The eukaryotic cell cycle is the sequence of events a cell undergoes from its formation until it divides into two new daughter cells. This sequence is fundamental to the life of an organism, serving as the mechanism for growth, tissue repair, and the replacement of aged or damaged cells. The process involves preparation and replication, followed by the physical separation of cellular components. This progression ensures that genetic material is accurately duplicated and distributed equally between the resulting cells. The entire cycle is divided into two major periods: the long preparatory phase, Interphase, and the relatively short division phase, the M phase.
Interphase: The Growth and Replication Phases
Interphase is the longest portion of the cell cycle, where the cell grows and prepares for division. This preparatory phase is subdivided into the G1, S, and G2 phases.
The G1 phase, or “First Gap,” immediately follows cell division and is characterized by intense cellular growth and metabolic activity. During this time, the cell synthesizes proteins, duplicates most cytoplasmic organelles, and increases in volume.
A major decision point occurs late in G1: if the cell receives the appropriate cues to divide, it transitions into the S phase. Cells that do not receive the signal typically exit the cycle and enter a quiescent state known as G0.
The S phase, or “Synthesis” phase, is dedicated to the duplication of the cell’s genetic material. DNA replication occurs, ensuring every chromosome is precisely copied. Each replicated chromosome consists of two identical DNA molecules, called sister chromatids, which remain tightly joined. In animal cells, the centrosome is also duplicated in preparation for division.
The G2 phase, or “Second Gap,” involves final preparations for physical division. The cell continues to grow and synthesizes proteins specifically needed for mitosis, such as those forming the mitotic spindle. G2 also serves as a final opportunity to check the integrity of the newly replicated DNA before the cell commits to separation.
M Phase: Nuclear and Cytoplasmic Division
The M phase is composed of two distinct, sequential processes: mitosis, the division of the nucleus, and cytokinesis, the division of the cytoplasm. Mitosis is a continuous process broken down into four stages, starting with Prophase.
In Prophase, the replicated chromatin fibers condense tightly, becoming visible as distinct chromosomes. The mitotic spindle begins to assemble from the duplicated centrosomes moving toward opposite poles of the cell.
In Prometaphase, the nuclear envelope completely breaks down, allowing spindle microtubules to invade the nuclear area. Microtubules capture the chromosomes by attaching to kinetochores, specialized protein structures located at the centromere of each sister chromatid. Metaphase is defined by the precise alignment of all chromosomes along the metaphase plate, an imaginary plane equidistant from the two spindle poles.
The cell proceeds to Anaphase once all chromosomes are correctly lined up and attached to microtubules from both poles. During Anaphase, cohesive proteins holding the sister chromatids together are cleaved, allowing the chromatids to separate completely. These individualized chromosomes are pulled toward the opposite spindle poles, ensuring an exact copy of the genome moves to each end of the cell.
The final stage of nuclear division is Telophase, where the chromosomes arrive at the poles and begin to decondense back into their diffuse chromatin state. New nuclear envelopes form around the two separate sets of chromosomes, creating two distinct nuclei.
Cytokinesis, the physical division of the cytoplasm, overlaps with the later stages of mitosis, typically beginning during Anaphase or Telophase. This process involves the formation of a contractile ring of actin and myosin filaments that pinches the cell membrane inward, known as cleavage furrow formation in animal cells. This action physically separates the parent cell into two genetically identical daughter cells, completing the cycle.
Gatekeepers of Progression: Cell Cycle Checkpoints
Cell cycle checkpoints operate at specific points in the sequence to monitor the cell’s internal and external environment. These control points prevent the transition to the next phase until all conditions are met.
The G1 checkpoint is the primary control mechanism determining whether a cell commits to division. It evaluates factors such as:
- Cell size
- The availability of nutrients
- The presence of growth factors
- Any evidence of DNA damage
If conditions are not optimal or if DNA damage is detected, the cell cycle is halted, preventing entry into the S phase.
The G2 checkpoint ensures that the cell is ready to initiate the M phase. Its main focus is confirming that DNA replication has been completed successfully and that no damage remains. If errors are found, the cell pauses progression to allow time for DNA repair mechanisms to correct the flaws before the nucleus divides.
The M checkpoint, also known as the spindle assembly checkpoint, operates during Metaphase. This mechanism monitors the alignment of all chromosomes at the metaphase plate and confirms that every kinetochore is properly attached to the spindle microtubules. The cell will not initiate Anaphase—the stage where sister chromatids separate—until the checkpoint confirms that all chromosomes are correctly positioned and attached.