The cell cycle is the ordered series of events a cell undergoes as it grows and divides into two daughter cells. This process drives growth, tissue repair, and reproduction. It is broadly divided into two main phases: interphase, a period of growth and preparation, and the mitotic (M) phase, when the cell divides. Interphase is the longest phase, during which the cell performs its normal functions while preparing for division. The G2 phase is a distinct part of this preparatory interphase.
What is the G2 Phase?
The G2 phase, the second growth phase, occurs after DNA replication (S phase) and immediately before cell division (M phase). This stage is a time of intense cellular activity, ensuring the cell is ready for mitosis. Its purpose is to serve as a final preparatory stage, allowing the cell to build resources and check for issues from earlier phases. This preparation helps prevent errors during cell division, ensuring accurate genetic material distribution.
Essential Preparations During G2
During the G2 phase, the cell undergoes substantial growth, expanding its cytoplasm and increasing its overall volume. Protein synthesis occurs, producing enzymes and structural proteins needed for mitosis. The cell also duplicates many organelles, such as mitochondria, endoplasmic reticulum, and the Golgi apparatus, ensuring each daughter cell receives complete cellular machinery. In animal cells, the centrosome replicates to organize the mitotic spindle. The cell replenishes its energy stores, accumulating adenosine triphosphate (ATP) to power cell division.
Quality Control and Repair in G2
The G2 phase involves rigorous quality control through the G2/M checkpoint. The cell meticulously inspects its replicated DNA for damage or errors from the S phase. Specialized proteins and enzymes detect DNA abnormalities. If issues like DNA breaks or incomplete replication are found, repair mechanisms engage and the cell cycle halts, allowing time for repairs before mitosis. This protective measure maintains genomic stability, preventing the transmission of mutations or abnormal chromosomes to new cells.
The G2 to M Phase Transition
The G2 phase culminates in the cell’s entry into the M phase, a transition orchestrated by specific molecular signals. Maturation Promoting Factor (MPF), a complex of Cyclin B and Cyclin-dependent kinase 1 (Cdk1), is central to this transition. As G2 progresses, Cyclin B levels increase, leading to active MPF formation. Once activated, MPF triggers events that prepare the cell for division, including chromosome condensation and mitotic spindle formation. The cell divides only after G2 preparations are complete and MPF reaches a threshold activity level.