The cell cycle describes the journey a cell takes from its creation until it divides into two new daughter cells. Interphase, the period of preparation, is composed of three stages: G1, S, and G2. The G2 phase, or “Gap 2,” is the final preparatory stage that immediately precedes the physical division of the cell. This phase ensures the cell makes its last checks and accumulates the resources necessary for a successful division.
Defining the G2 Phase
The G2 phase follows the S phase, where the cell’s entire genome was duplicated, and precedes the M phase (mitosis and cytokinesis). A cell entering G2 has twice the original amount of DNA, and the primary goal is to verify that all duplicated genetic material is intact and error-free. This is a period of rapid cell growth and extensive protein synthesis, ensuring the cell reaches an adequate size to divide into two viable daughter cells.
While G2 is sometimes the shortest part of interphase, its duration is highly variable depending on the cell type and external conditions. Some cells bypass G2 entirely, but for most cells, this time is used for quality control and stocking up on supplies. The successful completion of this phase is an irreversible commitment to cell division.
Cellular Preparation for Division
G2 is characterized by high biosynthetic activity, as the cell rapidly manufactures the tools and energy reserves needed for mitosis. The cell synthesizes a large number of specific proteins and enzymes required exclusively for division. For instance, structural proteins like tubulin, which form the spindle fibers to pull chromosomes apart, are produced in large quantities during G2.
Energy storage is also a major focus, with the cell accumulating high levels of adenosine triphosphate (ATP) to power chromosome movements and structural rearrangements. Furthermore, cellular organelles, such as mitochondria and the endoplasmic reticulum, continue to grow and duplicate so that each future daughter cell receives adequate machinery for survival. In animal cells, the duplicated centrosomes, which organize the mitotic spindle, begin to move away from each other, positioning themselves at opposite poles of the cell nucleus.
Monitoring Cell Integrity (The G2 Checkpoint)
A defining feature of the G2 phase is the G2/M checkpoint, a regulatory mechanism that acts as the cell’s final quality control station before entering mitosis. The main purpose of this checkpoint is to check the integrity of the duplicated DNA synthesized in the preceding S phase. Specialized molecular surveillance systems scan the genome for any signs of damage, breaks, or incomplete replication.
If damaged or unreplicated DNA is detected, these systems trigger a signal that halts the progression of the cell cycle, arresting the cell in G2. This arrest provides an opportunity for the cell’s repair machinery to fix the errors before division proceeds. If the damage proves too extensive, the cell may be directed toward programmed cell death, or apoptosis, preventing the damaged genetic material from being passed on. This function is a major defense against genomic instability, which is often a precursor to diseases like cancer.
Signaling the Start of Mitosis
Once preparatory tasks are complete and the G2 checkpoint confirms the DNA is sound and the cell size is adequate, a molecular signal commits the cell to division. This commitment is driven by the activation of specific cell cycle control molecules. These molecules are protein complexes formed by combining a regulatory protein (cyclin) with an enzyme (cyclin-dependent kinase).
As the G2 phase progresses, the level of a specific mitotic cyclin increases. Once the checkpoint is passed, this cyclin-kinase complex becomes highly active, acting as the “go” signal for mitosis. The surge in activity initiates the physical changes that mark the transition to the M phase, such as the initial condensation of chromosomes and the organization of the mitotic spindle.