What Are the Key Stages of Mitosis and the Cell Cycle?

Cell division is a regulated process that allows organisms to grow, develop, and replace old or damaged cells. This is accomplished through the cell cycle, a series of stages culminating in mitosis, the partitioning of genetic material. These processes ensure the formation of two genetically identical daughter cells from a single parent cell.

The Cell Cycle’s Preparatory Stages

Before a cell can divide, it passes through a preparatory period called interphase, which has three stages: G1, S, and G2. The G1 stage is a period of growth where the cell increases in size and synthesizes necessary proteins. The cell then advances to the S phase.

During the S (synthesis) phase, the cell’s primary task is to replicate its DNA, duplicating each chromosome into two identical sister chromatids. The cell then enters the G2 phase, where it continues to grow and assembles the materials required for division.

Prophase: The First Stage of Mitosis

Mitosis begins with prophase. Within the nucleus, the replicated DNA begins to condense and coil tightly, becoming visible as distinct chromosomes, each composed of two sister chromatids. Simultaneously, the mitotic spindle begins to form from microtubules at opposite poles of the cell. As prophase progresses, the nuclear envelope breaks down, allowing the spindle fibers to access the chromosomes.

Metaphase: Alignment at the Center

The mitotic spindle fibers attach to the centromere of each chromosome, leading to their alignment along the cell’s equator, an imaginary line called the metaphase plate. This alignment marks a checkpoint in the division process. The cell ensures that all chromosomes are properly attached to the spindle before proceeding to the next stage.

Anaphase: Separation of Chromatids

Anaphase begins with the splitting of the centromeres holding the sister chromatids together. Once separated, the chromatids are considered individual chromosomes. The spindle fibers attached to each chromosome then shorten, pulling them toward opposite poles of the cell. This movement ensures a complete set of genetic material is delivered to each end as the cell itself begins to elongate.

Telophase and Cytokinesis: Forming Two New Cells

The final stage of mitosis is telophase, which reverses many events from prophase. The chromosomes arrive at opposite poles of the cell and begin to uncoil and become less condensed. A new nuclear membrane forms around each set of chromosomes, creating two distinct nuclei within the single cell. Overlapping with telophase is cytokinesis, the physical division of the cytoplasm. In animal cells, a cleavage furrow pinches the cell in two, while plant cells form a cell plate that develops into a new wall.