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Mitosis

Mitosis

Mitosis is the process of dividing chromosomes during cell division in eukaryotic cells . Mitosis is followed by cytokinesis, the splitting of the cytoplasm . In cell division, a parent cell splits, producing two daughter cells that are identical to the parent. Eukaryotic unicellular organisms like the protist Amoeba use cell division in the production of new individuals, propagating their species. Multicellular eukaryotic organisms, including plants, animals, and fungi, rely on cell division to grow larger by adding new cells. They also use cell division to repair injured or worn-out tissues by replacing damaged cells with new cells.

The function of mitosis is to divide a cell's nucleus with its chromosomes into two daughter cell nuclei, each of which inherits the same number of chromosomes as the parent cell. Consider mitosis in human cells, each of which contains forty-six chromosomes.


FLEMMING, WALTHER (18431905)

German physician and cell biologist who first described the process by which cells divide and separate their chromosomes. He named this process "mitosis." At the time of his work, 1882, no one knew that the chromosomes carried the units of heredity, genes.


How does a parent cell with forty-six chromosomes divide to yield two daughter cells each with forty-six chromosomes identical to those of the parent? The eukaryotic parent cell first copies, or replicates, its chromosomes prior to mitosis. Rather than ninety-two chromosomes, however, this replication process yields forty-six chromosomes, each composed of two parts, called sister chromatids , that are genetically identical to each other. The sister chromatids are connected to each other at a point called the centromere .

During mitosis, the nuclear envelope dissolves, and sister chromatids separate at the centromere, becoming two individual daughter chromosomes, each now with only one chromatid. By the end of mitosis, these daughter chromosomes are segregated from each other to opposite poles of the cell and become enclosed within two separate daughter nuclei. Following mitosis, cytokinesis divides the cell into two, with two sets of organelles and two daughter nuclei, forming two separate but identical cells.

Specifics of Mitosis

Mitosis is a continuous process that is often divided into four sequential phases known as prophase, metaphase, anaphase, and telophase. These phases can be distinguished through microscopic analysis. Several critical steps in mitosis are controlled by phosphorylation or dephosphorylation of proteins.

Prophase. Prior to mitosis, chromosomes appear in the nucleus as a tangled mass of thin strands (chromatin) and are not distinguishable from each other as separate entities. During prophase, the chromosomes condense into shorter and thicker rodlike structures that can be easily seen to consist of two sister chromatids connected by a centromere. This is thought to be driven by addition of phosphate groups to the histone proteins of the chromosome.

Another major event in prophase is the organization of what is known as the mitotic spindle. This too is thought to be driven by phosphorylation. Prior to mitosis, a special area of the cytoplasm near the nucleus, known as the centrosome, contains a pair of small cylindrical bodies called centrioles. The centriole pairs replicate and then the two pairs of centrioles begin to move with their centrosomes to opposite poles of the cell. During prophase, they continue their migration to the cell's poles and organize parts of the cell's cytoskeleton (the scaffold that maintains the cell's shape) into the mitotic spindle. The spindle consists of microtubules that reach from each centriole pair across the cell toward the other pair.

Prometaphase

On either side of the centromere that connects the two sister chromatids of each chromosome, specialized complexes of proteins known as kinetochores form. These act as attachment points between chromosomes and the spindle fibers that are part of the mitotic spindle. Through these attachments, the spindle is able to physically move the chromosomes to opposite poles of the spindle. By the end of prometaphase, the nuclear membrane surrounding the chromosomes begins to break down (triggered by phosphorylation of membrane proteins) and the spindle fibers pull the chromosomes by their kinetochore attachments so that the chromosomes align at the midpoint between the spindle poles.

Metaphase. During metaphase the chromosomes are fully aligned end to end at the cell's midline at what is known as the metaphase plate. Each kinetochore is attached to spindle fibers emanating from centrioles at opposite poles.

Anaphase. The attachments between sister chromatids to each other split during anaphase, producing single-chromatid chromosomes. This is triggered by destruction of the phosphorylating proteins discussed earlier. For each pair of single chromatid chromosomes, one of the pair is pulled toward each of the two spindle poles. Meanwhile, the distance between the spindle poles also increases.

Telophase. During telophase, the nuclear membranes are dephosphorylated and begin to reform around the two sets of chromosomes at either pole, enclosing and separating them from the rest of the cytoplasm. The mitotic spindle disappears. The chromosomes decondense and become thinner and more difficult to distinguish from each other. Cytokinesis begins the process of separating the two daughter cells and is nearly complete by the end of telophase. The end result is the production of two new cells that are genetically identical to each other and to the parent cell.

Differences Between Plants and Animals

Plants use a similar process with a few differences. For example, although a plant cell creates a mitotic spindle and has a centrosome, it lacks centrioles. The other major difference in plants is the way in which cytokinesis occurs. In animal cells, the plasma membrane pinches in along the midline of the cell, creating a cleavage furrow that will separate the cytoplasm in two. Plant cells have rigid cell walls that prevent this. Instead, they use two different approaches for cytokinesis. The plasma membrane and cell wall grow inward together, eventually separating the parent cell into two. Alternatively, the cell wall that will separate the two daughter cells starts growing in the middle of the cell between the two nuclei and continues toward the periphery. This is known as the cell plate. It continues growing until its edges reach the cell's outer surface, separating the parent cell into two daughter cells.

see also Cell Cycle; Chromosome, Eukaryotic; Cytokinesis; Cytoskeleton; Meiosis; Nucleus; Replication; Sexual Reproduction

Michele D. Blum

Bibliography

Mader, Sylvia S. Biology, 6th ed. Boston: WCB McGraw-Hill, 1998.

McFadden, Carol H., and William T. Keeton. Biology: An Exploration of Life. New York: W. W. Norton and Company, Inc., 1995.

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Mitosis

Mitosis

Mitosis is the process by which all cells divide. Many cells have a limited life span, and mitosis allows them to be renewed on a regular basis. Mitosis is also responsible for generating the many millions of cells that are needed for an embryo to develop into a fetus, an infant, and finally an adult.

Most human cells continually undergo a cycle of different phases. The phases have distinct names but flow smoothly into one another. The mitotic (M) phase is the phase in which the cell's genetic material is split in two. Once the phase is completed, the cell is physically divided into two daughter cells, in a process called cytokinesis.

Before entering the M phase, cells are in interphase, the phase between two cell divisions. Interphase is itself divided into three phases: G1, S, and G2, where G stands for gap or growth, and S for synthesis. During the G1 phase, daughter cells formed in the previous M phase undergo active cell growth. During the S phase, the genetic material (DNA) contained in the chromosomes is duplicated so that both of the future daughter cells receive the same set of chromosomes. This ensures that they will be genetically identical to each other and to the cell from which they originated.

In human somatic cells, each of the forty-six chromosomes replicates to produce two daughter copies that are called sister chromatids. These two copies remain attached to each other at a single point, the centromere, which is a DNA sequence of about 220 nucleotides. The centromere has a disk-shaped protein molecule, called a kinetochore, attached to it. In interphase, the chromosomes are not visible as discrete entities under the light microscope. Interphase chromosomes are uncoiled threads composed of DNA and protein molecules. This noncondensed form of chromosomes is also called chromatin.

During the G2 phase of the cell cycle, the chromatin fibers start to condense, eventually turning into tightly coiled, compact bodies, visible as chromosomes. The cell also begins to manufacture protein fibers called microtubules, which will be used later to move the chromosomes to opposite poles of the cell, so two new daughter cells can form. Chromosome condensation and microtubule formation both begin in the G2 phase but occur mostly during the first stage of the M phase, which is called prophase. The microtubules are organized into a three-dimensional spindle apparatus, where each fiber of the spindle apparatus connects one cell pole to the other like a bridge.

During the next stage of mitosis, called prometaphase, the envelope surrounding the cell nucleus breaks down so that the chromosomes are free to migrate to the central plane of the spindle apparatus. A second group of microtubules grows out, to connect the two opposite sides of the kineto-chore to the two poles of the spindle. This arrangement is crucial for ensuring that the two sister chromatids end up in two separate daughter cells rather than being pulled into the same cell.

In the next stage of mitosis, metaphase, the chromosomes become maximally condensed and line up in an imaginary plane, called the metaphase plate, in the center of the cell and perpendicular to the spindle apparatus. All the centromeres are neatly arranged in a circle, about halfway between the two cell poles.

In human cells, at this point the twenty-three pairs of chromosomes, each made up of two condensed sister chromatids held together by a centromere, are visible under the microscope. Unlike in meiosis, the paternal and maternal copies in each pair of chromosomes align independently in the metaphase plate and are not associated with each other. At the end of metaphase, the centromeres that hold the two sister chromatids together all divide simultaneously.

During the next stage, anaphase, microtubules that are attached to the sister chromatids' kinetochores draw the chromatids quite rapidly to opposite poles of the spindle. The separation of sister chromatids completes the partitioning of the replicated genetic material.

The only task remaining during the final phase of mitosis, telophase, is to disassemble the spindle apparatus and re-form the nuclear envelope around each set of sister chromatids. The chromatids can be called chromosomes again, because they each have their own centromere. The chromosomes begin to uncoil, and the genes they carry begin to be expressed again. This is the end of mitosis.

The cell cycle is completed by cytokinesis, the physical division of the cytoplasm into two daughter cells. By the time cytokinesis occurs, other cytoplasmic organelles, such as mitochondria, already have been replicated during the S or G2 phases, and they have also been directed to the areas around the cell poles that will become the daughter cells. Cytokinesis is followed by the G1 phase, with active cell growth occurring in each of the two daughter cells.

Differences between Mitosis and Meiosis

Mitosis occurs in all eukaryotic cell tissues and produces genetically identical daughter cells with a complete set of chromosomes. In humans, mitosis produces somatic cells that are diploid, which means they contain two non-identical copies of each of the twenty-three chromosomes. One copy is derived from the person's mother and the other from the person's father.

Meiosis, on the other hand, occurs only in testis and ovary tissues, producing sperm and ova (eggs). The gametes that are produced by meiosis in humans are haploid, containing only one copy of each of the twenty-three chromosomes. Because of recombination and independent assortment of parental chromosomes, the daughter cells produced by meiosis are not genetically identical.

In mitosis, one round of DNA replication occurs per cell division. In meiosis, one round of DNA replication occurs for every two cell divisions. Prophase in mitosis typically takes about thirty minutes in human cells. Prophase in meiosis I can take years to complete.

see also Cell, Eukaryotic; Cell Cycle; Meiosis; Nucleus; Replication.

Silke Schmidt

Bibliography

Nasmyth, Kim. "Segregating Sister Genomes: The Molecular Biology of chromosome Separation." Science 297, no. 5581 (2002): 559.

Raven, Peter H., and George B. Johnson. Biology, 5th ed. New York: McGraw-Hill,1999.

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mitosis

mitosis (mītō´sĬs, mĬ–), process of nuclear division in a living cell by which the carriers of hereditary information, or the chromosomes, are exactly replicated and the two copies distributed to identical daughter nuclei. Mitosis is almost always accompanied by cell division (cytokinesis), and the latter is sometimes considered a part of the mitotic process. The pattern of mitosis is fundamentally the same in all cells. However, while animal cells apparently divide by pinching into two separate cells, plant cells develop a cell plate, which becomes a cellulose cell wall between the two daughter cells. The importance of mitosis is the maintenance of the chromosomal set; each cell formed receives chromosomes that are alike in composition and equal in number to the chromosomes of the parent cell.

The Stages of Mitosis

Mitosis is simply described as having four stages—prophase, metaphase, anaphase, and telophase; the steps follow one another without interruption. The entire four-stage division process averages about one hour in duration, and the period between cell divisions, called interphase or interkinesis, varies greatly but is considerably longer.

During interphase the chromosomes are dispersed in the nucleus and appear as a network of long, thin threads or filaments, called the chromatin. At some point before prophase begins, the chromosomes replicate themselves to form pairs of identical sister chromosomes, or chromatids; the deoxyribose nucleic acid (DNA) of the chromosomes is synthesized only during interphase, not while mitosis is in process.

During prophase the two chromatids remain attached to one another at a region called the centromere, but each contracts into a compact tightly coiled body; the nucleolus and, in most cases, the nuclear envelope break down and disappear. Also during prophase the spindle begins to form. In animal cells the centrioles separate and move apart, and radiating bundles of fibers, called asters, appear around them. Some sets of fiber run from one centriole to the other; these are the spindle fibers. In plant cells the spindle forms without centrioles.

During metaphase the chromosomes congregate at a plane midway between the two ends to which the spindle tapers. This is called the equatorial plane and marks the point where the whole cell will divide when nuclear division is completed; the ends of the spindle are the poles to which the chromatids will migrate. The chromatids are attached to the spindle fibers at the centromeres.

During anaphase the two chromatids of each chromosome separate and move to opposite poles, as if pulled along the spindle fibers by the centromeres. During telophase new nuclear envelopes form around the two groups of daughter chromosomes (as they are now called), the new nucleoli begin to appear, and eventually, as the formation of the two daughter nuclei is completed, the spindle fibers disappear. The chromosomes uncoil to assume their dispersed distribution within the interphased nucleus. Cytokinesis, which may begin before or after mitosis is completed, finally separates the daughter nuclei into two new individual daughter cells.

A considerable variance in the degree and timing of these stages exists across species, and cells can be classified by their mitotic characteristics. Despite the relative ease of observation of the physical stages of mitosis under the microscope (primarily because the chromosomes stain readily when in their coiled state), the exact chemical and kinetic nature of mitosis is not yet fully understood. For instance, the spindle has been determined to consist largely of thin, elongate tubules called microtubules, but their functions have yet to be understood.

Meiosis and Amitosis

Mitotic division is the method of nuclear division of the somatic (body) cells, as distinguished from the gametes, or sex cells (eggs and sperm). In sexual reproduction, i.e., by the union of two gametes, the complex process of meiosis takes place, which produces cells that each contain only half the normal number of chromosomes. Direct cell division, in which the nucleus simply cleaves in two (sometimes but not always followed by division of the cytoplasm), is called amitosis and is very rare.

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mitosis

mitosis The normal process of nuclear division by which two daughter nuclei are produced, each identical to the parent nucleus. Before mitosis begins each chromosome replicates; these daughter chromosomes then separate during mitosis so that one duplicate goes into each daughter nucleus. The result is two daughter nuclei, each with an identical complement of chromosomes and hence of genes. Mitosis is generally divided into four phases: prophase, metaphase, anaphase, and telophase. During prophase, chromosomes become visible within the nucleus because they shorten, thicken, and coil up as a spiral. Each chromosome is longitudinally double except in the region of the centromere, and each single strand of the chromosome is called a chromatid. The nucleolus and nuclear envelope disappear. In metaphase the chromosomes move within the spindle connecting the two centrosomes (formed by division of the centriole into two at the start of prophase, and the subsequent moving apart of these two daughter centrioles). The chromosomes finally arrange themselves along the equator of the spindle. During anaphase, the two chromatids making up each chromosome separate as the centromere becomes functionally double, and are thus converted to independent chromosomes moving to opposite poles. During telophase, the spindle disappears, a nuclear envelope reappears around each of the two groups of daughter chromosomes, the chromosomes return to their extended state, and the nucleoli reappear. This is usually followed by cytoplasmic division (cytokinesis) into two parts by a cleavage furrow. The final result is two daughter cells with identical nuclear contents. Compare MEIOSIS.

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mitosis

mitosis The normal process of nuclear division (occurring at cell division) by which 2 daughter nuclei are produced, each identical to the parent nucleus. Before mitosis begins each chromosome replicates to form 2 sister chromatids; these then separate during mitosis so that one duplicate goes into each daughter nucleus. The result is 2 daughter nuclei, each with an identical complement of chromosomes and hence of genes. Mitosis is conventionally divided into 4 phases: prophase, metaphase, anaphase, and telophase. During prophase, chromosomes become visible within the nucleus because they shorten, thicken, and coil up as a spiral. Each chromosome is longitudinally double except in the region of the centromere, and each single strand of the chromosome is called a chromatid. The nucleolus and nuclear membrane disappear. In lower plant cells, in metaphase the chromosomes move within the spindle connecting the 2 centrosomes (formed by division of the centriole into 2 at the start of prophase, and the subsequent moving apart of these 2 daughter centrioles). The chromosomes finally arrange themselves along the equator of the spindle. During anaphase, the 2 chromatids making up each chromosome separate as the centromere becomes functionally double, and are thus converted to independent chromosomes moving to opposite poles. During telophase, the spindle disappears, a nuclear envelope reappears around each of the 2 groups of daughter chromosomes, the chromosomes return to their extended state, and the nucleoli reappear. This may be followed by cytokinesis. The final result is 2 daughter cells with identical nuclear contents. Compare MEIOSIS.

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mitosis

mitosis A type of nuclear division that results in two daughter cells each having a nucleus containing the same number and kind of chromosomes as the mother cell. The changes during divisions are clearly visible with a light microscope. Each chromosome divides lengthwise into two chromatids, which separate and form the chromosomes of the resulting daughter nuclei. The process is divided into four stages, prophase, metaphase, anaphase, and telophase, which merge into each other (see illustration). Mitotic divisions ensure that all the cells of an individual are genetically identical to each other and to the original fertilized egg. See also cell cycle.

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mitosis

mi·to·sis / mīˈtōsəs/ • n. (pl. -ses / -sēz/ ) Biol. a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth.Compare with meiosis. DERIVATIVES: mi·tot·ic / mīˈtätik/ adj.

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mitosis

mitosis Cell division resulting in two genetically identical ‘daughter’ cells with the same number of chromosomes as the parent cell. Mitosis is the normal process of tissue growth, and is also involved in asexual reproduction. See also meiosis

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mitosis

mitosis (my-toh-sis) n. a type of cell division in which a single cell produces two genetically identical daughter cells. It is the way in which new body cells are produced for both growth and repair. Compare meiosis.
mitotic (my-tot-ik) adj.

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mitosis

mitosis The normal process of nuclear division by which two daughter nuclei are produced, each identical to the parent nucleus, resulting in two daughter cells with identical nuclear contents. Compare meiosis.

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Mitosis

Mitosis

Prophase

Metaphase

Anaphase

Telophase

Cytokinesis

Resources

Mitosis is a process that sorts and evenly distributes a cells 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 bodysuch as nerve and skeletal muscle cellscannot 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 elementsreplicate. 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 feet (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 microtubulescalled an aster because of its star-like patternis 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 cleanup 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.

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.

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.

See also Meiosis.

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: HarperCollins, 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.

OTHER

Cells Alive ! Mitosis: An Interactive Animation <http://www.cellsalive.com/mitosis.htm> (accessed December 4, 2006).

University of Arizona, Department of Biochemistry and Molecular Biophysics: The Biology Project. The Cell Cycle and Mitosis Tutorial <http://www.biology.arizona.edu/Cell_bio/tutorials/cell_cycle/cells3.html> (accessed December 4, 2006).

Elaine Friebele

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Mitosis

MITOSIS

Mitosis is the stage of the cell cycle at which chromosomal division occurs. This division precedes cytokinesis, or cell division, which leads to two daughter cells with identical nuclear DNA content. Between mitotic events, chromosomal DNA is replicated during the synthesis stage of the cell cycle. The process of mitosis is required whenever a somatic cell divides. In contrast to meiosis, which occurs in germ cells prior to the reproductive process, mitosis does not involve exchange of DNA between homologous chromosomes or a reduction in DNA content.

See also:MEIOSIS

Bibliography

Oak Ridge National Laboratory. "The Science behind the Human Genome Project." Available from http://www.ornl.gov/hgmis/project/info.html; INTERNET.

David W.Threadgill

Robert E.Boykin

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Mitosis

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.

See also Meiosis.


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.

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