The Mitotic Process
It may look like cells are living out their useful lives simply doing whatever specialized jobs they do best, but in truth mitosis is a continuous process. When the cell isn’t actively splitting itself in two, it’s actively preparing to do so.
DNA and centrioles (arrays of microtubules) are being replicated, and the cell is bulking up on cytoplasm to make sure there’s enough for both daughter cells. Mitosis may look like a waiting game, but there’s plenty going on behind the scenes.
Waiting for action: interphase
Interphase is the period when the cell isn’t dividing. It begins when the new cells are done forming and ends when the cell prepares to divide. Although it’s also called a “resting stage,” there’s constant activity in the cell during interphase.
Interphase is divided into subphases, each of which lasts anywhere from a few hours for those cells that divide frequently to days or years for those cells that divide less frequently (nerve cells, for example, can spend decades in interphase). The subphases are as follows:
G1, which stands for “gap” or “growth.” During G1, the cell creates its organelles, begins metabolism, grows, and synthesizes proteins.
S, which stands for “synthesis.” DNA synthesis or replication occurs during this subphase. The single double-helix DNA molecule inside the cell’s nucleus becomes two new “sister” chromatids, and the centrosome (a type of organelle) is duplicated.
G2, which stands for “gap.” Enzymes and proteins needed for cell division are produced during this subphase.
Sorting out the parts: prophase
As the first active phase of mitosis, prophase is when structures in the cell’s nucleus begin to disappear, including the nuclear membrane (or envelope), nucleoplasm, and nucleoli. The two centrosomes, duplicated in the synthesis process during interphase and each containing two centrioles, push apart to opposite ends of the nucleus, forming poles.
The centrioles produce protein filaments that form mitotic spindles between the poles as well as asters (or astral rays) that radiate from the poles into the cytoplasm.
At the same time, the chromatin threads (or chromonemata) shorten and coil, forming visible chromosomes. The chromosomes divide into chromatids that remain attached at an area called the centromere, which produces microtubules called kinetochore fibers. These interact with the mitotic spindles to assure that each daughter cell ultimately has a full set of chromosomes. The chromatids start to migrate toward the equatorial plane, an imaginary line between the poles.
Dividing at the equator: metaphase
After the chromosomes are lined up and attached along the cell’s newly formed equator, metaphase officially debuts. The nucleus itself is gone. The chromatids line up exactly along the centerline of the cell (or the equatorial plane), attaching to the mitotic spindles by the centromeres. The centromere also is attached by microtubules (spindles) to opposite poles in the cell.
Packing up to move out: anaphase
In anaphase, the centromeres split, separating the duplicate chromatids and forming two chromosomes. The spindles attached to the divided centromeres shorten, pulling the chromosomes toward the opposite poles. The cell begins to elongate. In late anaphase, as the chromosomes approach the poles, a slight furrow develops in the cytoplasm, showing where cytokinesis will eventually take place.
Pinching off: telophase
Telophase occurs as the chromosomes reach the poles and the cell nears the end of division. The spindles and asters of early mitosis disappear, and each newly forming cell begins to synthesize its own structure. New nuclear membranes enclose the separated chromosomes. The coiled chromosomes unwind, becoming chromonemata once again. There’s a more pronounced pinching, or furrowing, of the cytoplasm into two separate bodies, but there continues to be only one cell.
Splitting up: cytokinesis
Cytokinesis means it’s time for the big breakup. The furrow, formed by a contractile ring that will divide the newly formed sister nuclei, migrates inward until it cleaves the single, altered cell into two new cells. Each new cell is smaller and contains less cytoplasm than the mother cell, but the daughter cells are genetically identical to each other and to the original mother cell, and will grow to normal size during interphase.