28. Lesions V: Global Response (960)

Imagine the genome as a massive highway construction project. Thousands of crews are paving road at the same time, each working on its own short stretch. Each crew represents a replication fork.

Under normal circumstances, if one crew encounters a problem--a cracked pipe or a large rock in the path of a line--they handle it locally. They pause briefly, clear the obstruction, and continue paving.

Cells do something similar. When a replication fork encounters a lesion, local mechanisms such as translesion synthesis (TLS), re-priming, or fork reversal usually solve the problem. Most obstacles never affect the rest of the project.

But occasionally a problem is more serious--perhaps a much larger underground obstruction or a damaged utility line. The crew stops work and radios the central office.

At that point the issue is no longer local, and the project managers may slow work across the entire site, redirect equipment, and/or bring in specialized repair teams.

Initiating a global response

Recall that when a replication fork stalls, it generates a stretch of RPA-coated DNA (RPA-ssDNA) as the helicase continues unwinding DNA for a short time while synthesis pauses.

This stretch of RPA-ssDNA recruits and activates the protein kinase ATR. ATR then activates another protein kinase, CHK1, which helps coordinate the cell’s response to the stalled fork and stabilize replication under stress.

But sometimes a larger amount of RPA-ssDNA accumulates. This can occur during severe replication stress—for example, when forks remain stalled for long periods, when there are many stalled forks, or when DNA breaks occur during replication.

In these situations ATR activates many molecules of CHK1, strengthening the signal and allowing it to spread throughout the nucleus. At that point the response is no longer confined to a single fork. The cell begins coordinating replication across the entire genome.

The key point is that a sufficiently strong ATR–CHK1 signal is required to initiate a global response. And this makes sense. It shouldn't be easy to pause replication cell-wide.

The intra-S checkpoint

What is the global response? It's simply a long pause during S phase. This cell-wide reaction is known as the intra-S checkpoint. The checkpoint gives the cell more time to correct the serious problem that is impacting replication.

Let me say a few words about cell cycle checkpoints, generally. They are present in other phases of the cell cycle, too. Then I'll focus on the intra-S checkpoint.

As cells move through their cell cycle, they don't progress blindly from one stage to the next. At several points along the way the cell pauses and checks whether critical tasks have been completed correctly. These pause points are known as checkpoints.

A checkpoint isn't a permanent stop. It's a decision point. If everything looks normal, the cell proceeds to the next stage. If something is wrong, the cell extends the pause or arrests the cell cycle completely to address the problem.

One of the most important checkpoints--the intra-S checkpoint--operates during S phase. Because DNA replication is such a complex process, the cell continuously monitors it as it proceeds.

If the intra-S checkpoint is activated, replication slows down until the problem is resolved. The activation of new replication origins will be suppressed. Stalled replication forks will continue to be stabilized. And repair or other pathways needed to remove the underlying DNA damage or other replication problem will be mobilized.

Intra-S checkpoint. The cell gets a kind of "status report" at different points in the cell cycle called "checkpoints." The Intra-S checkpoint is focused on DNA replication. If there is significant replication stress within the cell as measured by the amount of activated CHK1 in the nucleus, the cell will arrest the cell cycle

A last resort: apoptosis

The intra-S checkpoint buys the cell time. In most cases this pause allows the cell to correct the problem. And once the problem is repaired, replication resumes and the cell cycle continues.

But sometimes the damage or other problem with replication cannot be repaired.

If damage remains unresolved, continuing the cell cycle would be dangerous. A cell carrying severe DNA errors could pass those errors to its daughter cells, creating mutations or chromosomal rearrangements and potentially cancer.

In such cases, the safest option for the organism is not to try to repair the cell, but rather to have it self-destruct, and so signaling pathways--often involving the protein p53, sometimes called the guardian of the genome--will trigger apoptosis.

Apoptosis is often called programmed cell death. It's not chaotic cell death but rather a carefully controlled biological program of choreographed steps.

Initially, a family of enzymes called caspases becomes activated. These enzymes function like molecular scissors, cutting specific proteins throughout the cell.

Then, as the process unfolds, the cell shrinks, the nucleus condenses, and the DNA is systematically fragmented. The cell then breaks into membrane-bound fragments, which are quietly removed by nearby immune cells.

Importantly, apoptosis occurs without spilling cellular contents into surrounding tissue, so it does not trigger inflammation.

Apoptosis is not a rare event. In the human body billions of cells undergo apoptosis every day. It's a normal part of organismal growth and development. And, as we've seen, it's the last resort if genome replication goes awry.

A hierarchy of safeguards

The responses described in the past few chapters form a hierarchy of safeguards.

Local mechanisms help replication forks bypass obstacles. Fork-protection systems stabilize stalled forks. The intra-S checkpoint coordinates the cell’s response when replication stress becomes widespread. And when the damage cannot be safely repaired, apoptosis removes the cell entirely.

Together these systems allow human cells to copy more than three billion letters of DNA with extraordinary reliability and accuracy.

In the final chapter, we'll take a step back and review how this intricate network of molecular machines works together to make accurate genome replication possible.