The Mitotic (M) phase represents the period of the cell cycle dedicated to cell division, following the preparation and DNA replication stages of interphase (G1, S, and G2 phases). The M phase ensures the faithful distribution of genetic material from a single parent cell into two genetically identical daughter cells. It is composed of two major events: mitosis, the division of the nucleus, and cytokinesis, the subsequent division of the cytoplasm and its contents. Mitosis is conventionally described through four distinct stages—Prophase, Metaphase, Anaphase, and Telophase.
Prophase: Chromosome Condensation and Spindle Formation
Prophase marks the beginning of mitosis, preparing the duplicated genetic material for separation. The diffuse genetic material, known as chromatin, condenses into compact structures called chromosomes. Each chromosome consists of two identical strands, the sister chromatids, which are held tightly together by cohesin proteins.
As the chromatin condenses, the centrosomes begin to move toward opposite ends of the cell. Microtubules extend from these separating centrosomes, forming the initial structure of the mitotic spindle. The nucleolus disappears, and the nuclear envelope, the membrane surrounding the nucleus, begins to break down, allowing spindle microtubules to access the chromosomes.
Metaphase: Alignment at the Equatorial Plate
Following the breakdown of the nuclear envelope, the cell enters metaphase, characterized by the arrangement of chromosomes at the cell’s center. Microtubules from the mitotic spindle attach to the kinetochore, a specialized protein complex on each sister chromatid. This attachment is bi-oriented: microtubules from one pole attach to one kinetochore, and microtubules from the opposite pole attach to the other.
The opposing pulling forces position the chromosomes in a single file line along the metaphase plate or equatorial plate. This alignment ensures the genetic material is evenly poised for segregation. The spindle assembly checkpoint verifies that every chromosome is correctly attached to the spindle from both poles before the process continues.
Anaphase: Separation of Sister Chromatids
Anaphase begins once the spindle assembly checkpoint signals that all chromosomes are properly aligned and attached. This stage is defined by the physical separation of the sister chromatids, which are now considered individual chromosomes. The cohesin proteins holding the sister chromatids together are cleaved by an enzyme called separase.
Once freed, the newly separated chromosomes are pulled toward the opposite spindle poles. This poleward movement is achieved primarily through the shortening of the kinetochore microtubules.
Telophase: Nuclear Envelope Reformation
Telophase represents the final stage of mitosis. The segregated chromosomes arrive at their respective poles and begin to decondense back into chromatin.
A new nuclear envelope forms around each set of chromosomes at the poles. The reformation of the nucleus creates two separate nuclei within the original cell boundary, completing nuclear division. Concurrently, the mitotic spindle apparatus begins to disassemble.
Cytokinesis: Division of the Cytoplasm
While telophase concludes mitosis, the M phase is completed by cytokinesis, the process that divides the cytoplasm between the two forming daughter cells. In animal cells, cytokinesis begins with the formation of a contractile ring composed of actin and myosin filaments, aligned with the former metaphase plate. The contraction of this ring pulls the plasma membrane inward, creating a visible indentation known as the cleavage furrow.
This furrow deepens until it pinches the parent cell in two. Plant cells, which possess a rigid cell wall, accomplish this division differently by forming a cell plate. The cell plate grows outward from the center to form a new cell wall between the two nuclei.