability, incomplete DNA replication, and cell death. Cells activate emergency repair pathways or checkpoint mechanisms to prevent propagation of incomplete or damaged DNA to daughter cells.
Q7: Why is fork restart critical for cell survival?
Fork restart ensures complete and accurate DNA replication, which is essential for cell viability and genomic stability. Without functional restart mechanisms, cells accumulate unrepaired damage, incomplete replication, and mutations. Proper fork restart prevents genomic instability and allows cells to complete S phase and proceed through the cell cycle successfully.