Mitosis is a fundamental process of cell division where a single parent cell divides to produce two genetically identical daughter cells. This process is how multicellular organisms grow, replace old or damaged cells, and how some single-celled organisms reproduce asexually. Through mitosis, the genetic material, organized into chromosomes, is precisely duplicated and distributed evenly between the two new cells, ensuring genetic consistency across cell generations.
The Cell Cycle and Mitosis’s Role
A cell’s life is structured into a series of events known as the cell cycle, which includes periods of growth, DNA replication, and cell division. The cell cycle is broadly divided into two main stages: Interphase and the Mitotic (M) phase. Interphase is the longest part of the cell cycle, accounting for about 90% of a cell’s life, and is further subdivided into three phases: G1, S, and G2. During G1, the cell grows and synthesizes proteins and organelles. The S phase involves DNA replication, resulting in two identical sister chromatids for each chromosome. G2 is a period of further growth and preparation for division, including organelle duplication and cytoskeleton dismantling. Following interphase, the cell enters the M phase, which encompasses both mitosis, the division of the nucleus, and cytokinesis, the division of the cytoplasm.
Distinct Stages of Mitosis
The mitotic phase itself consists of several distinct stages, each marked by specific changes to the chromosomes, nuclear envelope, and spindle fibers. These stages ensure the accurate segregation of genetic material.
Prophase
Prophase marks the beginning of visible changes within the cell’s nucleus as chromatin fibers condense and coil, becoming visible as discrete chromosomes. Each chromosome consists of two identical sister chromatids joined at the centromere. Concurrently, the nuclear envelope starts to break down and eventually disappears, while the nucleolus also diminishes. Outside the nucleus, the mitotic spindle, made of microtubules, begins to form and extend from centrosomes, which move towards opposite poles of the cell.
Metaphase
As the cell progresses into metaphase, the mitotic spindle becomes fully developed, with centrosomes positioned at opposite ends of the cell. The condensed chromosomes then align precisely along the cell’s equatorial plane, known as the metaphase plate. Each sister chromatid is firmly attached to spindle fibers originating from opposite poles of the cell. These spindle fibers connect to specialized protein structures called kinetochores, located at the centromere of each sister chromatid, ensuring proper alignment before separation.
Anaphase
Anaphase is a rapid stage characterized by the separation of sister chromatids. Cohesin proteins binding the sister chromatids at the centromere break down, allowing them to split apart. Once separated, each chromatid is considered an individual chromosome. These newly separated chromosomes are pulled by shortening spindle fibers towards opposite poles of the cell. Non-kinetochore spindle fibers also lengthen, contributing to cell elongation.
Telophase
Telophase represents the final stage of nuclear division, where prophase events are essentially reversed. The separated chromosomes arrive at their poles and begin to decondense, uncoiling into less compact chromatin. A new nuclear envelope forms around each chromosome cluster at both poles, derived from remnants of the parent nuclear envelope and endoplasmic reticulum components. The nucleoli reappear within the newly formed nuclei, and the mitotic spindle fibers disassemble.
Cytokinesis and Completing Cell Division
Cytokinesis is the final process that typically overlaps with telophase and completes cell division by dividing the cell’s cytoplasm and organelles into two distinct daughter cells.
In animal cells, cytokinesis involves the formation of a cleavage furrow, a visible indentation on the cell surface. This furrow deepens as a contractile ring of actin filaments, positioned inside the plasma membrane at the former metaphase plate, tightens and pinches the cell into two.
In plant cells, cytokinesis proceeds differently due to a rigid cell wall. Instead of a cleavage furrow, plant cells develop a cell plate in the middle of the cell. This cell plate forms from the fusion of Golgi apparatus vesicles, transported to the metaphase plate by microtubules. The cell plate grows outward until it fuses with the existing cell wall, creating a new cell wall that divides the parent cell into two daughter cells.
Identifying Mitotic Phases in a Cell
Identifying the specific phase of mitosis in a highlighted cell involves recognizing distinct visual cues related to the chromosomes, nuclear envelope, and spindle fibers.
Interphase: The cell has a visible nucleus with diffuse chromatin; individual chromosomes are not distinguishable. A nucleolus is often apparent.
Prophase: Chromosomes become visibly condensed, often appearing as X-shaped structures. The nuclear envelope begins to disintegrate, and the nucleolus disappears.
Metaphase: All condensed chromosomes align in a single line at the cell’s center, forming the metaphase plate. Spindle fibers are clearly attached.
Anaphase: Sister chromatids separate and move towards opposite poles, often appearing V-shaped as they are pulled.
Telophase: Chromosomes cluster at opposite ends and begin to decondense. New nuclear envelopes form, and the cell appears elongated, showing signs of cytoplasmic division (cleavage furrow or cell plate).