Meiosis
Meiosis
Meiosis, also known as reduction division, consists of two successive cell divisions in diploid cells. The two-cell divisions are similar to mitosis, but differ in that the chromosomes are duplicated only once, not twice. The end result of meiosis is four daughter cells, each of them haploid. Since meiosis only occurs in the sex organs (gonads), the daughter cells are the gametes (spermatozoa or ova), which contain hereditary material. By halving the number of chromosomes in the sex cells, meiosis assures that the fusion of maternal and paternal gametes at fertilization will result in offspring with the same chromosome number as the parents. In other words, meiosis compensates for chromosomes doubling at fertilization. The two successive nuclear divisions are termed as meiosis I and meiosis II. Each is further divided into four phases (prophase, meta-phase, anaphase, and telophase) with an intermediate phase (interphase) preceding each nuclear division.
Events of meiosis
The events that take place during meiosis are similar in many ways to the process of mitosis, in which one cell divides to form two clones (exact copies) of itself. It is important to note that the purpose and final products of mitosis and meiosis are very different.
Meiosis I
Meiosis I is preceded by an interphase period in which the DNA replicates (makes an exact duplicate of itself), resulting in two exact copies of each chromosome that are firmly attached at one point, the centromere. Each copy is a sister chromatid, and the pair are still considered as only one chromosome. The first phase of meiosis I, prophase I, begins as the chromosomes come together in homologous pairs in a process known as synapsis. Homologous chromosomes, or homologues, consist of two chromosomes that carry genetic information for the same traits, although that information may hold different messages (e.g., when two chromosomes carry a message for eyecolor, but one codes for blue eyes while the other codes for brown).
The fertilized eggs (zygotes) of all sexually reproducing organisms receive their chromosomes in pairs, one from the mother and one from the father. During synapsis, adjacent chromatids from homologous chromosomes “cross over” one another at random points and join at spots called chiasmata. These connections hold the pair together as a tetrad (a set of four chromatids, two from each homologue). At the chiasmata, the connected chromatids randomly exchange bits of genetic information so that each contains a mixture of maternal and paternal genes. This “shuffling” of the DNA produces a tetrad, in which each of the chromatids is different from the others, and a gamete that is different from others produced by the same parent. Crossing over does, in fact, explain why each person is a unique individual, different even from those in the immediate family.
Prophase I is also marked by the appearance of spindle fibers (strands of microtubules) extending from the poles or ends of the cell as the nuclear membrane disappears. These spindle fibers attach to the chromosomes during metaphase I as the tetrads line up along the middle or equator of the cell. A spindle fiber from one pole attaches to one chromosome while a fiber from the opposite pole attaches to its homologue. Anaphase I is characterized by the separation of the homologues, as chromosomes are drawn to the opposite poles. The sister chromatids are still intact, but the homologous chromosomes are pulled apart at the chiasmata.
Telophase I begins as the chromosomes reach the poles and a nuclear membrane forms around each set. Cytokinesis occurs as the cytoplasm and organelles are divided in half and the one parent cell is split into two new daughter cells. Each daughter cell is now haploid (n), meaning it has half the number of chromosomes of the original parent cell (which is diploid-2n). These chromosomes in the daughter cells still exist as sister chromatids, but there is only one chromosome from each original homologous pair.
Meiosis II
The phases of meiosis II are similar to those of meiosis I, but there are some important differences. The time between the two nuclear divisions (interphase II) lacks replication of DNA (as in interphase I). As the two daughter cells produced in meiosis I enter meiosis II, their chromosomes are in the form of sister chromatids. No crossing over occurs in prophase II because there are no homologues to synapse.
During metaphase II, the spindle fibers from the opposite poles attach to the sister chromatids (instead of the homologues as before). The chromatids are then pulled apart during anaphase II. As the centromeres separate, the two single chromosomes are drawn to the opposite poles. The end result of meiosis II is that by the end of telophase II, there are four haploid daughter cells (in the sperm or ova) with each chromosome now represented by a single copy. The distribution of chromatids during meiosis is a matter of chance, which results in the concept of the law of independent assortment in genetics.
Control of meiosis
The events of meiosis are controlled by a protein enzyme complex known collectively as maturation promoting factor (MPF). These enzymes interact with one another and with cell organelles to cause the breakdown and reconstruction of the nuclear membrane, the formation of the spindle fibers, and the final division of the cell itself. MPF appears to work in a cycle, with the proteins slowly accumulating during interphase, and then rapidly degrading during the later stages of meiosis. In effect, the rate of synthesis of these proteins controls the frequency and rate of meiosis in all sexually reproducing organisms from the simplest to the most complex.
Human gamete formation
Meiosis occurs in humans, giving rise to the haploid gametes, the sperm and egg cells. In males, the process of gamete production is known as spermatogenesis, where each dividing cell in the testes produces four functional sperm cells, all approximately the same size. Each is propelled by a primitive but highly efficient flagellum (tail). In contrast, in females, oogenesis produces only one surviving egg cell from each original parent cell. During cytokinesis, the cytoplasm and organelles are concentrated into only one of the four daughter cells—the one which will eventually become the female ovum or egg. The other three smaller cells, called polar bodies, die and are reabsorbed shortly after formation. The process of oogenesis may seem inefficient, but by donating all the cytoplasm and organelles to only one of the four gametes, the female increases the egg’s chance for survival, should it become fertilized.
Mistakes during meiosis
The process of meiosis does not work perfectly every time, and mistakes in the formation of gametes are a major cause of genetic disease in humans. Under normal conditions, the four chromatids of a tetrad will separate completely, with one chromatid going into each of the four daughter cells. In a disorder known as nondisjunction, chromatids do not separate and one of the resulting gametes receives an extra copy of the same chromosome. The most common example of this mistake in meiosis is the genetic defect known as Down syndrome, in which a person receives an extra copy of chromosome 21 from one of the parents.
KEY TERMS
Chiasmata —Points at which adjacent chromosomes overlap and connect.
Crossing over —In meiosis, a process in which adjacent chromosomes exchange pieces of genetic information.
Cytokinesis —The physical division of the cytoplasm of a eukaryotic cell to form two daughter cells, each housing a newly formed nuclei.
Diploid —Nucleus or cell containing two copies of each chromosome, generated by fusion of two haploid nuclei.
Haploid —Nucleus or cell containing one copy of each chromosome.
Homologue —The partner of a chromosome in a chromosome pair.
Nondisjunction —The failure of a chromosome pair to separate and go to different cells following cell division.
Sister chromatids —Two copies of the same chromosome produced by DNA replication.
Synapsis —Process in which homologues orient themselves side by side.
Tetrad —A set of four chromatids all belonging to the same homologues.
Another fairly common form of nondisjunction occurs when the sex chromosomes (XX, XY) do not divide properly, resulting in individuals with Klinefelter syndrome or Turner syndrome.
Other mistakes that can occur during meiosis include translocation, in which part of one chromosome becomes attached to another, and deletion, in which part of one chromosome is lost entirely. The severity of the effects of these disorders depends entirely on the size of the chromosome fragment involved and the genetic information contained in it. Modern technology can detect these genetic abnormalities early in the development of the fetus, but at present, little can be done to correct or even treat the diseases resulting from them.
See also Gene; Genetic disorders.
Resources
BOOKS
Haseltine, Florence P., and Susan Heyner. Meiosis II: Contemporary Approaches to the Study of Meiosis. Washington, DC: AAAS Press, 1993.
John, Bernard. Meiosis. New York: Cambridge University Press, 1990.
Jorde, L.B., J.C. Carey, M.J. Bamshad, and R.L. White. Medical Genetics. 2nd ed. New York: Year Book, Inc., 2000.
PERIODICALS
Murray, Andrew W., and Marc W. Kirschner. “What Controls the Cell Cycle.” Scientific American (March 1991).
OTHER
Estrella Mountain Community College. “Cell Division: Meiosis and Sexual Reproduction” <http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookmeiosis.html> (accessed December 4, 2006).
University of Arizona, Department of Biochemistry and Molecular Biophysics: The Biology Project. “Meiosis Tutorial” <http://www.biology.arizona.edu/CELL_BIO/tutorials/meiosis/main.html> (accessed December 4, 2006).
Cheryl Taylor
Meiosis
Meiosis
Meiosis is a type of cell division that, in humans, occurs only in male testes and female ovary tissue, and, together with fertilization, it is the process that is characteristic of sexual reproduction. Meiosis serves two important purposes: it keeps the number of chromosomes from doubling each generation, and it provides genetic diversity in offspring. In this it differs from mitosis, which is the process of cell division that occurs in all somatic cells.
Overview
All of our somatic cells except the egg and sperm cells contain twenty-three pairs of chromosomes, for a total of forty-six individual chromosomes. This number, twenty-three, is known as the diploid number. If our egg and sperm cells were just like our somatic cells and contained twenty-three pairs of chromosomes, their fusion during fertilization would create a cell with forty-six chromosome pairs, or ninety-two chromosomes total. To prevent that from happening and to ensure a stable number of chromosomes throughout the generations, a special type of cell division is needed to halve the number of chromosomes in egg and sperm cells. This special process is meiosis.
Meiosis creates haploid cells, in which there are twenty-three individual chromosomes, without any pairing. When gametes fuse at conception to produce a zygote, which will turn into a fetus and eventually into an adult human being, the chromosomes containing the mother's and father's genetic material combine to form a single diploid cell. The specialized diploid cells that will eventually undergo meiosis to produce the gametes are called primary oocytes in the female ovary and primary spermatocytes in the male testis. They are set aside from somatic cells early in the course of fetal development.
Even though meiosis is a continuous process in reality, it is convenient to describe it as occurring in two separate rounds of nuclear division. In the first round (meiosis I), the two versions of each chromosome, called homologues or homologous chromosomes, pair up along their entire lengths and thus enable genetic material to be exchanged between them. This exchange process is called crossing over and contributes greatly to the amount of genetic variation that we see between parents and their children. Subsequently, the two homologues are pulled toward opposite ends of their surrounding cell, thus creating a haploid cluster of chromosomes at each pole, at which point division occurs, separating the two clusters. Meiosis I is therefore the actual reduction division. At the end of meiosis I, each chromosome is still composed of two sister strands (chromatids) held together by a particular DNA sequence of about 220 nucleotides, called the centromere. The centromere has a disk-shaped protein molecule (kinetochore) attached to it that is important for the separation of the sister chromatids in the second round of meiosis (meiosis II). Meiosis II is essentially the same division process as mitosis. Through the separation of the two sister chromatids, a total of four daughter cells, each with a haploid set of chromosomes, are created.
Meiosis I
Meiosis must be preceded by the S phase of the cell cycle. This is when DNA replication (the copying of the genetic material) occurs. Thus, each chromosome enters meiosis consisting of two sister chromatids joined at the centromere. The first stage of meiosis is a stage called prophase I. First, the DNA of individual chromosomes coils more and more tightly, a process called DNA condensation. The sister chromatids then attach to specific sites on the nuclear envelope that are designed to bring the members of each homologous pair of chromosomes close together. The sister chromatids line up in a fashion that is precise enough to pair up each gene on the DNA molecule with its corresponding "sister gene" on the homologous chromosome. This four-stranded structure of maternal and paternal homologues is also called a bivalent.
Next in prophase I is the process of crossing over, in which fragments of DNA are exchanged between the homologous sister chromatids that form the paired DNA strands. Crossing over involves the physical breakage of the DNA double helix in one paternal and one maternal chromatid and joining of the respective ends. Under the light microscope, the points of this exchange can often be seen as an X-shaped structure called a chiasma.
The exchange of genetic material means that new combinations of genes are created on two of the four chromatids: Stretches of DNA with maternal gene copies are mixed with stretches of DNA with paternal copies. This creation of new gene combinations is called "recombination" and is very important for evolution, since it increases the amount of genetic material that evolution can act upon. A statistical technique known as linkage analysis uses the frequency of recombination to infer the location of genes, such as those that increase a person's risk for certain diseases.
At the beginning of metaphase I, the nuclear envelope has dissolved, and specialized protein fibers called microtubules have formed a spindle apparatus, as also occurs in the metaphase of mitosis. These microtubules then attach to the kinetochore protein disks on the two centromeres of the homologous pair of chromosomes. However, there is an important difference between mitosis and meiosis in the way this attachment occurs. In mitosis, microtubules attach to both faces of the kinetochore and thus separate sister chromatids when they pull apart. In meiosis, because the chiasma structures still hold the homologous sister chromatids together, only one face of each kinetochore is accessible to the microtubules. Since the microtubules can only attach to one face of the kinetochore, the sister chromatids will be drawn to opposite poles as a pair, without separation of the individual chromatids.
At the end of metaphase I, the pairs of homologues line up on the metaphase plate in the center of the cell, the spindle apparatus is fully developed, and the microtubules of the spindle fibers are attached to one side of each of the two kinetochores. In anaphase I, the microtubules begin to shorten, thus breaking apart the chiasmata and pulling the centromeres with their respective sister chromatids toward the two cell poles. The centromeres do not divide, as they do in mitosis. At the final stage of meiosis I, called telophase I, each cell pole has a cluster of chromosomes that corresponds to a complete haploid set, one member of each homologous chromosome pair.
The Sources of Genetic Diversity
It is completely random whether the maternal or paternal chromosome of each pair ends up at a particular pole. The orientation of each pair of homologous chromosomes on the metaphase plate is random, and a mixture of maternal and paternal chromosomes will be drawn toward the same cell pole by chance. This phenomenon is often called "independent assortment," and it creates new combinations of genes that are located on different chromosomes. Thus, we have two levels of gene reshuffling occurring in meiosis I. The first occurs during recombination in prophase I, which creates new combinations of genes on the same chromosome. In contrast to mitosis, the sister chromatids of a chromosome are not genetically identical because of the crossing-over process. Anaphase I then adds the independent assortment of chromosomes to create new combinations of genes on different chromosomes. A total of 223 (8.4 million) possible combinations of parental chromosomes can be produced by one person, and recombination further increases this to an almost unlimited number of genetically different gametes.
Meiosis II
Once both cell poles have a haploid set of chromosomes clustered around them, these chromosomes divide mitotically (without reshuffling or reducing the number of chromosomes during division) during the second part of meiosis. This time, the spindle fibers bind to both faces of the kinetochore, the centromeres divide, and the sister chromatids move to opposite cell poles. At the end of meiosis II, therefore, the cell has produced four haploid groups of chromosomes. Nuclear envelopes form around each of these four sets of chromosomes, and the cytoplasm is physically divided among the four daughter cells in a process known as cytokinesis .
In males, the four resulting haploid sperm cells all go on to function as gametes (spermatozoa). They are produced continuously from puberty onwards. In females, all primary oocytes enter meiosis I during fetal development but then arrest at the prophase I stage until puberty. During infancy and early childhood, the primary oocytes acquire various functional characteristics of the mature egg cell. After puberty, one oocyte a month completes meiosis, but only one mature egg is produced, rather than the four mature sperm cells in males. The other daughter cells, called polar bodies, contain little cytoplasm and do not function as gametes.
Comparison with Mitosis
In summary, the main differences between meiosis and mitosis are that meiosis occurs only in specialized cells rather than in every tissue; it produces haploid gametes rather than diploid somatic cells; and each daughter cell is genetically different from the others due to recombination and independent assortment of homologues, rather than genetically identical. The pairing of homologous chromosomes and crossing over occur only in meiosis.
Chromosomal Aberrations
Meiosis is a very intricate process that requires, among other things, the precise alignment of homologous chromosome pairs and correct attachment of microtubules. During meiosis, errors in chromosome distribution may occur and lead to chromosomal aberrations in the offspring. One example is Down syndrome, where affected children carry three copies of chromosome 21 (trisomy 21). This may be explained by the failure of paired chromosomes or sister chromatids to separate in either sperm or egg, leading to the presence of two copies of chromosome 21. After fertilization with a normal gamete, the zygote will carry three copies, which leads to several phenotypic abnormalities, including mental retardation.
see also Cell, Eukaryotic; Chromosomal Aberrations; Crossing Over; Down Syndrome; Fertilization; Linkage and Recombination; Mitosis; Replication.
Silke Schmidt
Bibliography
Alberts, Bruce, et al. Molecular Biology of the Cell, 3rd ed. New York: Garland Publishing, 1994.
Curtis, Helena, and Susan Barnes. Invitation to Biology, 5th ed. New York: WorthPublishers, 1994.
Raven, Peter H., and George B. Johnson. Biology, 2nd ed. St. Louis, MO: Times Mirror/Mosby College Publishing, 1989.
Robinson, Richard. Biology. Farmington Hills, MI: Macmillan Reference USA, 2001.
Meiosis
Meiosis
Meiosis, also known as reduction division, consists of two successive cell divisions in diploid cells. The two cell divisions are similar to mitosis , but differ in that the chromosomes are duplicated only once, not twice. The end result of meiosis is four daughter cells, each of them haploid. Since meiosis only occurs in the sex organs (gonads), the daughter cells are the gametes (spermatozoa or ova), which contain hereditary material. By halving the number of chromosomes in the sex cells, meiosis assures that the fusion of maternal and paternal gametes at fertilization will result in offspring with the same chromosome number as the parents. In other words, meiosis compensates for chromosomes doubling at fertilization. The two successive nuclear divisions are termed as meiosis I and meiosis II. Each is further divided into four phases (prophase, metaphase, anaphase, and telophase) with an intermediate phase (interphase) preceding each nuclear division.
Events of meiosis
The events that take place during meiosis are similar in many ways to the process of mitosis, in which one cell divides to form two clones (exact copies) of itself. It is important to note that the purpose and final products of mitosis and meiosis are very different.
Meiosis I
Meiosis I is preceded by an interphase period in which the DNA replicates (makes an exact duplicate of itself), resulting in two exact copies of each chromosome that are firmly attached at one point, the centromere. Each copy is a sister chromatid, and the pair are still considered as only one chromosome. The first phase of meiosis I, prophase I, begins as the chromosomes come together in homologous pairs in a process known as synapsis. Homologous chromosomes, or homologues, consist of two chromosomes that carry genetic information for the same traits, although that information may hold different messages (e.g., when two chromosomes carry a message for eye color , but one codes for blue eyes while the other codes for brown). The fertilized eggs (zygotes) of all sexually reproducing organisms receive their chromosomes in pairs, one from the mother and one from the father. During synapsis, adjacent chromatids from homologous chromosomes "cross over" one another at random points and join at spots called chiasmata. These connections hold the pair together as a tetrad (a set of four chromatids, two from each homologue). At the chiasmata, the connected chromatids randomly exchange bits of genetic information so that each contains a mixture of maternal and paternal genes. This "shuffling" of the DNA produces a tetrad, in which each of the chromatids is different from the others, and a gamete that is different from others produced by the same parent. Crossing over does, in fact, explain why each person is a unique individual, different even from those in the immediate family. Prophase I is also marked by the appearance of spindle fibers (strands of micro-tubules) extending from the poles or ends of the cell as the nuclear membrane disappears. These spindle fibers attach to the chromosomes during metaphase I as the tetrads line up along the middle or equator of the cell. A spindle fiber from one pole attaches to one chromosome while a fiber from the opposite pole attaches to its homologue. Anaphase I is characterized by the separation of the homologues, as chromosomes are drawn to the opposite poles. The sister chromatids are still intact, but the homologous chromosomes are pulled apart at the chiasmata. Telophase I begins as the chromosomes reach the poles and a nuclear membrane forms around each set. Cytokinesis occurs as the cytoplasm and organelles are divided in half and the one parent cell is split into two new daughter cells. Each daughter cell is now haploid (n), meaning it has half the number of chromosomes of the original parent cell (which is diploid-2n). These chromosomes in the daughter cells still exist as sister chromatids, but there is only one chromosome from each original homologous pair.
Meiosis II
The phases of meiosis II are similar to those of meiosis I, but there are some important differences. The time between the two nuclear divisions (interphase II) lacks replication of DNA (as in interphase I). As the two daughter cells produced in meiosis I enter meiosis II, their chromosomes are in the form of sister chromatids. No crossing over occurs in prophase II because there are no homologues to synapse . During metaphase II, the spindle fibers from the opposite poles attach to the sister chromatids (instead of the homologues as before). The chromatids are then pulled apart during anaphase II. As the centromeres separate, the two single chromosomes are drawn to the opposite poles. The end result of meiosis II is that by the end of telophase II, there are four haploid daughter cells (in the sperm or ova) with each chromosome now represented by a single copy. The distribution of chromatids during meiosis is a matter of chance, which results in the concept of the law of independent assortment in genetics .
Control of meiosis
The events of meiosis are controlled by a protein enzyme complex known collectively as maturation promoting factor (MPF). These enzymes interact with one another and with cell organelles to cause the breakdown and reconstruction of the nuclear membrane, the formation of the spindle fibers, and the final division of the cell itself. MPF appears to work in a cycle, with the proteins slowly accumulating during interphase, and then rapidly degrading during the later stages of meiosis. In effect, the rate of synthesis of these proteins controls the frequency and rate of meiosis in all sexually reproducing organisms from the simplest to the most complex.
Human gamete formation
Meiosis occurs in humans, giving rise to the haploid gametes, the sperm and egg cells. In males, the process of gamete production is known as spermatogenesis, where each dividing cell in the testes produces four functional sperm cells, all approximately the same size. Each is propelled by a primitive but highly efficient flagellum (tail). In contrast, in females, oogenesis produces only one surviving egg cell from each original parent cell. During cytokinesis, the cytoplasm and organelles are concentrated into only one of the four daughter cells—the one which will eventually become the female ovum or egg. The other three smaller cells, called polar bodies, die and are reabsorbed shortly after formation. The process of oogenesis may seem inefficient, but by donating all the cytoplasm and organelles to only one of the four gametes, the female increases the egg's chance for survival, should it become fertilized.
Mistakes during meiosis
The process of meiosis does not work perfectly every time, and mistakes in the formation of gametes are a major cause of genetic disease in humans. Under normal conditions, the four chromatids of a tetrad will separate completely, with one chromatid going into each of the four daughter cells. In a disorder known as nondis-junction, chromatids do not separate and one of the resulting gametes receives an extra copy of the same chromosome. The most common example of this mistake in meiosis is the genetic defect known as Down syndrome , in which a person receives an extra copy of chromosome 21 from one of the parents. Another fairly common form of nondisjunction occurs when the sex chromosomes (XX, XY) do not divide properly, resulting in individuals with Klinefelter syndrome or Turner syndrome . Other mistakes that can occur during meiosis include translocation, in which part of one chromosome becomes attached to another, and deletion, in which part of one chromosome is lost entirely. The severity of the effects of these disorders depends entirely on the size of the chromosome fragment involved and the genetic information contained in it. Modern technology can detect these genetic abnormalities early in the development of the fetus, but at present, little can be done to correct or even treat the diseases resulting from them.
See also Gene; Genetic disorders.
Resources
books
Edwards, Gabrielle I. Biology the Easy Way. 2nd ed. New York: Barrons Educational Service, Inc., 1990.
Haseltine, Florence P., and Susan Heyner. Meiosis II: Contemporary Approaches to the Study of Meiosis. Washington, DC: AAAS Press, 1993.
John, Bernard. Meiosis. New York: Cambridge University Press, 1990.
Jorde, L.B., J.C. Carey, M.J. Bamshad, and R.L. White. Medical Genetics. 2nd ed. New York: Year Book, Inc., 2000.
periodicals
Murray, Andrew W., and Marc W. Kirschner. "What Controls the Cell Cycle." Scientific American (March 1991).
Cheryl Taylor
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Chiasmata
—Points at which adjacent chromosomes overlap and connect.
- Crossing over
—In meiosis, a process in which adjacent chromosomes exchange pieces of genetic information.
- Cytokinesis
—The physical division of the cytoplasm of a eukaryotic cell to form two daughter cells, each housing a newly formed nuclei.
- Diploid
—Nucleus or cell containing two copies of each chromosome, generated by fusion of two haploid nuclei.
- Haploid
—Nucleus or cell containing one copy of each chromosome.
- Homologue
—The partner of a chromosome in a chromosome pair.
- Nondisjunction
—The failure of a chromosome pair to separate and go to different cells following cell division.
- Sister chromatids
—Two copies of the same chromosome produced by DNA replication.
- Synapsis
—Process in which homologues orient themselves side by side.
- Tetrad
—A set of four chromatids all belonging to the same homologues.
Meiosis
Meiosis
Meiosis is the two-step series of specialized cell divisions that makes sexual reproduction possible. Meiosis produces haploid cells, which contain just one member of every chromosome pair characteristic of an organism. In all animals, specialized cells in the reproductive organs, called germ cells, undergo meiosis to produce haploid gametes (sperm and egg), which then fuse during sexual reproduction to create new diploid embryos. For example, human gametes are haploid and contain twenty-three different chromosomes. All other cells in the human body are diploid, containing two versions of each chromosome for a total of forty-six. Fusion of gametes to form a new embryo restores the diploid number characteristic of the organism, and it mixes maternal and paternal genes to give new combinations of traits. Meiosis itself also yields great genetic diversity in the resultant gametes through two mechanisms: (1) independent assortment of chromosomes at both of the meiotic divisions; and (2) physical exchange of chromosomal regions through a process called crossing over. Both processes create new chromosomal combinations, resulting in an array of genetically diverse gametes from a single individual.
Plants, fungi, and some protists also perform meiosis. In plants, meiosis creates a multicellular haploid organism, called a gametophyte , which in some groups is independent of the diploid plant. Gametes are produced by mitosis of the gametophyte, which then fuse to form the embryo. This cycle is called alternation of generations.
Chromosome Basics and Meiosis Overview
As noted, diploid cells contain pairs of chromosomes, each member of which carries the same set of genes. One member of each pair is inherited from the mother, and one from the father. The two pair members are called homologous chromosomes , or homologs.
Prior to meiosis, the diploid cell replicates its deoxyribonucleic acid (DNA). During replication, each chromosome duplicates itself to form two identical copies, which remain attached at a region known as the centromere . Each copy is known as a chromatid ; thus, each chromosome is composed of two identical sister chromatids.
During meiosis, homologous chromosomes line up and exchange segments, a process called crossing over. Following this, homologs are separated from each other in the first meiotic division. Next, in the second meiotic division, chromatids are separated from each other, in a process which is mechanically identical to mitosis. The result is four haploid cells. The coordination of the two meiotic chromosomal divisions gives meiosis its distinctive characteristics: a reduction in the number of chromosomes by half, accompanied by mixing of parental chromosomes, and swapping of regions between homologous chromosomes.
Meiosis I
Consider a spermatocyte or oocyte about to embark on meiosis. This diploid cell contains one set of chromosomes contributed by its mother and one set of chromosomes contributed by its father. Following DNA replication, the unique aspects of the first division of meiosis (meiosis I) begin. Because meiosis reduces chromosome content, a mechanism must ensure that every final haploid gamete has both the correct number and the correct set of chromosomes, with one member of each homologous pair. Meiosis I guarantees this by keeping each chromatid pair together and aligning homologous pairs of duplicated sister chromosomes prior to the first chromosomal division. The alignment and subsequent separation of pairs of homologous chromosomes during meiosis I thus sets up the mechanism that ensures that all four haploid gametes will contain the correct complement of chromosomes. Interestingly, the mechanism whereby meiosis aligns homologs also results in reciprocal exchanges of DNA between aligned chromosomes.
Alignment of homologous chromosome pairs begins before meiosis I, when each duplicated set of chromosomes seeks its homologous partner pair within the oocyte or spermatocyte. The underlying DNA sequence homology of the similar maternal and paternal chromosome pairs guides this search and eventual alignment along the entire length of each chromosome. The alignment is further mediated and cemented by a three-dimensional zipperlike structure surrounding each set of paired homologous chromosomes, the synaptonemal complex. In the process of these alignment steps, specific enzymes nick and then rejoin DNA at different places along the paired chromosomes. This process of genetic exchange is called meiotic recombination, or crossing over. Crossing over provides an attachment that holds homologous chromosomes temporarily in place and, at the same time, produces progeny chromosomes consisting of a patchwork of material from each of the originals. Thus, the two central characteristics of meiosis, reduction in chromosome number and genetic rearrangements, are intimately intertwined.
Once all sets of chromosome pairs have established at least one such crossing over, correct assortment of chromosomes at meiosis I is ensured. The synaptonemal complexes dissolve and the newly rearranged chromosomes proceed through the second mechanism that generates genetic diversity at meiosis I: They assort independently of one another to opposite poles of the cell pulled by spindle fibers. Whereas one chromosome pair might divide so that its predominantly maternal chromosome moves to the cell's "north" pole, another pair of chromosomes will move its predominantly paternal chromosome to that same north pole. These chromosomal movements are randomly determined, yielding great genetic diversity of gametes in an organism with multiple chromosomes. In an organism with three homologous pairs, there are four different possible chromosome arrangements at the end of meiosis I. In humans there are more than 4 million possible arrangements.
Thus, overall, the first division of meiosis provides two major mechanisms for new genetic combinations: (1) cutting apart and pasting together various segments of homologous chromosomes to yield unique hybrid chromosomes; and (2) independent assortment of maternal and paternal chromosomes.
Meiosis II and Cytokinesis
As meiosis II begins, each daughter nucleus contains the haploid number of chromosomes (for humans, twenty-three). Each chromosome is composed of two chromatids attached at the centromere. The second division of meiosis separates the chromatids. Once again, spindle fibers provide the pulling power. Once chromatids are separated, they are called chromosomes, and so at the end of meiosis II, each of the four new cells has the haploid number of chromosomes. Following this, cytokinesis occurs, in which the cytoplasm of the original cell is divided and membranes form to separate the new cells. Cytoplasm is divided evenly in sperm, but unevenly in eggs. During egg formation, most of the cytoplasm is allotted to one of the cell products, leaving one functional egg and several "polar bodies" that contain DNA and membrane, but little else. This unequal division gives the single egg a larger store of food to supply the developing embryo after fertilization .
Meiosis versus Mitosis
The alignment of homologous chromosome pairs in meiosis I and the accompanying physical exchanges between aligned chromosomes is unique to meiosis. In mitosis, by contrast, homologous chromosome pairs never or very rarely interact. Each mitotic chromosome duplicates, forming two sister chromatids, and then these two identical sister chromatids separate to opposite poles. While mitosis is specialized to produce entirely identical progeny, meiosis is specialized to produce a wide range of distinctive haploid progeny.
Mistakes in Meiosis
Among the many potential causes of infertility are problems with meiosis. If a person's spermatocytes or oocytes consistently produce sperm or eggs that contain an incorrect number or complement of chromosomes, then there will be great difficulty in producing a viable embryo.
A much more common situation arises from the rare, sporadic occurrence in a normally fertile person of an improper chromosome separation. When two chromosomes fail to separate as they should, a "nondisjunction" event has occurred. Such nondisjunctions are almost always lethal to the egg or sperm, or to the resultant embryo. There are exceptions, however. For example, approximately one out of one hundred men is the result of such a nondisjunction, which gave him an extra X chromosome. Such XXY individuals have Klinefelter's syndrome, a sex chromosome trisomy (three sex chromosomes instead of the normal two) with minor outward manifestations. Down syndrome individuals possess three copies of chromosome twenty-one instead of the normal two; their extra copy resulted from a nondisjunction of those chromosomes during one of the meiotic divisions of one of the parents.
see also Alternation of Generations; Chromosome, Eukaryotic; Cytokinesis; Mitosis; Sex Chromosomes; Sexual Reproduction
Wendy E. Raymond
Bibliography
Griffiths, Anthony J. F., et al. Modern Genetic Analysis. New York: W. H. Freeman and Company, 1999.
"Meiosis and Genetic Recombination." MIT's Biology Hypertextbook. <http://esg-.www.mit.edu:8001/esgbio/mg/meiosis.html>.
"Meiosis Tutorial." The Biology Project. <http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/main.html>.
meiosis
meiosis
Meiosis
Meiosis
Meiosis (may-OH-sis) is a specialized form of cell division that takes place only in the reproductive cells. The goal of meiosis is to produce sex cells (sperm and egg) that have only one set of twenty-three chromosomes. When a sex cell unites with another sex cell, the zygote (fertilized egg) will have the proper total of forty-six chromosomes.
It is important to distinguish meiosis from mitosis (my-TOH-sis). Although both are a form of cell division, mitosis produces two identical cells, while meiosis produces four different cells. Mitosis makes new, identical cells so that an organism will be able to replace damaged and dead cells and be able to grow. Nearly all the cell division in an organism can be described as mitosis. Meiosis only occurs in an organism's sex cells and is structured so that it deliberately produces different rather than identical cells. Without meiosis producing differing cells, there would be no variation in offspring, who also would have twice the number of chromosomes than they should have.
If mitosis occurred in reproductive cells the way it does in all other cells of the body, the new cell produced would have twice the number of chromosomes that it should have. For example, the exact amount of chromosomes needed to be human is forty-six. Without meiosis cutting the number of chromosomes in half, that number would be ninety-two chromosomes after fertilization has taken place. Meiosis splits in half the number of chromosomes in sperm and egg cells, so when the cells unite, the zygote will get half the number of chromosomes from each parent.
Besides halving the number of chromosomes, meiosis also performs another very important function. It allows genetic material to be "shuffled" as the chromosomes cross over each other and swap genes before the cell divides. This is a random exchange of genetic material that guarantees 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. This assures that no two sperm or egg cells have the same exact combination of genes. This also partly explains why brothers and sisters (except identical twins) of the same parents have different characteristics.
Eventually, when two of these unique sex cells are joined as sperm and egg and form a new individual (thus further mixing the genetic instructions), an entirely unique organism is created unlike any other existing organism. The variations or differences caused by meiosis are very important to evolution, since the process of natural selection (the process of survival and reproduction of organisms that are best suited to their environment) needs genetic variety from which to "select." If there were no differences, there would be no evolution.
In humans, meiosis occurs in the gametes or sex cells (sperm and egg). In males, the process of gamete production is known as spermato-genesis. During this process, each dividing cell in the testes produces four functional sperm cells, all basically the same size. In contrast, the female process of producing eggs, called oogenesis, makes four eggs, only one of which survives. This is because nature gives all the necessary cytoplasm (living material) and organelles (structures with particular functions) to only one egg, thereby increasing its chances of survival should it become fertilized.
[See alsoCell Division; Fertilization; Reproduction, Sexual ]
meiosis
meiosis
mei·o·sis / mīˈōsəs/ • n. (pl. -ses / -sēz/ ) 1. Biol. a type of cell division that results in two daughter cells each with half the chromosome number of the parent cell, as in the production of gametes.Compare with mitosis.2. another term for litotes.DERIVATIVES: mei·ot·ic / mīˈätik/ adj.mei·ot·i·cal·ly / -ik(ə)lē/ adv.
Meiosis
MEIOSIS
Two types of nuclear division, mitosis and meiosis, occur in cell biology. Most human cells (called diploid cells) are formed through mitosis and contain forty-six chromosomes in twenty-three matched pairs. By contrast, meiosis produces haploid cells, each containing a single set of twenty-three unpaired chromosomes. Sex cells (sperm and ovum) are haploid.
Prior to meiosis, DNA is replicated within a diploid cell, resulting in four copies of each chromosome (now numbering ninety-two). Two successive divisions of the nuclear material occur during meiosis. As part of the process, homologous chromosomes—paired maternal and paternal chromosomes—exchange segments, thus recombining their genes. Four daughter haploid cells are formed, each with one-quarter of the genetic material (twenty-three chromosomes) of the original diploid cell. When haploid cells unite in sexual reproduction, each contributes half of the genetic material that creates the offspring. Meiosis, therefore, contributes to the genetic diversity within species.
See also:MITOSIS
Bibliography
Alberts, Bruce, Dennis Bray, Alexander Johnson, Julian Lewis, Peter Walter, Keith Roberts, and Martin Raff. Essential Cell Biology: An Introduction to the Molecular Biology of the Cell. New York: Garland, 1998.
Maryann WzorekRossi