Mitosis is a biological process where a single cell divides to produce two genetically identical daughter cells. It is responsible for the growth of multicellular organisms, tissue repair, and asexual reproduction in single-celled organisms.
Preparing for Cell Division
Before mitosis, a cell undergoes interphase, a preparatory stage. During interphase, the cell grows and copies its DNA, ensuring each new daughter cell receives a complete set of genetic material. Interphase is divided into three sub-phases.
In G1 (first gap), the cell grows and duplicates organelles. Following G1, the cell enters the S phase, where DNA replication occurs. Each chromosome is duplicated, resulting in two identical sister chromatids that remain joined. The final stage is G2 (second gap), where the cell continues to grow and synthesizes proteins and organelles needed for mitosis.
The Four Phases of Mitosis
Mitosis involves four distinct phases, each ensuring the accurate distribution of genetic material. These phases occur in a strict sequential order to produce two identical nuclei.
Prophase
Prophase is the initial stage of mitosis. Chromatin within the nucleus condenses into distinct, compact chromosomes. Each chromosome consists of two sister chromatids joined at the centromere. The nucleolus disappears, and the nuclear envelope breaks down. The mitotic spindle, a framework of microtubules, begins to form outside the nucleus, moving towards opposite ends of the cell.
Metaphase
Following prophase, metaphase begins with the precise alignment of chromosomes. Condensed chromosomes align along the metaphase plate, an imaginary central plane. Spindle fibers, which are microtubules extending from opposite poles, facilitate this alignment. Each sister chromatid has a kinetochore at its centromere, where spindle fibers attach. The spindle fibers exert pulling forces on the chromosomes, maintaining their position at the cell’s equator.
Anaphase
Anaphase is where the duplicated genetic material is separated. The centromeres holding sister chromatids together split. Each separated chromatid is now an individual chromosome. These chromosomes are pulled towards opposite poles by the shortening of the spindle fibers. The cell elongates, which helps to further separate the two sets of genetic material.
Telophase
Telophase is the final stage of nuclear division in mitosis, reversing many prophase changes. Chromosomes at their poles decondense into a diffuse chromatin state. New nuclear envelopes reform around each set of chromosomes, creating two distinct nuclei. Nucleoli reappear. The mitotic spindle disassembles and disappears.
Completing Cell Division
After mitosis, the cell undergoes cytokinesis, which involves the division of the cytoplasm. Cytokinesis separates the parent cell into two distinct daughter cells, each containing one of the newly formed nuclei. This cytoplasmic division differs significantly between animal and plant cells due to their structural variations.
In animal cells, cytokinesis begins with a cleavage furrow. This indentation forms on the cell’s surface, typically appearing during late anaphase or telophase. A contractile ring, composed of actin and myosin filaments, forms inside the plasma membrane. This ring contracts, pinching the cell inward until it divides the cytoplasm and separates the cell into two independent daughter cells.
Plant cells, with their rigid cell walls, employ a different mechanism for cytokinesis. Instead of a cleavage furrow, a cell plate forms in the center. This cell plate is formed by the fusion of vesicles from the Golgi apparatus, which transport cell wall and membrane components to the equatorial plane. The cell plate grows outwards, fusing with the existing parental cell wall and plasma membrane, creating a new cell wall that divides the plant cell into two separate daughter cells.