Cell division is a fundamental biological process for all living organisms, enabling growth, repairing damaged tissues, and facilitating reproduction. This intricate process ensures that genetic material is accurately passed from one cell to its descendants. Mitosis is a specific type of cell division responsible for producing two genetically identical daughter cells from a single parent cell.
The Four Phases of Mitosis
Mitosis, the process of nuclear division, comprises four distinct phases. These stages ensure the precise segregation of duplicated chromosomes into two new nuclei. The four phases are Prophase, Metaphase, Anaphase, and Telophase. These phases specifically relate to the division of the cell’s nucleus, rather than the entire cell.
The Preparatory Stage: Interphase
Before a cell must first prepare itself during a period called Interphase. This preparatory stage is not part of mitosis itself, but it is essential for successful cell division. During Interphase, the cell grows, synthesizes new proteins and organelles, and replicates its entire DNA content.
Interphase is subdivided into three main stages: G1 phase, S phase, and G2 phase. The G1 phase involves cell growth and the production of new organelles, preparing the cell for DNA replication. During the S phase, the cell’s DNA is synthesized, resulting in duplicated chromosomes, each consisting of two identical sister chromatids. The G2 phase involves continued cell growth and production of proteins necessary for mitosis.
Early Stages of Nuclear Division: Prophase and Metaphase
The initial stage of mitosis is Prophase. Within the nucleus, the chromatin (loosely organized DNA and protein complex) condenses to form compact, visible chromosomes. Each chromosome at this point consists of two identical sister chromatids joined at the centromere. Concurrently, the nuclear envelope (the membrane surrounding the nucleus) begins to break down.
As Prophase progresses, the mitotic spindle starts to form outside the nucleus. This structure, composed of microtubules, is responsible for separating the chromosomes. Following Prophase, the cell enters Metaphase. During Metaphase, the condensed chromosomes align along the cell’s equatorial plane, an imaginary line equidistant from the two spindle poles, often referred to as the metaphase plate. Spindle fibers, specialized microtubules emanating from the spindle poles, attach to the centromeres of each chromosome, preparing them for separation.
Completing Nuclear Division: Anaphase and Telophase
The cell transitions into Anaphase. During Anaphase, the sister chromatids of each chromosome separate at their centromeres. Once separated, each chromatid is considered an individual chromosome. These newly independent chromosomes are then pulled by the shortening spindle fibers towards opposite poles of the cell. This movement ensures that each developing daughter cell receives a complete and identical set of genetic information.
The final stage of nuclear division is Telophase, which begins once the separated chromosomes arrive at the opposite poles of the cell. At each pole, the chromosomes begin to decondense, returning to their less compact chromatin state. Concurrently, new nuclear envelopes form around each set of chromosomes, creating two distinct nuclei within the single parent cell. The mitotic spindle fibers also disassemble and disappear during this phase.
Dividing the Cell: Cytokinesis
The entire cell divides through a separate yet closely associated process called Cytokinesis. Cytokinesis involves the division of the cytoplasm and its contents, resulting in two distinct daughter cells. This process begins during late Anaphase or Telophase, overlapping with the final stages of nuclear division.
The mechanism of cytokinesis differs between animal and plant cells. In animal cells, a contractile ring of actin and myosin filaments forms just inside the plasma membrane at the metaphase plate. This ring constricts, forming a cleavage furrow that deepens until the cell pinches into two separate daughter cells. Plant cells, with their rigid cell walls, form a new cell wall between the two daughter nuclei. This involves the formation of a cell plate, which originates from vesicles derived from the Golgi apparatus, expanding outwards until it fuses with the existing parent cell wall, effectively dividing the cell.