Mitosis

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Mitosis

Mitosis is a process that sorts and evenly distributes a cell's genetic instructions to the nuclei of two daughter cells during cell division . Mitosis distributes identical DNA instructions to new cells when the old cell divides.

Growth is based on cell division and mitosis. Some cells in the body—such as nerve and skeletal muscle cells—cannot divide, and they stay with us for life. But most tissues of the body grow and replace themselves by cell division. These cells go through a cell cycle from the time they are formed until the time when they divide. During the period of mitosis, the nucleus and cytoplasm divide to form two new cells. The rest of the cell cycle is known as interphase.

In the nucleus of a cell, DNA comprises a code carrying all the instructions that the cell needs to live. In the nuclei of eukaryotic cells, the DNA molecules, coiled like microscopic spaghetti, form compact, bulky structures called chromosomes. Under the light microscope, chromosomes have the appearance of solid, flexible rods. Early microscopists applied the word chromosome , meaning "colored body," to structures that took up basic red or purple dyes in the nucleus of a dividing cell. Humans have 23 chromosomes, but other organisms, like the fruit fly, have as few as four, and others, such as the dog, as many as 39.

As chromosomes uncoil at the beginning of interphase, the thin threads of DNA, termed chromatin , become invisible to an observer looking through a light microscope. Each chromosome then makes a copy of itself, stringing together basic units available in the cytoplasm according to its own sequence. Now two identical DNA strands, called sister chromatids, are attached to each other by a structure known as a centromere. The DNA begins to twist and coil, curling more and more tightly.

At this point, the cell begins to assemble the scaffolding it needs for moving the chromosomes to opposite ends of the cell. In animal cells, the two centrioles—small cylinders containing tiny tubular elements—replicate. The cells of higher plants do not have centrioles.

Mitosis is a continuous process, but for convenience, scientists divide it into four stages: prophase, metaphase, anaphase, and telophase.

Prophase

The cell enters prophase as the long tangled DNA molecules, or chromatin, coil into the compact bodies of chromosomes. This coiling process is comparable to taking a thin strand 656 ft (200 m) long and coiling it into a cylinder 1 mm wide by 8 mm long. A structure called the kinetochore is formed on each chromatid at the outer face of the centromere region (see Figure 1). The nuclear membrane breaks down.

During prophase, the cells' cytoskeleton or structural framework made of the protein tubulin breaks down into subunits. From these subunits, a bridge of microtubules called the spindle apparatus forms between the two pairs of centrioles as they move apart. When the centrioles reach opposite ends of the cell, they extend microtubules in all directions. Like a boat moored to a dock with multiple lines, the centriole anchors itself to the cell membrane. This arrangement of microtubules—called an aster because of its star-like pattern—is thought to mechanically strengthen the spindle apparatus. Mitosis does not appear to depend upon the presence of centrioles, however. Destroying centrioles in animal cells with a laser beam does not prevent mitosis. Plant cells equipped with rigid cell walls, but neither centrioles nor asters, also grow by mitotic division.

As prophase continues, a set of microtubules grows from the kinetochore of each sister chromatid. The microtubules extending from each chromatid become attached to opposite poles of the spindle.

Metaphase

In the second stage of mitosis, called metaphase, the pairs of sister chromatids line up in the center of the cell like couples taking their place on a dance floor. The centromeres of the chromosomes appear to be aligned within an imaginary plate midway between the centrioles and dissecting the cell at right angles to the spindle. Responding to some unknown cue, the centromeres divide in unison, freeing the sister chromatids to separate in the next phase.

Anaphase

As if following a neatly choreographed dance, the sister chromatids separate, rapidly moving toward the pole to which their microtubule is attached. The cell appears "stretched" as the spindle fibers slide past one another, elongating the spindle apparatus and further separating the poles. Shortening of the microtubules by removal of tubulin units pulls the chromosomes closer and closer to the pole. The movement of sister chromatids to opposite sides of the cell completes the equal division and distribution of genetic material.

Telophase

During telophase, or the "clean-up" stage of mitosis, the spindle apparatus is broken down, and the tubulin subunits stand ready to form the cytoskeleton of a new cell. The chromosomes, now clustered in two groups around the poles, uncoil into tangled threads again, and a new nuclear envelope forms around them. At this point, each new nucleus contains one copy of each chromosome. Mitosis is complete.

Cytokinesis

Cell division is not finished, however. During cytokinesis, the cytoplasm of a cell is physically divided to form two daughter cells housing the newly formed nuclei. In addition to dividing up the cytoplasm, cytokinesis distributes cellular organelles equally to the daughter cells. The binding of some molecules or organelles to the chromosomes or spindle microtubules ensures that each daughter cell will receive a fair share of cytoplasmic components.

A belt of microfilaments constricts the cell, pinching it in two. In plants, a cell plate forms, growing outward until it reaches the cell membrane and fuses with it. Cellulose is laid down on the new membranes, forming a strong new cell wall.

Resources

books

Arms, Karen, and Pamela S. Camp. Biology. 3rd ed. Philadelphia: Saunders College Publishing, 1987.

Beck, William S., Karel F. Liem, and George G. Simpson. Life, an Introduction to Biology. 3rd ed. New York: Harper-Collins, 1991.

Campbell, N., J. Reece, and L. Mitchell. Biology. 5th ed. Menlo Park: Benjamin Cummings, Inc. 2000.

Starr, Cecie, and Ralph Taggart. Biology, The Unity and Diversity of Life. Belmont, CA: Wadsworth Publishing Company, 1992.

Elaine Friebele

KEY TERMS

Centriole: —An arrangement of microtubules found in most animal cells and in cells of some lower plants and fungi.
Centromere: —A constricted region of the chromosome joining two sister chromatids. The centromere is composed of highly repeated DNA sequences approximately 220 units in length.
Chromatin: —The name given to loose tangle of DNA strands in the nuclei of cells during periods when they are not dividing. As a cell prepares to divide, chromatin strands condense into compact chromosomes.
Chromosomes: —Structures in the eukaryotic cell nucleus consisting of heavily coiled DNA and proteins and carrying genetic information.
Cytokinesis: —The physical division of the cytoplasm of a eukaryotic cell to form two daughter cells, each housing a newly formed nuclei.
Cytoskeleton: —A network of assorted protein filaments attached to the cell membrane and to various organelles that makes up the framework for cell shape and movement.
DNA: —Strands of DNA, or deoxyribonucleic acid, are like long sentences of words composed of a four letter alphabet of nucleotide base pairs: A (adenine); T (thymine); G (guanine); and C (cytosine). The "words" contain the instructions for sequences of amino acids making up proteins.
Eukaryotic cell: —A cell whose genetic material is carried on chromosomes inside a nucleus encased in a membrane. Eukaryotic cells also have organelles that perform specific metabolic tasks and are supported by a cytoskeleton which runs through the cytoplasm, giving the cell form and shape. In contrast, the more primitive prokaryotic cells are smaller than eukaryotes, and have no nucleus, distinct organelles, or cytoskeleton.
Kinetochore: —A disk of protein bound to the centromere to which microtubules attach during mitosis, linking each chromatid to the spindle.
Microtubule: —A hollow protein cylinder, about 25 nanometers in diameter, composed of subunits of the protein tubulin. Microtubules grow in length by the addition of tubulin subunits at the end and are shortened by their removal.
Nucleotide: —The "letters" or basic units of DNA, containing a phosphate group, a 5-carbon sugar, and a ring-shaped nitrogenous base.
Spindle apparatus: —An axis of microtubules formed between centrioles in animal cells that aids the equal distribution of chromosomes to new cells being formed.