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Lesson 4 - Mitosis

Student Performance Objectives
1. Draw the following stages of mitosis in a series of labeled diagrams in a cell with 4 chromosomes: early prophase,
    late prophase (prometaphase), metaphase, anaphase, early telophase, late telophase.
2. Identify the following structures with their functions in short answer question format:
    cell membrane, nuclear membrane, chromosomes, centrioles, spindle fibers, asters,
    centromere, chromatids, cleavage furrow, and daughter cells.

Lesson Outline
A. Introduction to the Process
     1. In general, mitosis is the division of one cell into two cells. It is the fundamental process by which life creates more life. The two resulting cells are called daughter cells. The two daughter cells may not be equivalent. One of the two daughter cells may have the ability to undergo mitosis again. The other daughter cell may differentiate into a more specialized cell that will never again undergo mitosis. We observe this situation in many adult tissues with rapid mitotic rates like red marrow within osseus tissue (bone substances), the germinative layer of the epidermis, and the lining of the digestive, respiratory and genitourinary tracts.
     2. Growth and repair: Mitosis is responsible for the growth of a single fertilized human egg cell into an embryo and then a fetus. Mitosis increases the mass of children as they grow. Mitosis is responsible for the repair of a broken leg as bone and other connective tissue cells reproduce and repair the damaged area.
     3. Maintenance: Mitosis is responsible for the maintenance of the correct number of erythrocytes in the blood. These cells have a life span of approximately 120 days. It is estimated that 2 million erythrocytes spontaneously rupture each second. An equivalent number of red cells are produced in the body's red marrow each second to replace this loss. Similarity, there is a loss of many thousands of leukocytes (white blood cells) per second which must be balanced by mitosis of various leukocyte progenitor cells in bone marrow and in lymphatic tissue. Mitosis is also responsible for the replacement of the lining of the digestive, respiratory and urogenital tracts during the course of each 4-6 day period.
     4. Cell cycle: mitosis occurs during the mitotic phase of the cell's life cycle, called, in general, the cell cycle. As an example, a cell may divide once each 24 hours. One to three hours may be spent in mitosis. The cell is busy growing and doing metabolically what it is genetically programmed to do for the other 20-21 hours of its existence. The cell is said to be in interphase when it is not dividing. More detail is considered in the next lesson concerning the cell cycle.
     5. Mitotic stages: mitosis, although a continuous process, is divided into several phases: prophase, metaphase, anaphase, and telophase. Each of these phases can be subdivided into "early" and "late," like early and late prophase and early and late telophase. Cytokinesis is completed after telophase and two cells are observed where only one existed previously..
          a. Prophase - the DNA-histone complexes (nucleosomes) in the nucleus coil and super coil. The result is that the threads that displayed staining properties in the interphase nucleus and had been referred to as "chromatin," is now observed to be condensed into discreet chromosomes, the exact number being characteristic for a species. Since the DNA is already replicated (it did so during the "S" phase of interphase) each chromosome is actually a "duplicated chromosome" with each identical part called a chromatid. The chromatids are held together at a spot called the centromere.
The centromere is within a protein complex called the kinetochore. As the chromosomes appear, the nucleoli disappear. As prophase proceeds into late prophase (sometimes called prometaphase), the nuclear membrane disappears and the centrioles separate, moving toward the poles of the cell, and microtubules called spindle fibers radiate out from the centrioles and other microtubules attach to the kinetochores of the chromosomes of the cell.
         b. Metaphase - during metaphase the chromosomes are pushed and pulled by the microtubules (spindle fibers) into a central region of the cell (now without a nuclear membrane) called the metaphase plate. The chromosomes are now poised for movement to opposite poles of the cell which occurs in the next phase.
         c. Anaphase - When the centromere splits and the two chromatids become separated from each other, anaphase begins as the chromosomes now move to opposite ends of the cell due to a mechanism involving the spindle fibers and the kinetochore region of each "daughter" chromosome. Anaphase ends as the chromosomes reach the region of the centrioles at opposite ends of the cell.
         d. Telophase - cell changes during telophase bring it back to its interphase appearance: the chromosomes uncoil and become filamentous chromatin once more, the nuclear membrane reforms, nucleoli reappear.
         e. Cytokinesis - this division of the cytoplasm into two roughly equal portions actually begins during anaphase. It is like tightening a belt around the cell. It occurs at the level of the metaphase plate and is accomplished through the action of cytoskeletal elements. The resulting "cleavage furrow" deepens and results in splitting off of two daughter cells, each with a complete set of chromosomes,
during late telophase (around when the nuclear membrane is reformed).

B. Observe animations of the cell cycle and mitosis on the web:

C. Review mitotic stages through drawings on the board in a cell with 4 chromosomes (diploid number = 4).

Biomedical Terminology:

cleavage furrow
daughter cells
spindle fibers