A cell in the G2 phase of the cell cycle is found to have incompletely replicated DNA. Which of the following outcomes is most likely?

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

The G2/M checkpoint serves as a critical quality-control gate that prevents cells containing incompletely replicated or damaged DNA from entering mitosis. This surveillance mechanism depends on the precise regulation of M-Phase Promoting Factor (MPF), a heterodimeric kinase complex composed of cyclin B1 and cyclin-dependent kinase 1 (CDK1/Cdc2). During normal G2 progression, Cdc25 phosphatase removes inhibitory phosphate groups from threonine-14 and tyrosine-15 residues on CDK1, thereby activating MPF and permitting the G2-to-M transition. When DNA replication remains incomplete, single-stranded DNA and stalled replication forks activate the phosphatidylinositol 3-kinase–related kinase ATR (ataxia telangiectasia and Rad3-related). ATR phosphorylates the effector kinase Chk1, which in turn phosphorylates Cdc25C on serine-216. This phosphorylation event creates a binding site for 14-3-3σ proteins, which sequester Cdc25C in the cytoplasm, preventing it from activating nuclear MPF. Simultaneously, the kinase Wee1 maintains the inhibitory phosphorylation on CDK1, further locking MPF in its inactive conformation. The net electrochemical and spatial consequence is that MPF remains catalytically dormant, and the cell cannot phosphorylate the mitotic substrates—lamin A/C, condensin complexes, and microtubule-associated proteins—required for nuclear envelope breakdown, chromosomal condensation, and spindle assembly.

Why Other Options Are Wrong

PILLAR 2 — STEP-BY-STEP LOGIC

The question stem specifies that a cell in G2 phase harbors incompletely replicated DNA. Because the S-phase synthesis program has not concluded, replication forks persist as physical structures bound by replication protein A (RPA)-coated single-stranded DNA. These structures are the molecular ligands that recruit and activate ATR at stalled forks through its partner protein ATRIP. The ATR→Chk1→Cdc25C inactivation cascade ensues as described above, arresting the cell at the G2/M boundary. The cell therefore cannot transit into prophase because MPF remains inhibited. This arrest is reversible: once replication completes and RPA-ssDNA signals dissipate, phosphatases such as PP2A dephosphorylate Chk1, Cdc25C is dephosphorylated, returns to the nucleus, and activates MPF. The correct answer, option C, reflects this checkpoint-dependent arrest: the cell will not proceed to mitosis until replication fidelity is achieved.

PILLAR 3 — DISTRACTOR ANALYSIS

Option A traps students who conflate cell-cycle progression with an irreversible, time-driven sequence rather than a checkpoint-regulated process. This reflects a misunderstanding of the feedback mechanisms that halt the cycle at specific surveillance nodes. The G2 checkpoint actively prevents MPF activation, making mitotic entry impossible until the molecular conditions are satisfied.

Option B attracts students who overgeneralize apoptosis as the default response to any cellular irregularity. While sustained, unrepairable replication stress can eventually trigger p53-mediated apoptotic pathways through mitochondrial cytochrome c release and caspase-9 activation, the immediate and most probable outcome is reversible cell-cycle arrest, not programmed cell death. The checkpoint provides an opportunity for repair before apoptosis is initiated.

Option D exploits confusion about cell-cycle directionality. Some students assume a cell can reverse course from G2 back into S phase to complete replication. However, the cell cycle operates as a unidirectional, vector-like process driven by the sequential degradation of cyclins via the anaphase-promoting complex/cyclosome (APC/C) and SCF ubiquitin ligases. No known molecular mechanism permits backward movement from G2 to S phase; the cell must resolve the deficit within G2 or undergo senescence and apoptosis.