Cell Division

Definition

The process by which a cell distributes its genetic material (DNA) and cytoplasm to daughter cells.

Description

In higher organisms, including humans, there are two types of cell division, mitosis and meiosis. Strictly speaking, mitosis and meiosis refer to division of the DNA and associated materials in the nucleus of the cell. In mitosis, the cells produced by division have exactly the same genetic information as the original cell, while in meiosis the cells produced by division have only half the genetic information as the original cell. Both processes are accompanied by cytokinesis, the division of the cytoplasm.

Mitosis

Mitosis produces two daughter cells, each of which has the same genetic information as the parent cell. The entire process includes a series of precise steps to insure that the genetic material is accurately duplicated and distributed. The life of a cell is generally made up of two parts, interphase and mitosis. During interphase, DNA synthesis occurs. Since the process uses the original DNA as a template, the copy is exact (or nearly exact if mutations occur). After a pause, the cell then enters mitosis. Although the life-span of a cell varies in length depending on the cell type, mitosis itself usually takes about one to two hours and involves four stages: prophase, metaphase, anaphase, and telophase.

PROPHASE. During prophase, the chromosomes, which contain the DNA, condense in length and become visible under a microscope. Humans have 23 chromosome pairs, for a total of 46 chromosomes. Since DNA duplication has already occurred, each of the 46 chromosomes at this stage is present in two copies referred to as sister chromatids. The two sister chromatids of a pair are attached to each other at a point called the centromere. As the chromosomes condense, the membrane surrounding the nucleus disappears, and fibers appear, which come together to form a spindle within the cell. The spindle has two opposite poles and a mid-section, the equatorial plate.

METAPHASE. At the beginning of metaphase, the chromosomes line up individually on the equatorial plate. Fibers emanating from the poles of the spindle attach to the centromeres of the sister chromatids. One member of each pair of sister chromatids is attached to a spindle fiber that radiates from one pole, and the other is attached to a fiber that radiates from the opposite pole.

ANAPHASE. After all chromosomes (92 sister chromatids in 46 pairs) have aligned on the equatorial plane of the spindle, the centromere of each chromosome splits, and the fibers begin to contract. One sister chromatid of each pair is pulled to one pole of the spindle and the other is pulled to the opposite pole.

TELOPHASE. Separate membranes form around the chromosome sets at each pole to form two nuclei. The chromosomes elongate and the spindle disappears. Cytokinesis then occurs, resulting in two daughter cells each with 46 chromosomes and roughly half the cytoplasm of the parent cell.

Function and role in human health

Mitosis is the process by which a single human zygote (fertilized egg cell) becomes a complex organism consisting of over 100 trillion cells. During the lifetime of an individual, mitosis continues. In some tissues such as epithelium (skin, mucous membranes), mitosis actively occurs to replace cells and repair damage. Other cell types such as nerve cells do not readily undergo mitosis after a certain point in development. Thus the capacity for mitosis is programmed into each cell type and is cell-specific. In addition, there are many molecules within the body that can influence cell division. Scientists are just beginning to learn about some of these and their possible roles in human health. For example, cancer occurs when the normal pattern of cell division within a tissue or organ is disrupted, and the cells begin to repeatedly undergo mitosis. Changes within the cell as well as external influences can play a part in disrupting the normal control of mitosis.

Meiosis

Meiosis is a special type of cell division that, in higher organisms, occurs only in cells of the ovaries or testes. Within these organs, cells destined to become eggs and sperm undergo meiosis in order to halve the amount of DNA that will be packaged into an egg or sperm. As with all the other cells in the body, these precursor cells are diploid; that is, they have the full complement of 46 chromosomes (23 pairs). Whereas mitosis creates two diploid cells from one existing diploid cell, meiosis results in eggs and sperm that have only one member of each pair of chromosomes. Thus these cells, collectively known as germ cells, have only 23 chromosomes and are said to be haploid. At fertilization, the union of one egg and one sperm produces a diploid zygote (fertilized egg) with 46 chromosomes, half from the mother and half from the father. This zygote then begins the many mitotic divisions that will take it from a single cell to a complex, fully differentiated organism.

The steps in meiosis are similar in many ways to those in mitosis, but there are several important differences. One obvious distinction is that, unlike mitosis which includes only one division of the nucleus and cytoplasm, meiosis is actually composed of two divisions, meiosis I and meiosis II. As in a mitotic division, DNA duplication occurs during interphase before meiosis so that the cells begin meiosis I with double the diploid amount of DNA (92 sister chromatids).

MEIOSIS I PROPHASE. Prophase of meiosis I (prophase I) includes several significant features. As the chromosomes condense, chromosome pairing occurs. This is an important phenomenon that occurs only during meiosis. Higher organisms receive half of their genetic material from their mother and half from their father; that is, one set of chromosomes is maternal in origin and the other set is paternal. During interphase of a cell cycle, as well as during mitotic divisions, these various chromosomes from the maternal and paternal sets do not associate in pairs. Pairing only occurs in prophase of meiosis I. This pairing brings the same chromosome from the mother and father together in close association. This pairing is essential for the important step that happens next.

A process of crossing over occurs between the maternal and paternal member of each chromosome pair. These crossover points, which can be seen through the microscope, are the places where maternal and paternal chromosomes have exchanged sections of genetic material in a process known as recombination. This essential step occurs during meiosis and serves to recombine the genetic material an individual received from their mother and father. That individual can then pass on new combinations of the genes from their parents to their offspring. This greatly increases the possible combinations of genetic traits and helps create diversity in the offspring. At the end of prophase, recombination is complete and the chromosome pairs, still attached at their cross-over points, move to the equatorial plate of the spindle that is beginning to form.

In females the process of meiosis begins while the individual herself is still an embryo. The eggs within that early embryo complete prophase I up to a certain point and then go into an arrested state. Eggs only begin to be released from that arrest many years later after a woman has reached puberty. Each month as one egg is ovulated (released from the ovary), meiosis resumes.

MEIOSIS I METAPHASE. During metaphase I, the 23 chromosome pairs line up on the equatorial plate of the spindle with one member of each pair attached by a spindle fiber to one pole and the other member attached to the other pole. At this point the two members of a pair (each of which is itself composed of a pair of sister chromatids) are being held together only at the anchor points created by the cross-overs. When all chromosome pairs are properly aligned on the equatorial plate of the spindle, the anchors release and anaphase I begins.

MEIOSIS I ANAPHASE. During this stage, the two members of a chromosome pair travel to opposite spindle poles. Unlike anaphase of mitosis, the centromeres do not separate. Thus, each chromosome at a pole is composed of a pair of sister chromatids attached at their centromeres. An important point to understand is that the pairs of chromosomes do not line up on the spindle with all of the individual's mother's chromosomes pointing toward one pole and the father's pointing to the other. The alignment is random, so the function of meiosis I is similar to the shuffling of a deck of cards before dealing a hand. The half set of 23 chromosomes that collects at one spindle pole during anaphase will have chromosomes, and thus genetic information, from both the individual's mother and father. This is another way in which meiosis increases diversity in the offspring. When the Austrian monk Gregor Mendel put forth his principles of heredity in 1865, the process of meiosis had not been discovered. However, scientists later came to realize that the inheritance pattern Mendel described for specific traits such as color and shape in the garden pea, were due to the events of the first meiotic division.

MEIOSIS I TELOPHASE. At the poles, a separate nuclear membrane forms around each haploid chromosome set and cytokinesis occurs, resulting into two daughter cells. In females, cytokinesis produces one large cell with the bulk of the cytoplasm, and one very small cell, the first polar body. The larger cell proceeds to meiosis II. In males, cytoplasmic division is equal and both cells enter meiosis II. Because meiosis I has reduced the diploid number of 46 chromosomes to 23, meiosis I is often referred to as the reduction division.

MEIOSIS II INTERPHASE. Unlike in mitosis, there is no further DNA duplication and interphase is brief.

MEIOSIS II PROPHASE. The nuclear membrane breaks down and a new spindle begins to form.

MEIOSIS II METAPHASE. The haploid set of 23 chromosomes, each consisting of a pair of sister chromatids, moves to the equatorial plate of the spindle. Fibers from the two poles attach at each centromere pair and exert tension to align the chromosomes.

MEIOSIS II ANAPHASE. The centromeres separate, and the sister chromatids are pulled to opposite poles. In this regard meiosis II is very similar to mitosis. In females, anaphase II is triggered by the sperm entering the recently ovulated egg.

MEIOSIS II TELOPHASE. The chromosomes begin to de-condense, a nuclear membrane forms around each set, and cytokinesis occurs. In sperm, cytokinesis is again equal and the result is the production of four haploid spermatids, which will go through a process of maturation to become sperm. In males, there is no arrest of meiosis and the entire meiotic process takes about 60 days. In females, meiosis II produces a small second polar body containing one set of chromosomes and a small amount of cytoplasm. The majority of the cytoplasm together with the other set of chromosomes comprises the ovum (mature egg). Since a sperm has already penetrated the envelope of the egg, all that remains is for the haploid chromosome sets from the egg and sperm to merge to produce the diploid zygote.

Common diseases and disorders

In humans, errors in chromosome division occur frequently during meiosis. Although these errors can take place either during the formation of the egg or the sperm, most errors occur during meiosis in the female for reasons that are not yet clearly understood. If mistakes occur during meiosis, eggs and sperm can be formed with either too many or too few chromosome. Fertilization then results in a fertilized egg than has less than or more than 46 chromosomes, a situation with major health consequences. For example, roughly 20% of all clinically recognized pregnancies result in miscarriage. Half of these are due to an extra or missing chromosome(s) in the developing embryo. Among live births, one in 150 infants has some type of chromosome abnormality. One of the more common is Down syndrome. Most cases of Down syndrome are due to an error in meiosis that results in an extra chromosome (extra chromosome 21) being present in the fertilized egg. This condition is called trisomy 21. The individual who develops from this egg will have the clinical features of Down syndrome including mental retardation. Trisomy 21, as well as other similar chromosome errors, occurs more often in the pregnancies of women as they get older. For example, older women have a higher risk for miscarriages associated with chromosome errors. They also have a higher risk of giving birth to an infant with trisomy 21 Down syndrome or a similar chromosome abnormality. For this reason, women in their mid-30s or older are usually referred to a geneticist or genetic counselor to learn about prenatal testing options such as amniocentesis and chorionic villus sampling (CVS).

KEY TERMS

Amniocentesis— A procedure performed around the fourth month of pregnancy in which a needle is inserted through a woman's abdomen into her uterus to draw out a small sample of the amniotic fluid from around the baby. Fetal cells in the fluid can be used to check the chromosome make-up of the baby.

Chorionic villus sampling (CVS)— A procedure used for prenatal diagnosis at eight to 10 weeks gestation. Under ultrasound guidance a needle is inserted either through the mother's vagina or abdominal wall and a sample of cells is collected from around the early embryo. These cells can be used to study the chromosomes of the fetus.

Chromosomes— Structures in the nucleus of a cell that contain a thread of DNA containing the genetic information (genes). Humans have 46 chromosomes in 23 pairs.

Cytoplasm— The portion of the cell that surrounds the nucleus.

DNA— Deoxyribonucleic acid, the molecule that encodes the genes.

Genetic counselor— An individual, usually with an advanced degree and board certification, who specializes in assessing genetic risk and informing patients about these risks and the options for dealing with them.

Geneticist— A individual with an advanced degree (MS, MD, PhD) in genetics. Human geneticists and medical geneticists specialize in genetic issues pertaining to humans. Many geneticists are certified by specialty boards.

Nucleus— The membrane-bound body within a cell that contains the chromosomes.

Resources

BOOKS

Carlson, Bruce M. Human Embryology and Developmental Biology. 2nd ed. St. Louis: Mosby, 1999.

Endow, Sharyn A., and David M. Glover, eds. Dynamics of Cell Division. New York: Oxford University Press, 1998.

Jorde, Lynn B., et al. Medical Genetics. 2nd ed. New York: Mosby, 1999.

Tortora, Gerard, and Sandra Reynolds Grabowski. Principles of Anatomy and Physiology. 9th ed. New York: HarperCollins, 2000.

OTHER

The Biology Project. 〈http://www.biology.arizona.edu〉.

Mitosis Animation. 〈http://galileo.physiology.uiowwa.edu/animations/mitosis.htm〉.

Cell division

Cell division is the process where a single living cell splits to become two or more distinct new cells. All cells divide at some point in their lives. Cell division occurs in single-celled organisms like bacteria , in which it is the major form of reproduction (binary fission), or in multicellular organisms like plants, animals, and fungi . Many cells continually divide, such as the cells that line the human digestive tract or the cells that make up human skin. Other cells divide only once.

There are two major ways in which biologists categorize cell division. The first, mitosis, is simple cell division that creates two daughter cells that are genetically identical to the original parent cell. The process varies slightly between prokaryotic and eukaryotic organisms. In eularyotes, mitosis begins with replication of the deoxyribonucleic acid (DNA) within the cell to form two copies of each chromosome . Once two copies are present, the cell splits to become two new cells by cytokinesis, or formation of a fissure. Mitosis occurs in most cells and is the major form of cell division.

The second process, called meiosis is the production of daughter cells having half the amount of genetic material as the original parent cell. Such daughter cells are said to be haploid. Meiosis occurs in human sperm and egg production in which four haploid sex cells are produced from a single parent precursor cell. In both mitosis and meiosis of nucleated cells, shuffling of chromosomes creates genetic variation in the new daughter cells. These very important shuffling processes are known as independent assortment and random segregation of chromosomes.

Cell division is stimulated by certain kinds of chemical compounds. Molecules called cytokines are secreted by some cells to stimulate others to begin cell division. Also, contact with adjacent cells can control cell division. The phenomenon of contact inhibition is a process where the physical contact between neighboring cells prevents cell division from occurring. When contact is interrupted, however, cell division is stimulated to close the gap between cells. Cell division is a major mechanism by which organisms grow, tissues and organs maintain themselves, and wound healing occurs. Cancer is potentially a deadly form of uncontrolled cell division.

Eukaryotic cell division

Although prokaryotes (i.e., non-nucleated unicellular organisms) divide through binary fission, eukaryotes undergo a more complex process of cell division because DNA is packed in several chromosomes located inside a cell nucleus. In eukaryotes, cell division may take two different paths, in accordance with the cell type involved. Mitosis is a cellular division resulting in two identical nuclei is performed by somatic cells. The process of meiosis results in four nuclei, each containing half of the original number of chromosomes. Sex cells or gametes (ovum and spermatozoids) divide by meiosis. Both prokaryotes and eukaryotes undergo a final process, known as cytoplasmatic division, which divides the parental cell in new daughter cells.

The series of stages that a cell undergoes while progressing to division is known as cell cycle. Cells under-going division are also termed competent cells. When a cell is not progressing to mitosis, it remains in phase G0("G" zero ). Therefore, the cell cycle is divided into two major phases: interphase and mitosis. Interphase includes the phases (or stages) G1, S, and G2 whereas mitosis is subdivided into prophase, metaphase, anaphase and telophase.

The cell cycle starts in G1, with the active synthesis of RNA and proteins , which are necessary for young cells to grow and mature. The time G1 lasts, varies greatly among eukaryotic cells of different species and from one tissue to another in the same organism . Tissues that require fast cellular renovation, such as mucosa and endometrial epithelia, have shorter G1 periods than those tissues that do not require frequent renovation or repair, such as muscles or connective tissues.

The cell cycle is highly regulated by several enzymes, proteins, and cytokines in each of its phases, in order to ensure that the resulting daughter cells receive the appropriate amount of genetic information originally present in the parental cell. In the case of somatic cells, each of the two daughter cells must contain an exact copy of the original genome present in the parental cell. Cell cycle controls also regulate when and to what extent the cells of a given tissue must proliferate, in order to avoid abnormal cell proliferation that could lead to dysplasia or tumor development. Therefore, when one or more of such controls are lost or inhibited, abnormal overgrowth will occur and may lead to impairment of function and disease .

Cells are mainly induced into proliferation by growth factors or hormones that occupy specific receptors on the surface of the cell membrane , being also known as extra-cellular ligands. Examples of growth factors are as such: epidermal growth factor (EGF), fibroblastic growth factor (FGF), platelet-derived growth factor (PDGF), insulin-like growth factor (IGF), or by hormones. PDGF and FGF act by regulating the phase G2 of the cell cycle and during mitosis. After mitosis, they act again stimulating the daughter cells to grow, thus leading them from G0 to G1. Therefore, FGF and PDGF are also termed competence factors, whereas EGF and IGF are termed progression factors, because they keep the process of cellular progression to mitosis going on. Growth factors are also classified (along with other molecules that promote the cell cycle) as pro-mitotic signals. Hormones are also pro-mitotic signals. For example, thyrotrophic hormone, one of the hormones produced by the pituitary gland, induces the proliferation of thyroid gland's cells. Another pituitary hormone, known as growth hormone or somatotrophic hormone (STH), is responsible for body growth during childhood and early adolescence, inducing the lengthening of the long bones and protein synthesis. Estrogens are hormones that do not occupy a membrane receptor, but instead, penetrate the cell and the nucleus, binding directly to specific sites in the DNA, thus inducing the cell cycle.

Anti-mitotic signals may have several different origins, such as cell-to-cell adhesion, factors of adhesion to the extra-cellular matrix, or soluble factor such as TGF beta (tumor growth factor beta), which inhibits abnormal cell proliferation, proteins p53, p16, p21, APC, pRb, etc. These molecules are the products of a class of genes called tumor suppressor genes. Oncogenes, until recently also known as proto-oncogenes, synthesize proteins that enhance the stimuli started by growth factors, amplifying the mitotic signal to the nucleus, and/or promoting the accomplishment of a necessary step of the cell cycle. When each phase of the cell cycle is completed, the proteins involved in that phase are degraded, so that once the next phase starts, the cell is unable to go back to the previous one. Next to the end of phase G1, the cycle is paused by tumor suppressor gene products, to allow verification and repair of DNA damage. When DNA damage is not repairable, these genes stimulate other intracellular pathways that induce the cell into suicide or apoptosis (also known as programmed cell death ). To the end of phase G2, before the transition to mitosis, the cycle is paused again for a new verification and "decision:" either mitosis or apoptosis.

Along each pro-mitotic and anti-mitotic intra-cellular signaling pathway, as well as along the apoptotic pathways, several gene products (proteins and enzymes) are involved in an orderly sequence of activation and inactivation, forming complex webs of signal transmission and signal amplification to the nucleus. The general goal of such cascades of signals is to achieve the orderly progression of each phase of the cell cycle.

Interphase is a phase of cell growth and metabolic activity, without cell nuclear division, comprised of several stages or phases. During Gap 1 or G1 the cell resumes protein and RNA synthesis, which was interrupted during mitosis, thus allowing the growth and maturation of young cells to accomplish their physiologic function. Immediately following is a variable length pause for DNA checking and repair before cell cycle transition to phase S during which there is synthesis or semi-conservative replication or synthesis of DNA. During Gap 2 or G2, there is increased RNA and protein synthesis, followed by a second pause for proofreading and eventual repairs in the newly synthesized DNA sequences before transition to Mitosis.

At the start of mitosis the chromosomes are already duplicated, with the sister-chromatids (identical chromosomes) clearly visible under a light microscope . Mitosis is subdivided into prophase, metaphase, anaphase and telophase.

During prophase there is a high condensation of chromatids, with the beginning of nucleolus disorganization and nuclear membrane disintegration, followed by the start of centrioles' migration to opposite cell poles. During metaphase the chromosomes organize at the equator of a spindle apparatus (microtubules), forming a structure termed metaphase plate. The sister-chromatids are separated and joined to different centromeres, while the microtubules forming the spindle are attached to a region of the centromere termed kinetochore. During anaphase there are spindles, running from each opposite kinetochore, that pull each set of chromosomes to their respective cell poles, thus ensuring that in the following phase each new cell will ultimately receive an equal division of chromosomes. During telophase, kinetochores and spindles disintegrate, the reorganization of nucleus begins, chromatin becomes less condensed, and the nucleus membrane start forming again around each set of chromosomes. The cytoskeleton is reorganized and the somatic cell has now doubled its volume and presents two organized nucleus.

Cytokinesis usually begins during telophase, and is the process of cytoplasmatic division. This process of division varies among species but in somatic cells, it occurs through the equal division of the cytoplasmatic content, with the plasma membrane forming inwardly a deep cleft that ultimately divides the parental cell in two new daughter cells.

Terry Watkins

Cell Division

Cell division is the process by which one cell divides to make two. It is the mechanism that enables an organism to grow, repair damaged tissue, and replace dead cells. There are two different forms of cell division: mitosis and meiosis. Mitosis (my-TOH-sis) is the division of a cell nucleus (a cell's control center) to produce two identical cells. Meiosis (may-OH-sis) is a form of cell division that produces differing sex cells. Mitosis is used to grow, replace, and repair with exact copies. Meiosis is used to produce an entirely new individual.

Without cell division, an organism could no longer grow, reproduce, or repair itself. Every day, the human body makes billions of new cells. Yet each human being began life as a single cell that was formed by the union of a sperm and an egg. Once that single cell began dividing (first into two, then into four, then eight, and so on), the process continued until a complete individual was formed. In organisms that are still growing, like seedlings or children, cells divide very rapidly, but as an organism grows older, many of its cells lose their ability to divide. Thus when cells are dying faster than they can be replaced, the organism begins to feel and show the effects of the aging process, and it looks and acts older. Older people develop wrinkles because their skin and muscle tone is lost as fewer cells are replaced. Older people also cannot heal themselves as quickly as they did when young. All cells have a basic cycle of life that they go through, according to their specialty. A healthy young person's skin cells complete one cycle every twenty-four hours, but a person's brain cells go through only so many cycles and then stop forever.

PHASES OF MITOSIS

Despite the duration of an individual cell's life cycle, each cell goes through the same process when it divides. Most cells are produced by mitosis. In mitosis, a single cell goes through a process in which it eventually produces an identical cell called a "daughter cell." Each daughter cell then grows and soon becomes capable of dividing and producing yet another daughter cell. Mitosis takes place in four stages—prophase, metaphase, anaphase, and telophase—during which each chromosome copies itself, the nucleus divides in two, and the whole cell splits into two identical daughter cells. Each new cell receives a set of chromosomes identical to those of the original cell.

During the first phase of mitosis, called the prophase, the cell's chromosomes become shorter and thicker and duplicate themselves, appearing as double-stranded structures. These joined copies are called chromatids. The membrane around the nucleus also begins to disintegrate. During the metaphase, each pair of joined chromosomes line up across the center of the cell and attach themselves to tiny tubes called spindle fibers. In anaphase, the third stage of mitosis, the chromatids (joined chromosome pairs) are pulled apart by the spindle fibers and move toward opposite ends of the cell as it begins to divide. Actual division occurs in the telophase, when an envelope surrounds each set of chromatids to form a new nucleus in each. Finally, the cell splits in two as the cyotplasm (the cell's jelly-like fluid) turns inward and pinches together, resulting in the production of two new, identical cells. Despite minor differences, mitosis is basically the same for plant and animal cells.

PHASES OF MEIOSIS

Where mitosis makes two identical cells, meiosis produces differing cells. Meiosis takes place whenever reproductive cells such as sperm, pollen, or egg cells are produced. The goal of meiosis is to reduce by half the number of chromosomes, so that when two different reproductive cells join together to form a new organism, it will have the exact same number of chromosomes as its parent. If this halving of chromosomes did not happen, the new cell produced would have twice the number of chromosomes that it should have. For example, in order to be human, an individual must have forty-six chromosomes. Without meiosis, that number would be ninety-two chromosomes after fertilization. Because of meiosis, fertilization will result in an offspring with the exact same number of chromosomes as the parents, getting one-half from each.

Unlike mitosis, meiosis has only two major stages that result in the creation of four reproductive cells. Besides halving the number of chromosomes, meiosis also performs another major function. It allows genetic material to be "shuffled" since chromosomes cross over each other and swap genes before the cell divides. This is a random exchange of genetic

material that assures that an entirely new individual will be produced after fertilization. Because of this shuffle of genetic instructions, each reproductive cell is given its own unique set of instructions (making sure, for example, that no two egg cells will have the exact same combination of genes). This partly explains why brothers or sisters of the same parents have different characteristics. Eventually, when two of these unique cells are joined sexually (sperm and egg) to form a new individual (which further mixes the genetic instructions), a unique organism is created. The exception to this is, of course, the case of identical twins (two complete individuals with the exact same genetic makeup). Identical twins occur after fertilization when a human embryo spontaneously splits during the first cleavage (division) and forms two separate cells.

[See alsoCell; Chromosome; DNA; Meiosis; Mitosis ]

Cell Division

Cell division is the basis of life itself; it is how animals grow and reproduce. When cells divide, two daughter cells are produced from one mother cell. Each new cell has exactly the same genetic material (DNA) as the cell that produced it.

Cellular division has three main functions: (1) the reproduction of an entire unicellular organism, (2) the growth and repair of tissues in multicellular animals, and (3) the formation of gametes (eggs and sperm) for sexual reproduction in multicellular animals. The process of mitosis produces identical cells for the first two functions listed above; the process of meiosis forms gametes.

Cellular division has two steps. First, the genome is divided up inside the nucleus by either mitosis or meiosis. Second, the cytoplasm (the rest of the content of the cell) is divided. The cell is actually split in two in a process called cytokinesis, in which the cellular membrane is pinched in the middle like a balloon squeezed in the center.

Most of the life of a cell is spent growing and replicating DNA. This phase in the cell cycle is called interphase. Cells grow with materials produced from within the cell, using specialized structures called organelles . Before cell division takes place, the entire genome (the genetic material) has been copied, and there are now two complete copies in the cell nucleus.

Diploid eukaryotes have two copies of DNA on two sets of chromosomes. The DNA of eukaryotic animals is packaged into chromosomes. Chromosomes come in pairs. Like pairs of shoes, they are almost the same but with slight variations. Humans have forty-six chromosomes, or twenty-three pairs. When DNA is replicated before the cell divides, each chromosome has two identical copies of DNA called sister chromatids. Sister chromatids can be compared to two left and two right shoes.

Mitosis

Mitosis is the process of cellular division that produces identical daughter cells from one mother cell. In single-cell organisms like protists, mitosis produces two whole organisms. In multicellular organisms, mitosis is the process by which the animal grows and repairs its tissues.

There are five steps in mitosis.

1. Prophase. The shape of the DNA changes. Other changes take place in the cytoplasm.
2. Prometaphase. Chromosomes start to move because microtubules are attaching to them.
3. Metaphase. Chromosomes line up in the middle of the cell, pulled there by microtubules. Sister chromatids line up on each side of the metaphase plate. This can be compared to putting one left shoe on one side of the plate and one right shoe on the other side of the plate.
4. Anaphase. Pairs of sister chromatids split and are pulled to opposite sides of the cell by the microtubules. This is like putting the left shoes into different sides of the cell; the same thing happens with the right shoes. At the end of anaphase, there is one complete set of chromosomes on each side of the cell and the sets are identical.
5. Telophase. DNA returns to the state it was in during interphase.

Cytokinesis then divides the rest of the cell, and two identical cells result.

Meiosis

Meiosis is the process of cellular division that produces the gametes which take part in sexual reproduction. Where mitosis produces two daughter cells from one mother cell, meiosis produces four daughter cells from one mother cell. The end products of meiosis, the gametes, contain only half the genome of a organism. This is like each cell ending up with only a single shoe; there are not pairs in these cells anymore. The two gametes fuse to produce a zygote . Because each gamete has half the genetic material of the mother cell, this fusion results in a zygote with the correct amount of genetic material.

There are two stages in meiosis, meiosis I and meiosis II. There are five steps in meiosis I.

• Interphase I. Chromosomes replicate, resulting in two identical sister chromatids for each chromosome.
• Prophase I. Chromosomes change shape. Homologous pairs of chromosomes, each with two sister chromatids, come together in a process called synapsis. This tetrad of chromatids is joined in several places, called chiasmata, and crossing-over occurs.
• Metaphase I. Tetrads line up on the metaphase plate, still joined.
• Anaphase I. Homologous chromosomes split apart. Sister chromatids remain together. Microtubules pull each homologue to opposite sides of the cell. This is like putting the left shoes on one side and the right shoes on the other.
• Telophase I and Cytokinesis. The cell divides. Each cell contains a pair of sister chromatids.

Meiosis II is similar to mitosis—sister chromatids split apart into new cells—and the same steps occur in the same order. Pairs of chromosomes were split in meiosis I, and sister chromatids are split in meiosis II. Meiosis II results in four separate chromosomes (two pairs of sister chromatids), each packaged separately. Crossing-over produces slight variations among all four cells. These four cells are gametes, either eggs or sperm.

Laura A. Higgins

Bibliography

Campbell, Neil A., Jane B. Reece, and Lawrence G. Mitchell. Biology, 5th ed. Menlo Park, CA: Addison Wesley Longman, Inc., 1999.

Griffiths, Anthony J. F., Jeffrey H. Miller, David T. Suzuki, Richard C. Lewontin, and William M. Gelbart. An Introduction to Genetic Analysis, 6th ed. New York: W. H. Freeman and Company, 1996.

Purves, William K., Gordon H. Orians, H. Craig Heller, and David Sadava. Life: The Science of Biology, 5th ed. Sunderland, MA: Sinauer Associates Inc. Publishers, 1998.

Cell division

Cell division is the process by which a single living cell splits to become two cells. All cells divide at some point in their lives. Cell division occurs in single-celled organisms like bacteria, and in multicellular organisms like plants, animals, and fungi. Many cells continually divide; examples include the cells that line the human digestive tract and skin cells.

Cell division is can occur by means of mitosis or meiosis. Mitosis is simple cell division that creates two daughter cells that are genetically identical to the original parent cell. The process varies slightly between prokaryotic organisms (organisms whose genetic material is not enclosed within a membrane) and eukaryotic organisms (whose genetic material is enclosed within a specialized membrane). In eukaryotes, mitosis begins with replication of the deoxyribonucleic acid (DNA) within the cell to form two copies of each chromosome. Once two copies are present, the cell splits to become

two new cells by cytokinesis, or formation of a fissure. Mitosis occurs in most cells and is the major form of cell division.

The process of meiosis is the production of daughter cells having half the amount of genetic material as the original parent cell. Such daughter cells are designated as haploid. Meiosis occurs in human sperm and egg production in which four haploid sex cells are produced from a single parent precursor cell. In both mitosis and meiosis of nucleated cells, shuffling of chromosomes creates genetic variation in the new daughter cells. These very important shuffling processes are known as independent assortment and random segregation of chromosomes.

Cell division is stimulated by certain kinds of chemical compounds. Molecules called cytokines are secreted by some cells to stimulate others to begin cell division. Also, contact with adjacent cells can control cell division. The phenomenon of contact inhibition is a process where the physical contact between neighboring cells prevents cell division from occurring. When contact is interrupted, however, cell division is stimulated to close the gap between cells. Cell division is a major mechanism by which organisms grow, tissues and organs maintain themselves, and wound healing occurs. Cancer is a potentially deadly form of uncontrolled cell division.

while prokaryotes divide via mitosis that esentially involves the duplication of the genetic material and the separation of each copy into the newly divided cells, eukaryotes undergo a more complex process of cell division because DNA is packed in several chromosomes located inside a cell nucleus. In eukaryotes, cell division may take two different paths. Mitosis is a cellular division resulting in two identical nuclei is performed by somatic cells. The process of meiosis results in four nuclei, each containing half of the original number of chromosomes (the haploid cells). Sex cells or gametes (ovum and spermatozoids) divide by meiosis. Both prokaryotes and eukaryotes undergo a final process, known as cytoplasmatic division, which divides the parental cell in new daughter cells.

The series of stages that a cell undergoes while progressing to division is known as the cell cycle. Cells undergoing division are also termed competent cells. When a cell is not progressing to mitosis, it remains in phase G0. Therefore, the cell cycle is divided into two major phases: interphase and mitosis. Interphase includes the phases (or stages) G1, S, and G2, whereas mitosis is subdivided into prophase, metaphase, anaphase and telophase.

The cell cycle starts with the active synthesis of RNA and proteins, which are necessary for young cells to grow and mature. This phase varies greatly among eukaryotic cells of different species and from one tissue to another in the same organism. Tissues that require fast cellular renovation such as mucosa and endometrial epithelia have a shorter G1 period than those tissues that do not require frequent renovation or repair, such as muscles or connective tissues.

The cell cycle is highly regulated by several enzymes, proteins, and cytokines in each of its phases, in order to ensure that the resulting daughter cells receive the appropriate amount of genetic information originally present in the parental cell. In the case of somatic cells, each of the two daughter cells must contain an exact copy of the original genome present in the parental cell. Cell cycle controls also regulate when and to what extent the cells of a given tissue must proliferate, in order to avoid abnormal cell proliferation that could lead to dysplasia or tumor development. Therefore, when one or more of such controls are lost or inhibited, abnormal overgrowth will occur and may lead to impairment of function and disease.

When occurring properly, the various phases of the cell cycle result in the duplication of the genetic material and the segregation of the material in the correct amounts into the newly divided cell.

Terry Watkins