Why does a eukaryotic cell need to undergo the specialized process of mitosis? After S phase, there are two complete sets of chromosomes. If the cell simply divided in half, each daughter cell would receive a random assortment of chromosomes. Mitosis is required to precisely distribute the duplicated sets of chromosomes to the daughter cells.

Eukaryotic chromosomes are made of chromatin, which consists of DNA wrapped around histone proteins. During interphase, chromosomes are extended and can’t be seen under the light microscope. Early in mitosis, the chromatin condenses, making the chromosomes visible. The two copies of the duplicated chromosomes are called sister chromatids and are linked at a region of the chromosome called the centromere. Within the centromere is a region called a kinetochore, which functions as an attachment site for the mitotic spindle. The mitotic spindle is an array of microtubules responsible for the precise distribution of chromosomes to the daughter cells. It’s function is extremely dependent on its function.

The microtubules of the spindle are called spindle fibers, which consist of repeated tubulin sub-units arranged end-to-end. This modular nature allows the fibers to lengthen and shorten through the addition and removal of tubulin sub-units. Spindle fibers are assembled by microtubule-organizing center called centrosomes located at the ends of the spindle.

In animal cells, centrosomes contain a pair of organelles called centrioles that help organize the spindle. Microtubules arrays called asters radiate from the centrosomes bracing the centrioles during spindle retraction. Assembly of the spindle begins in G2, as the centrosome replicates and tubulin is synthesized.

Mitosis is divided into four stages called prophase, metaphase, anaphase, and telophase. Cytokinesis occurs after mitosis. Prophase begins as the chromatin condenses into visible chromosomes and the nucleolus disappears. For simplicity, we’ll observe mitosis in a cell with four chromosomes. The centrosomes move toward opposite ends of the cell, forming spindle fibers between them. The cell will eventually divide at a right angle to the spindle axis. The nuclear envelope disappears, and the spindle fibers attach to the kinetochores of the chromosomes. Each chromosome becomes connected to both poles of the spindle. Other microtubules called nonkinetochore microtubules grow from each centrosome toward the center of the cell without attaching to chromosomes.

In metaphase, the spindle fibers align the chromosomes in the middle of the cell in a region called the metaphase plate, which is an imaginary plane cutting through the nucleus that predicts where it will divide. The mitotic spindle makes sure that the sister chromatids are equally spaced from the two poles by lining up the chromosomes along the exact midpoint of the cell. As metaphase ends, each centromere divides in two, separating the two sister chromatids.

During anaphase, the poles move apart as nonkinetochore microtubules from opposite poles slide past each other, lengthening the cell. Simultaneously, the spindle fibers contract, pulling sister chromatids away from each other toward opposite poles. Tubulin subunits are removed close to the kinetochore ends of the microtubules.

In telophase, a nuclear envelope forms around each set of chromosomes and the nucleoli reappear. Finally, the spindle apparatus is disassembled.

Now that we’ve seen how the nucleus divides, let’s look at cytokinesis - the division of the cytoplasm. During animal cytokinesis, a ring of actin microfilaments called the contractile ring constricts around the cell, like the tightening of an imaginary belt. This produces a groove in the cell surface called the cleavage furrow approximately at the site of the old metaphase plate. The cleavage furrow deepens until the cell is literally pinched into two daughter cells, each with one nucleus containing an identical set of chromosomes.

In plant cells, the cell wall is too rigid to be contracted by actin microfilaments. Instead, a double membrane called the cell plate composed of excess membrane components forms at about the same place as the old metaphase plate. A new cell wall is then made between the two membranes of the cell plate.

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