Mitosis is a form of cell division that occurs in somatic (nonsex) cells, and produces two new cells from an original cell (fig. 3.37). These new cells are genetically identical, each with the full complement of 46 (23 pairs of) chromosomes. In contrast is meiosis, another form of cell division that occurs only in sex cells (sperm and eggs). Meiosis halves the chromosome number, a mechanism that ensures that when sperm meets egg, the total number of 46 chromosomes is restored. Chapter 22 (pp. 884-887) considers meiosis in detail.

During mitosis, the nuclear contents divide, an event called karyokinesis, and then the cytoplasm is apportioned into the two cells, a process called cytokinesis. Mitosis must be very precise, because the nucleus contains the information, in the form of DNA molecules, that "tells" the cell how to function. Each new cell must have a complete copy of this information in order to survive. Although the chromosomes have already been copied in interphase, it is in mitosis that the chromosome sets are evenly distributed between the two forming cells.

Mitosis is a continuous process, but it is described in stages that indicate the sequence of major events, as follows:

1. Prophase. One of the first indications that a cell is going to divide is the appearance of chromosomes. These structures form as fibers of chromatin condense into tightly coiled rods. During interphase, the DNA molecules replicate so that each chromosome is composed of two identical structures, called chromatids, that are temporarily attached by a region on each called a centromere.

The centrioles of the centrosome replicate just before the onset of mitosis, and during prophase, the two newly formed pairs of centrioles move to opposite sides of the cell. Soon the nuclear envelope and the nucleolus disperse and are no longer visible. Microtubules are assembled from tubulin proteins in the cytoplasm, and these structures associate with the centrioles and chromosomes (figs. 3.38 and 3.39). A spindle-shaped array of microtubules (spindle fibers) forms between the centrioles as they move apart.

2. Metaphase. Spindle fibers attach to the centromeres of the chromosomes so that a fiber accompanying one chromatid attaches to one centromere and a fiber accompanying the other chromatid attaches to its centromere (fig. 3.40). The chromosomes move along the spindle fibers and align about midway between the centrioles as a result of microtubule activity.

3. Anaphase. Soon the centromeres of the chromatids separate, and these identical chromatids are now considered individual chromosomes. The separated chromosomes move in opposite directions, and once again the movement results from microtubule activity. The spindle fibers shorten and pull their attached chromosomes toward the centrioles at opposite sides of the cell (fig. 3.41).

4. Telophase. The final stage of mitosis begins when the chromosomes complete their migration toward the centrioles. It is much like prophase, but in reverse. As the identical sets of chromosomes approach their respective centrioles, they begin to elongate and unwind from rodlike structures to threadlike structures. A nuclear envelope forms around each chromosome set, and nucleoli become visible within the newly formed nuclei. Finally, the microtubules disassemble into free tubulin molecules (fig. 3.42).

Table 3.4 summarizes the stages of mitosis. Cytoplasmic Division

Cytoplasmic division (cytokinesis) begins during anaphase when the cell membrane starts to constrict around the middle, which it continues to do through telophase. The musclelike contraction of a ring of actin microfilaments pinches off two cells from one. The microfilaments assemble in the cytoplasm and attach to the inner surface of the cell membrane. The contractile ring forms at right angles to the microtubules that pulled the chromosomes to opposite ends of the cell during mitosis. As the ring pinches, it separates the two newly formed nuclei and apportions about half of the organelles into each of the daughter cells. The newly formed cells may differ slightly in size and number of organelles and inclusions, but they have identical chromosomes and thus contain identical DNA information. (fig. 3.43).

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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