The cell cycle is the ordered series of events that leads to a cell dividing and duplicating its contents to create two daughter cells. This cycle is divided into two main phases: Interphase, where the cell grows and copies its DNA, and the Mitotic (M) phase, where the cell physically divides. The G2 phase, or “Gap 2,” is the final stage of Interphase, acting as the cell’s last opportunity for preparation before division begins.
During the G2 phase, the cell continues to grow and synthesizes the proteins and organelles necessary for separation. The cell ensures that the DNA replicated in the preceding S phase is complete and undamaged. Once preparations are finished, the cell enters the M phase, the process of nuclear and cellular division.
The G2/M Regulatory Checkpoint
Before the cell can commit to division, it must successfully pass through a rigorous quality control point known as the G2/M checkpoint. This checkpoint acts as a molecular gatekeeper, preventing the cell from entering the M phase with an incomplete or damaged genome. The cell checks for two primary conditions: the successful completion of DNA replication and the repair of any existing DNA damage.
The transition is governed by a complex of regulatory proteins, principally Cyclin B bound to Cyclin-Dependent Kinase (CDK1). The accumulation and activation of this Cyclin B-CDK1 complex, sometimes referred to as Maturation Promoting Factor (MPF), triggers the cellular changes required for entry into mitosis. If DNA damage is detected, signaling pathways are activated that inactivate the complex, pausing the cell cycle to allow time for repair. This “go/no-go” signal maintains genomic stability by ensuring that only fully prepared cells proceed to divide.
The Steps of Nuclear Division (Mitosis)
With the checkpoint successfully passed, the cell enters Mitosis, the process of nuclear division which is divided into four main sequential stages.
Prophase
Prophase involves the condensation of the cell’s genetic material, transforming the diffuse chromatin into compact, visible chromosomes. Simultaneously, the mitotic spindle begins to assemble, which is a structure made of microtubules that will facilitate chromosome movement.
Metaphase
The nuclear envelope completely breaks down, and the spindle fibers attach to the chromosomes. The chromosomes are then maneuvered until they line up exactly along the cell’s equatorial plane, called the metaphase plate. This alignment is a momentary pause during which a spindle assembly checkpoint confirms that every chromosome is correctly attached to fibers from both poles.
Anaphase
Anaphase is a rapid event where the paired sister chromatids suddenly separate. Once separated, each chromatid is considered a full chromosome. The spindle fibers shorten to pull these chromosomes toward opposite ends of the cell, while non-attached fibers lengthen to elongate the entire cell.
Telophase
Telophase marks the conclusion of nuclear division. The separated chromosomes arrive at the opposite poles and begin to decondense back into diffuse chromatin. A new nuclear envelope forms around each set of chromosomes, creating two distinct nuclei within the single parent cell. The mitotic spindle is disassembled.
Cytokinesis and Final Cell Separation
Following the separation of the genetic material, Cytokinesis is the physical process that divides the cytoplasm, organelles, and cell membrane to complete the formation of two daughter cells. This process often begins during the late stages of Anaphase or Telophase, ensuring the final separation follows closely behind the nuclear division. The mechanism for this physical separation differs between major cell types.
In animal cells, Cytokinesis occurs through the formation of a cleavage furrow, which is an indentation that encircles the cell at the metaphase plate. This furrow is pulled inward by a contractile ring composed of actin and myosin filaments located just beneath the plasma membrane. This ring constricts like a drawstring until the cell is pinched into two separate daughter cells.
Plant cells, which possess a rigid cell wall, cannot form a cleavage furrow, so they divide using a different mechanism. Vesicles carrying cell wall materials gather at the center of the cell, where they fuse to form a structure called the cell plate. This cell plate grows outward from the center until it connects with the existing cell walls, ultimately dividing the cell into two. The entire process results in two genetically identical daughter cells, each now ready to enter the G1 phase or a non-dividing G0 phase.