The Cell Cycle

In eukaryotes, the cell cycle comprises two main phases: interphase and the M phase. Interphase (G1, S, G2) prepares the cell for division by growing, repairing, and replicating its DNA. The M phase—mitosis followed by cytokinesis—produces two genetically identical daughter cells.

Mitosis: Nuclear Division

Mitosis is the process by which a cell’s nucleus divides. It is divided into five distinct phases:

• Prophase: Chromosomes condense, the nuclear envelope dissolves, and the mitotic spindle forms from centrioles that migrate to opposite poles.
• Prometaphase: Microtubules attach to kinetochores on each sister chromatid, pulling them toward the cell’s equatorial plane.
• Metaphase: Chromatids align precisely on the metaphase plate, with one sister chromatid on each side.
• Anaphase: Sister chromatids separate at the centromere and are pulled to opposite poles by the spindle apparatus.
• Telophase: Nuclear membranes reform around each set of chromosomes, and the chromosomes begin to decondense. The cell is not yet fully divided.

Cytokinesis: Cytoplasmic Division

Following telophase, cytokinesis physically separates the two daughter cells. In animal cells, a contractile ring of actin filaments forms around the cell’s equator, tightening to create a cleavage furrow that pinches the membrane inward until two distinct cells are fully formed. In plant cells, a cell plate forms at the midline, guided by microtubules, to build a new cell wall between the daughters.

Overlap Between Mitosis and Cytokinesis

While mitosis and cytokinesis are distinct processes, they overlap temporally. Cytokinesis typically begins during anaphase and completes after telophase, ensuring that chromosome segregation is finished before the cytoplasm is divided.

Understanding these steps is crucial for fields ranging from developmental biology to cancer research, where dysregulation of mitosis or cytokinesis can lead to aneuploidy or uncontrolled cell proliferation.