What Is the G2 Checkpoint? A Critical Step in Cell Cycle Control

Cells must regulate division to maintain genomic integrity and prevent harmful mutations. The G2 checkpoint ensures cells are ready for mitosis by verifying DNA integrity and proper replication.

Disruptions in this checkpoint can lead to uncontrolled growth or genetic instability, contributing to cancer and other diseases. Understanding its function provides insight into potential therapeutic targets.

Role in Cell Cycle Progression

The G2 checkpoint acts as a regulatory barrier between DNA replication and mitosis, ensuring cells do not proceed with errors that could compromise genomic stability. Positioned at the G2-to-mitosis transition, it assesses whether DNA replication has been completed accurately. If errors are detected, the checkpoint halts progression, allowing time for corrections before division.

A central component of this regulation is cyclin-dependent kinase 1 (CDK1), which, when activated, drives mitotic entry. Under normal conditions, CDK1 pairs with cyclin B to form the maturation-promoting factor (MPF). However, if replication stress or structural abnormalities are detected, inhibitory signals prevent CDK1 activation, pausing the cycle. This prevents the propagation of mutations that could lead to genomic instability.

Beyond preventing premature mitotic entry, the checkpoint integrates stress response signals to determine whether a cell should proceed with division, undergo repair, or initiate programmed cell death. If DNA damage is extensive, apoptosis is triggered to eliminate defective cells, preventing mutation accumulation that could contribute to tumorigenesis.

DNA Damage Surveillance

Cells constantly encounter threats to DNA integrity, from endogenous replication errors to exogenous sources like radiation and mutagens. The G2 checkpoint employs a surveillance system to detect damage and initiate appropriate responses.

Sensor proteins scan the genome for irregularities, activating signaling cascades that determine whether the cell should pause for repair or undergo apoptosis. ATM and ATR kinases serve as primary detectors. ATM responds to double-strand breaks, while ATR detects single-stranded regions from stalled replication forks. These kinases phosphorylate checkpoint kinase 1 (CHK1) and checkpoint kinase 2 (CHK2), which regulate DNA repair pathways and delay mitotic entry, allowing time for correction.

The tumor suppressor protein p53 plays a key role in determining cell fate after genotoxic stress. If damage is detected, p53 promotes repair or, in severe cases, triggers programmed cell death. Mutations in the TP53 gene, which encodes p53, are common in cancers, highlighting the checkpoint’s role in preventing malignant transformation. Loss of p53 function allows unchecked cell cycle progression despite DNA damage, increasing the risk of chromosomal abnormalities and tumorigenesis.

Key Regulatory Proteins

The G2 checkpoint relies on regulatory proteins that control the transition from G2 to mitosis by modulating CDK1 activity. Their coordinated function ensures cells only proceed when DNA replication is complete and genomic integrity is maintained.

Cyclin B

Cyclin B binds to CDK1 to form MPF, driving mitotic entry. Its levels increase throughout G2, peaking before mitosis. The accumulation of cyclin B is tightly regulated to ensure precise timing of mitotic initiation. Activation of the cyclin B-CDK1 complex is controlled by inhibitory phosphorylation, preventing premature mitotic entry. Once the checkpoint confirms DNA replication is complete, this inhibition is lifted, allowing full MPF activation. After mitosis, cyclin B is rapidly degraded by the anaphase-promoting complex (APC/C), ensuring proper cycle progression.

CDC25

CDC25 is a phosphatase that activates CDK1 by removing inhibitory phosphate groups. CDC25C, the isoform relevant to the G2 checkpoint, counteracts the effects of inhibitory kinases like Wee1. Under normal conditions, CDC25C remains inactive until DNA replication is verified. Once activated, it enables CDK1 activation, driving mitotic entry.

CDC25C regulation is dynamic, subject to both activating and inhibitory phosphorylation. In response to DNA damage, CHK1 phosphorylates CDC25C, leading to its sequestration in the cytoplasm and preventing premature mitotic progression. This ensures cells do not enter mitosis with unresolved DNA lesions.

CHK1

CHK1, a serine/threonine kinase, enforces the G2 checkpoint by inhibiting CDC25 and stabilizing cell cycle arrest in response to DNA damage. Activated primarily by ATR in response to replication stress, CHK1 phosphorylates CDC25C, inactivating it and preventing CDK1 activation. CHK1 also stabilizes stalled replication forks, preventing their collapse and allowing replication to resume once stress is resolved.

Beyond checkpoint enforcement, CHK1 enhances DNA repair capacity, reinforcing genomic stability. Given its role in maintaining cell cycle fidelity, CHK1 inhibitors have been explored as cancer treatments, sensitizing tumor cells to DNA-damaging agents by disabling their checkpoint response.

Wee1

Wee1 is a kinase that prevents premature mitotic entry by phosphorylating CDK1 at inhibitory sites. This regulation ensures cells complete DNA replication and repair before progressing. Under replication stress, Wee1 extends the G2 phase, allowing time for corrective mechanisms. In response to DNA damage, Wee1 expression increases, reinforcing checkpoint-mediated arrest.

The balance between Wee1 and CDC25C determines whether a cell remains in G2 or proceeds to mitosis. Wee1 inhibitors have been explored in cancer therapy to force tumor cells with defective checkpoints into mitotic catastrophe, exploiting their reliance on G2 arrest for survival.

Consequences of Checkpoint Failure

Failure of the G2 checkpoint allows cells to enter mitosis with damaged or unreplicated DNA, leading to genomic instability. This increases the risk of mutations, including chromosomal rearrangements, deletions, and amplifications, frequently observed in cancer cells. Many tumors exhibit defects in checkpoint regulatory proteins, contributing to unchecked progression and aneuploidy, a condition where cells gain or lose entire chromosomes.

Beyond cancer, checkpoint failure is implicated in developmental disorders and age-related diseases. Cells bypassing the G2 checkpoint without adequate repair may enter premature senescence, leading to tissue dysfunction. In rapidly dividing tissues, such as bone marrow or the intestinal epithelium, this impairs regeneration and can cause organ failure.

Neurodegenerative conditions like ataxia-telangiectasia, caused by mutations in DNA damage response genes, further highlight the consequences of defective checkpoint signaling. The inability to halt the cell cycle in response to genotoxic stress compromises both individual cell viability and overall tissue homeostasis.