Mitosis is cell division resulting in two daughter cells genetically identical to the parent cell. Deoxyribonucleic acid (DNA) is the genetic material carrying instructions for an organism’s development. Before division, DNA is organized into chromosomes, which are tightly packaged bundles of DNA and associated proteins. Mitosis does not involve chromosome duplication; the copying occurs beforehand. Mitosis separates the already duplicated chromosomes, ensuring each new cell receives a complete set.
The Cell Cycle: Preparation for Division
A cell’s life is organized into the cell cycle, a regulated sequence of growth and division. Most of a cell’s existence is spent in Interphase, a preparatory phase divided into three sub-phases. The first gap phase (G1) is a period of general cell growth, where the cell synthesizes proteins and increases mass. The cell must pass a regulatory checkpoint after G1 to commit to dividing.
Once committed to division, the cell enters the Synthesis phase (S phase), where DNA replication takes place. The cell then moves into the second gap phase (G2), continuing to grow and synthesizing structures required for cell division. After completing Interphase, the cell enters the final stage, the Mitotic phase (M phase).
DNA Replication: When Chromosomes Duplicate
Duplication of the cell’s genetic material occurs exclusively during the S phase of Interphase, not during mitosis. Replication involves unwinding the DNA double-helix and using each strand as a template to synthesize a new, complementary strand. This semi-conservative replication results in two identical copies of every DNA molecule.
After duplication, each original chromosome is composed of two identical DNA strands, known as sister chromatids. These chromatids are held together tightly at the centromere, a constricted region. Although the DNA amount has doubled, the connected structure is still counted as a single, duplicated chromosome. This pairing ensures two copies are ready for separation when the cell divides.
The Separation Process: What Mitosis Actually Does
Mitosis (M phase) begins after the genome is duplicated and the cell is ready for division. The primary function of mitosis is the precise segregation of sister chromatids into two separate nuclei. The process is divided into four sequential stages: Prophase, Metaphase, Anaphase, and Telophase.
In Prophase, the duplicated chromosomes condense, becoming tightly coiled and visible as distinct, compact structures. The nuclear envelope breaks down, and the mitotic spindle, a framework of protein fibers, begins to form. During Metaphase, spindle fibers attach to the centromere of each duplicated chromosome and maneuver them to align along the cell’s central plane, known as the metaphase plate.
The separation event occurs in Anaphase. Specialized proteins holding the sister chromatids together at the centromere cleave, allowing them to pull apart. Once separated, each chromatid is considered an individual chromosome. Spindle fibers actively pull these newly separated chromosomes toward opposite ends of the cell.
In Telophase, the separated chromosomes arrive at the opposite poles, and the mitotic spindle disassembles. A new nuclear envelope forms around each complete set of chromosomes, creating two distinct nuclei within the parent cell. The chromosomes then begin to uncoil, returning to their less-condensed state.
The Result: Two Identical Daughter Cells
The culmination of S phase duplication and mitotic separation is the creation of two genetically identical nuclei. Immediately following nuclear division, the final step of cell division, called cytokinesis, occurs. Cytokinesis is the physical division of the cytoplasm and the cell membrane, which splits the parent cell into two separate daughter cells.
Each resulting daughter cell contains a full and identical complement of chromosomes, exactly matching the parent cell’s original genetic content. This outcome is why mitosis is the basis for growth in multicellular organisms, as well as the replacement of old or damaged cells throughout life. The entire process ensures genetic stability is maintained across generations of cells.