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Encyclopedia > Meiosis

In biology or life science, Meiosis (pronounced mi-o-sis or me-o-sis) is a process of reduction division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes. The word "meiosis" comes from the Greek meioun, meaning "to make small," since it results in a reduction in chromosome number in the gamete cell. A figure of speech, sometimes termed a rhetoric, or locution, is a word or phrase that departs from straightforward, literal language. ... Meiosis is a figure of speech which intentionally understates something or implies that it is less in significance, size, than it really is. ... Miosis should not be confused with meiosis, the cellular division process involved in sexual reproduction. ... For the song by Girls Aloud see Biology (song) Biology studies the variety of life (clockwise from top-left) E. coli, tree fern, gazelle, Goliath beetle Biology (from Greek: Βιολογία - βίος, bio, life; and λόγος, logos, speech lit. ... A gamete (from Ancient Greek γαμετης; translated gamete = wife, gametes = husband) is a cell that fuses with another gamete during fertilization (conception) in organisms that reproduce sexually. ...


Meiosis is essential for sexual reproduction and therefore occurs in all eukaryotes (including single-celled organisms) that reproduce sexually. A few eukaryotes, notably the Bdelloid rotifers, have lost the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis. Meiosis does not occur in archaea or bacteria, which reproduce via asexual processes such as mitosis or binary fission. Each cell has half the number of chromosomes as the parent cell. Sexual reproduction is a union that results in increasing genetic diversity of the offspring. ... Kingdoms Animalia - Animals Fungi Plantae - Plants Chromalveolata Protista Alternative phylogeny Unikonta Opisthokonta Metazoa Choanozoa Eumycota Amoebozoa Bikonta Apusozoa Cabozoa Rhizaria Excavata Corticata Archaeplastida Chromalveolata Animals, plants, fungi, and protists are eukaryotes (IPA: ), organisms whose cells are organized into complex structures by internal membranes and a cytoskeleton. ... The bdelloids (Bdelloidea) are a class of rotifers, found in freshwater and moist soil. ... Classes Seisonoidea Bdelloidea Monogononta The rotifers make up a phylum of microscopic, pseudocoelomate animals. ... For the religious belief, see Virgin Birth of Jesus. ... Phyla Crenarchaeota Euryarchaeota Korarchaeota Nanoarchaeota ARMAN The Archaea (pronounced ) are a group of prokaryotic and single-celled microorganisms. ... Phyla Actinobacteria Aquificae Chlamydiae Bacteroidetes/Chlorobi Chloroflexi Chrysiogenetes Cyanobacteria Deferribacteres Deinococcus-Thermus Dictyoglomi Fibrobacteres/Acidobacteria Firmicutes Fusobacteria Gemmatimonadetes Lentisphaerae Nitrospirae Planctomycetes Proteobacteria Spirochaetes Thermodesulfobacteria Thermomicrobia Thermotogae Verrucomicrobia Bacteria (singular: bacterium) are unicellular microorganisms. ... Mitosis divides genetic information during cell division. ... Binary fission Binary fission is the form of asexual reproduction in single-celled organisms by which one cell divides into two cells of the same size, used by most prokaryotes. ...


During meiosis, the genome of a diploid germ cell, which is composed of long segments of DNA packaged into chromosomes, undergoes DNA replication followed by two rounds of division, resulting in four haploid cells. Each of these cells contain one complete set of chromosomes, or half of the genetic content of the original cell. If meiosis produces gametes, these cells must fuse during fertilization to create a new diploid cell, or zygote before any new growth can occur. Thus, the division mechanism of meiosis is a reciprocal process to the joining of two genomes that occurs at fertilization. Because the chromosomes of each parent undergo genetic recombination during meiosis, each gamete, and thus each zygote, will have a unique genetic blueprint encoded in its DNA. Together, meiosis and fertilization constitute sexuality in the eukaryotes, and generates genetically distinct individuals in populations. In biology the genome of an organism is the whole hereditary information of an organism that is encoded in the DNA (or, for some viruses, RNA). ... Diploid (meaning double in Greek) cells have two copies (homologs) of each chromosome (both sex- and non-sex determining chromosomes), usually one from the mother and one from the father. ... A germ cell is part of the germline and is involved in the reproduction of organisms. ... The structure of part of a DNA double helix Deoxyribonucleic acid, or DNA, is a nucleic acid molecule that contains the genetic instructions used in the development and functioning of all known living organisms. ... A scheme of a condensed (metaphase) chromosome. ... Haploid (meaning simple in Greek) cells have only one copy of each chromosome. ... This article is about the biological chromosome. ... Categories: Biology stubs ... For other meanings see Zygote (disambiguation). ... Genetic recombination is the process by which a strand of the genetic material (usually DNA; but can also be RNA) is broken and then joined to the end of a different DNA molecule. ...


In all plants, and in many protists, meiosis results in the formation of haploid cells that can divide vegetatively without undergoing fertilization. In these groups, gametes are produced by mitosis.


Meiosis uses many of the same biochemical mechanisms employed during mitosis to accomplish the redistribution of chromosomes. There are several features unique to meiosis, most importantly the pairing and genetic recombination between homologous chromosomes. Mitosis divides genetic information during cell division. ... Homologous chromosomes are chromosomes in a biological cell that pair (synapse) during meiosis, or alternatively, non-identical chromosomes that contain information for the same biological features and contain the same genes at the same loci but possibly different genetic information, called alleles, at those genes. ...

Contents

History

Meiosis was discovered and described for the first time in sea urchin eggs in 1876, by noted German biologist Oscar Hertwig (1849-1922). It was described again in 1883, at the level of chromosomes, by Belgian zoologist Edouard Van Beneden (1846-1910), in Ascaris worms' eggs. The significance of meiosis for reproduction and inheritance, however, was described only in 1890 by German biologist August Weismann (1834-1914), who noted that two cell divisions were necessary to transform one diploid cell into four haploid cells if the number of chromosomes had to be maintained. In 1911 the American geneticist Thomas Hunt Morgan (1866-1945) observed crossover in Drosophila melanogaster meiosis and provided the first true genetics. Subclasses Subclass Perischoechinoidea Order Cidaroida (pencil urchins) Subclass Euechinoidea Superorder Atelostomata Order Cassiduloida Order Spatangoida (heart urchins) Superorder Diadematacea Order Diadematoida Order Echinothurioida Order Pedinoida Superorder Echinacea Order Arbacioida Order Echinoida Order Phymosomatoida Order Salenioida Order Temnopleuroida Superorder Gnathostomata Order Clypeasteroida (sand dollars) Order Holectypoida Wikispecies has information related to... In most birds and reptiles, an egg (Latin ovum) is the zygote, resulting from fertilization of the ovum. ... Year 1876 (MDCCCLXXVI) // January 31 - United States orders all Indigenous peoples in the United States to move onto reservations February 2 - The National League of Professional Baseball Clubs of Major League Baseball is formed. ... Oskar Hertwig (April 21, 1849, Friedberg, Hessen - October 25, 1922, Berlin) was a German zoologist. ... Year 1883 (MDCCCLXXXIII) was a common year starting on Monday (link will display the full calendar) of the Gregorian calendar (or a common year starting on Saturday of the 12-day slower Julian calendar). ... National motto: Dutch: Eendracht maakt macht; French: Lunion fait la force; German: Einigkeit macht stark (English: Strength lies in unity) Official language Dutch, French, German Capital Brussels Largest City Brussels King Albert II Prime Minister Guy Verhofstadt Area  - Total  - % water Ranked 148th 30,528 km² 6. ... Edouard Van Beneden (1846-1910) was a professor of zoology at the university of Liège; he has been at the basis of the genetic studies of the 20th century, thanks to his works on a parasite of the horse, that made him discover how chromosomes combined during reproduction. ... Species Ascaris lumbricoides Ascaris suum Ascaris is a genus of parasitic nematodes. ... Year 1890 (MDCCCXC) was a common year starting on Wednesday (link will display the full calendar) of the Gregorian calendar (or a common year starting on Monday of the Julian calendar). ... August Weismann Friedrich Leopold August Weismann (b. ... Year 1911 (MCMXI) was a common year starting on Sunday (link will display the full calendar) of the Gregorian calendar (or a common year starting on Saturday of the 13-day-slower Julian calendar). ... Thomas Hunt Morgan (September 25, 1866 – December 4, 1945) was an American geneticist and embryologist. ... Thomas Hunt Morgans illustration of crossing over (1916) Homologous Recombination is the process by which two chromosomes, paired up during prophase I of meiosis, exchange some distal portion of their DNA. Crossover occurs when two chromosomes, normally two homologous instances of the same chromosome, break and then reconnect but... Binomial name Meigen, 1830[1] Drosophila melanogaster (from the Greek for black-bellied dew-lover) is a two-winged insect that belongs to the Diptera, the order of the flies. ...


Evolution

Meiosis is thought to have appeared 1.4 billion years ago. The only supergroup of eukaryotes which does not have meiosis in all organisms is excavata. The other five major supergroups, opisthokonts, amoebozoa, rhizaria, archaeplastida and chromalveolates all seem to have genes for meiosis universally present, even if not always functional. Some excavata species do have meiosis which is consistent with the hypothesis that excavata is an ancient, paraphyletic grade. An example of eukaryotic organism in which meiosis does not exist is euglenoid. Kingdoms Eukaryotes are organisms with complex cells, in which the genetic material is organized into membrane-bound nuclei. ... This article is about the protist group called excavates. ... Subgroups Kingdom Animalia Kingdom Fungi Choanozoa Choanoflagellates Corallochytrids Mesomycetozoea Nucleariids The opisthokonts (Greek: (opisthō-) = rear, posterior + (kontos) = pole i. ... Subgroups Mycetozoa(slime moulds) Archamoebae    Pelobiontida    Entamoebida Gymnamoebia Various others The Amoebozoa are a major group of amoeboid protozoa, including the majority that move by means of internal cytoplasmic flow. ... Phyla Cercozoa Foraminifera Radiolaria The Rhizaria are a major line of protists. ... The Archaeplastida are a major line of eukaryotes, comprising the land plants, green and red algae, and a small group called the glaucophytes. ... Phyla Heterokontophyta Haptophyta Cryptophyta Alveolata Ciliophora Apicomplexa Dinoflagellata Chromalveolata is a eukaryote supergroup first proposed by Thomas Cavalier-Smith as a refinement of his kingdom Chromista, which was first proposed in 1981. ... The excavates are a major line of protists, often known as Excavata. ... This article is about the protist group called excavates. ... Paraphyletic - Wikipedia /**/ @import /skins-1. ... The euglenids are one of the best-known groups of flagellates, commonly found in freshwater especially when rich in organic materials, with a few marine and endosymbiotic members. ...


Occurrence of meiosis in eukaryotic life cycles

Gametic life cycle.
Gametic life cycle.
Zygotic life cycle.
Zygotic life cycle.
Sporic life cycle.
Main article: Biological life cycle

Meiosis occurs in all eukaryotic life cycles involving sexual reproduction, comprising of the constant cyclical process of meiosis and fertilization. This takes place alongside normal mitotic cell division. In multicellular organisms, there is an intermediary step between the diploid and haploid transition where the organism grows. The organism will then produce the germ cells that continue in the life cycle. The rest of the cells, called somatic cells, function within the organism and will die with it. Image File history File links Drawn by self for Biological life cycle Scan black/white/grey outline PSP Reduce size (by 20%) Brightness/contrast to get rid of artifacts Copy-&-paste the multicellular balls Fill-in colours Labelling Re-fix details by going back to Layer 1. ... Image File history File links Drawn by self for Biological life cycle Scan black/white/grey outline PSP Reduce size (by 20%) Brightness/contrast to get rid of artifacts Copy-&-paste the multicellular balls Fill-in colours Labelling Re-fix details by going back to Layer 1. ... Image File history File links Drawn by self for Biological life cycle Scan black/white/grey outline PSP Reduce size (by 20%) Brightness/contrast to get rid of artifacts Copy-&-paste the multicellular balls Fill-in colours Labelling Re-fix details by going back to Layer 1. ... Image File history File links Drawn by self for Biological life cycle Scan black/white/grey outline PSP Reduce size (by 20%) Brightness/contrast to get rid of artifacts Copy-&-paste the multicellular balls Fill-in colours Labelling Re-fix details by going back to Layer 1. ... Image File history File links Drawn by self for Biological life cycle Cut-&-merge halves Image:Zygotic_meiosis. ... Image File history File links Drawn by self for Biological life cycle Cut-&-merge halves Image:Zygotic_meiosis. ... A life cycle is a period involving one generation of an organism through means of reproduction, whether through asexual reproduction or sexual reproduction. ... Sexual reproduction is a union that results in increasing genetic diversity of the offspring. ... Mitosis divides genetic information during cell division. ... A germ cell is part of the germline and is involved in the reproduction of organisms. ... This article does not cite any references or sources. ... For other uses, see Death (disambiguation). ...


Cycling meiosis and fertilisation events produces a series of transitions back and forth between alternating haploid and diploid states. The organism phase of the life cycle can occur either during the diploid state (gametic life cycle), or during the haploid state (zygotic life cycle), or both (sporic life cycle, in which there two distinct organism phases, one during the haploid state and the other during the diploid state). In this sense, there are three types of life cycles that utilize sexual reproduction, differentiated by the location of the organisms phase(s). In the gametic life cycle, the species is diploid, grown from a diploid cell called the zygote. In the zygotic life cycle the species is haploid instead, spawned by the proliferation and differentiation of a single haploid cell called the gamete. Humans, for example, are diploid creatures. Human stem cells undergo meiosis to create haploid gametes, which are spermatozoa for males or ova for females. These gametes then fertilize in the Fallopian tubes of the female, producing a diploid zygote. The zygote undergoes progressive stages of mitosis and differentiation, turns into a blastocyst and then gets implanted in the uterus endometrium to create an embryo. For other meanings see Zygote (disambiguation). ... A gamete (from Ancient Greek γαμετης; translated gamete = wife, gametes = husband) is a cell that fuses with another gamete during fertilization (conception) in organisms that reproduce sexually. ... Schematic diagram of a sperm cell, showing the (1) acrosome, (2) cell membrane, (3) nucleus, (4) mitochondria, and (5) flagellum (tail) A sperm cell, or spermatozoon ( spermatozoa) (in Greek: sperm = semen and zoon = alive), is the haploid cell that is the male gamete. ... A human ovum An ovum (loosely, egg or egg cell) is a female sex cell or gamete. ... Female internal reproductive anatomy The Fallopian tubes or oviducts are two very fine tubes leading from the ovaries of female mammals into the uterus. ... Differentiation can mean the following: In biology: cellular differentiation; evolutionary differentiation; In mathematics: see: derivative In cosmogony: planetary differentiation Differentiation (geology); Differentiation (logic); Differentiation (marketing). ... The blastocyst is an early stage of the human (or any other mammal) development early in pregnancy. ... This article is about female reproductive anatomy. ... The endometrium is the inner membrane of the mammalian uterus. ... For other uses, see Embryo (disambiguation). ...


In the gametic life cycle, of which humans are a part, the living organism is diploid in nature. Here, we will generalize the example of human reproduction stated previously. The organism's diploid germ-line stem cells undergo meiosis to create haploid gametes, which fertilize to form the zygote. The diploid zygote undergoes repeated cellular division by mitosis to grow into the organism. Mitosis is a related process to meiosis that creates two cells that are genetically identical to the parent cell. The general principle is that mitosis creates somatic cells and meiosis creates germ cells. Mitosis divides genetic information during cell division. ...


In the zygotic life cycle, the living organism is haploid. Two organisms of opposing gender contribute their haploid germ cells to form a diploid zygote. The zygote undergoes meiosis immediately, creating four haploid cells. These cells undergo mitosis to create the organism. Many fungi and many protozoa are members of the zygotic life cycle. Mitosis divides genetic information during cell division. ... For the fictional character, see Fungus the Bogeyman. ... Leishmania donovani, (a species of protozoan) in a bone marrow cell (in Greek proto = first and zoa = animals) are one-celled eukaryotes (that is, unicellular microbes whose cells have membrane-bound nuclei) that commonly show characteristics usually associated with animals, mobility and heterotrophy. ...


Finally, in the sporic life cycle, the living organism alternates between haploid and diploid states. Consequently, this cycle is also known as the alternation of generations. The diploid organism's germ-line cells undergo meiosis to produce gametes. The gametes proliferate by mitosis, growing into a haploid organism. The haploid organism's germ cells then combine with another haploid organism's cells, creating the zygote. The zygote undergoes repeated mitosis and differentiation to become the diploid organism again. The sporic life cycle can be considered a fusion of the gametic and zygotic life cycles. Sporic or diplohaplontic life cycle. ...


Process

Because meiosis is a "one-way" process, it cannot be said to engage in a cell cycle as mitosis does. However, the preparatory steps that lead up to meiosis are identical in pattern and name to the interphase of the mitotic cell cycle. The cell cycle, or cell-division cycle, is the series of events that take place in a eukaryotic cell leading to its replication. ...


Interphase is divided into three phases:

  • Growth 1 (G1) phase: Immediately follows cytokinesis. This is a very active period, where the cell synthesizes its vast array of proteins, including the enzymes and structural proteins it will need for growth. In G1 stage each of the 46 human chromosomes consists of a single (very long) molecule of DNA. At this point cells are 46,2N, identical to somatic cells.
  • Synthesis (S) phase: The genetic material is replicated: each of its chromosomes duplicates (46,2N). The cell is still diploid, however, because it still contains the same number of centromeres. However, the identical sister chromatids are in the chromatin form because spiralisation and condensation into denser chromosomes have not taken place yet. It will take place in prophase I in meiosis.
  • Growth 2 (G2) phase: G2 phase is absent in Meiosis

Interphase is immediately followed by meiosis I and meiosis II. Meiosis I consists of segregating the homologous chromosomes from each other, then dividing the diploid cell into two haploid cells each containing one of the segregates. Meiosis II consists of decoupling each chromosome's sister strands (chromatids), segregating the DNA into two sets of strands (each set containing one of each homologue), and dividing both haploid, duplicated cells to produce four haploid, unduplicated cells. Meiosis I and II are both divided into prophase, metaphase, anaphase, and telophase subphases, similar in purpose to their analogous subphases in the mitotic cell cycle. Therefore, meiosis encompasses the interphase (G1, S, G2), meiosis I (prophase I, metaphase I, anaphase I, telophase I), and meiosis II (prophase II, metaphase II, anaphase II, telophase II). The G1 phase is a period in the cell cycle during interphase, after cytokinesis and before the S phase. ... DNA replication. ... Chromosome. ... G2 phase is a the 3rd and final subphase in interphase of the cell cycle. ... Homologous chromosomes are chromosomes in a biological cell that pair (synapse) during meiosis, or alternatively, non-identical chromosomes that contain information for the same biological features and contain the same genes at the same loci but possibly different genetic information, called alleles, at those genes. ... A chromatid forms one part of a chromosome after it has coalesced for the process of mitosis or meiosis. ... Newt lung cell in Prophase, with the mitotic spindles stained green and the cell nucleus and chromatin stained blue. ... An image of a newt lung cell stained with fluorescent dyes during metaphase. ... Newt lung cell during late anaphase. ... A cell during telophase that has almost completed cytokinesis. ...


Meiosis I

In meiosis I, chromosomes in a diploid cell disintegrates, producing two haploid cells (23, 1N). This is the step which generates genetic diversity.


Prophase I

Homologous chromosomes pair and form synapsis, a step unique to meiosis. The paired chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent. At this stage, non-sister chromatids may cross-over at points called chiasmata, which increase genetic randomness.


Leptotene

The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads."[1] During this stage, individual chromosomes begin to condense into long strands within the nucleus. However the two sister chromatids are still so tightly bound that they are indistinguishable from one another. The chromosomes in the leptotene stage show a specific arrangement where the telomeres are oriented towards the nuclear membrane. Hence this stage is called, "bouquet stage".


Zygotene

The zygotene stage, also known as zygonema, from Greek words meaning "paired threads,"[1] occurs as the chromosomes approximately line up with each other into homologous chromosomes. The combined homologous chromosomes are said to be bivalent. They may also be referred to as a tetrad, a reference to the four sister chromatids. The two homologous chromosomes become "zipped" together, forming the synaptonemal complex, in a process known as synapsis. A bivalent is sometimes referred to as a tetrad. ... Synaptonemal complex: A protein structure that forms between two homologous chromosomes during meiosis and that is thought to mediate chromosome pairing, synapsis, and recombination. ... It has been suggested that this article or section be merged into meiosis. ...


Pachytene

The pachytene stage, also known as pachynema, from Greek words meaning "thick threads,"[1] contains the following chromosomal crossover. Nonsister chromatids of homologous chromosomes randomly exchange segments of genetic information over regions of homology. (Sex chromosomes, however, are not identical, and only exchange information over a small region of homology.) Exchange takes place at sites where recombination nodules or chiasmata (chiasma,sigular) have formed. The exchange of information between the non-sister chromatids results in a recombination of information; each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the process. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope. Thomas Hunt Morgans illustration of crossing over (1916) Homologous Recombination is the process by which two chromosomes, paired up during prophase I of meiosis, exchange some distal portion of their DNA. Crossover occurs when two chromosomes, normally two homologous instances of the same chromosome, break and then reconnect but... A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. ...


Diplotene

During the diplotene stage, also known as diplonema, from Greek words meaning "two threads,"[1] the synaptonemal complex degrades and homologous chromosomes separate from one another a little. The chromosomes themselves uncoil a bit, allowing some transcription of DNA. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing over occurred. Synaptonemal complex: A protein structure that forms between two homologous chromosomes during meiosis and that is thought to mediate chromosome pairing, synapsis, and recombination. ... A micrograph of ongoing gene transcription of ribosomal RNA illustrating the growing primary transcripts. ... A chiasma (Greek χίασμα, related to the Greek letter Χ) means crossing. ...


In fetal oogenesis all developing oocytes develop to this stage and stop before birth. This suspended state is referred to as the dictyotene stage and remains so until puberty. In males, only spermatogonia exist until meiosis begins at puberty. Oogenesis or rarely oögenesis is the creation of an ovum (egg cell). ... Puberty refers to the process of physical changes by which a childs body becomes an adult body capable of reproduction. ... The introduction of this article does not provide enough context for readers unfamiliar with the subject. ...


Diakinesis

Chromosomes condense further during the diakinesis stage, from Greek words meaning "moving through."[1] This is the first point in meiosis where the four parts of the tetrads are actually visible. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. Other than this observation, the rest of the stage closely resembles prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form. In early prometaphase, the nuclear membrane has just degraded, allowing the microtubules to quickly interact with the kinetochores on the chromosomes, which have just condensed. ... In biology, the nucleolus is, strictly speaking, a suborganelle of the cell nucleus, which is an organelle. ... The nuclear envelope refers to the double membrane of the nucleus that encloses genetic material in eukaryotic cells. ...


Synchronous processes

During these stages, centrosomes, each containing a pair of centrioles are migrating to the two poles of the cell. These centrosomes, which were duplicated during S-phase, function as microtubule organizing centers nucleating microtubules, essentially cellular ropes and poles, during crossing over. They invade the nuclear membrane after it disintegrates, attaching to the chromosomes at the kinetochore. The kinetochore functions as a motor, pulling the chromosome along the attached microtubule toward the originating centriole, like a train on a track. There are four kinetochores on each tetrad, but the pair of kinetochores on each sister chromatid fuses and functions as a unit during meiosis I. [2][3] Microtubules are one of the components of the cytoskeleton. ... The kinetochore is the protein structure in eukaryotes which assembles on the centromere and links the chromosome to microtubule polymers from the mitotic spindle during mitosis. ...


Microtubules that attach to the kinetochores are known as kinetochore microtubules. Other microtubules will interact with microtubules from the opposite centriole. These are also nonkinetochore microtubules.


Meiosis-phases

Metaphase I

Homologous pairs move together along the phase plate: as kinetochore microtubules from both centrioles attach to their respective kinetochores, the homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along the metaphase poop.


Anaphase I

Kinetochore microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart. Since each chromosome only has one functional unit of a pair of kinetochores[3], whole chromosomes are pulled toward opposing poles, forming two haploid sets. Each chromosome still contains a pair of sister chromatids. Nonkinetochore microtubules lengthen, pushing the centrioles further a part. The cell elongates in preparation for division down the middle.


Telophase I

The last mitosis division effectively ends when the centromeres arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. This effect produces a variety of responses from the neuro-synchromatic enzyme, also known as NSE. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells.


Cells enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage.


telophase I contains no nucleus, two daughter cells, and chromosomes remain attached.


Meiosis II

Meiosis II is the second part of the meiotic process. Much of the process is similar to mitosis and meiosis I. End result is production of four haploid cells (23,1N) from the two haploid cells (23,1N) produced in meiosis I.


Prophase II takes an inversely proportional time compared to telophase I. In this prophase we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrioles move to the polar regions and arrange spindle fibers for the second meiotic division. This article is about proportionality, the mathematical relation. ... The nuclear envelope (also known as the perinuclear envelope, nuclear membrane, nucleolemma or karyotheca) is the double membrane of the nucleus that encloses genetic material in eukaryotic cells. ...


In metaphase II, the centromeres contain two kinetochores, that attach to spindle fibers from the centrosomes (centrioles) at each pole. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate.


This is followed by anaphase II, where the centromeres are cleaved, allowing microtubules attached to the kinetochores to pull the sister chromatids apart. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles.


The process ends with telophase II, which is similar to telophase I, and is marked by uncoiling and lengthening of the chromosomes and the disappearance of the microtubules. Nuclear envelopes reform and cleavage or cell wall formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes. Meiosis is now complete.


Significance of meiosis

Meiosis facilitates stable sexual reproduction. Without the halving of ploidy, or chromosome count, fertilization would result in zygotes that have twice the number of chromosomes than the zygotes from the previous generation. Successive generations would have an exponential increase in chromosome count, resulting in an unwieldy genome that would cripple the reproductive fitness of the species. Polyploidy, the state of having three or more sets of chromosomes, also results in developmental abnormalities or lethality [4]. Polyploidy is poorly tolerated in animal species. Plants, however, regularly produce fertile, viable polyploids. Polyploidy has been implicated as an important mechanism in plant speciation. Ploidy is the number of homologous sets of chromosomes in a biological cell. ... Polyploidy refers to cells or organisms that contain more than two copies of each of their chromosomes. ...


Most importantly, however, meiosis produces genetic variety in gametes that propagate to offspring. Recombination and independent assortment allow for a greater diversity of genotypes in the population. As a system of creating diversity, meiosis allows a species to maintain stability under environmental changes.


Nondisjunction

The normal separation of chromosomes in Meiosis I or sister chromatids in meiosis II is termed disjunction. When the separation is not normal, it is called nondisjunction. This results in the production of gametes which have either more or less of the usual amount of genetic material, and is a common mechanism for trisomy or monosomy. Nondisjunction can occur in the meiosis I or meiosis II, phases of cellular reproduction, or during mitosis. A trisomy means the presence of three (instead of the normal two) chromosomes of a particular numbered type in an organism. ... Aneuploidy is a chromosomal state where abnormal numbers of specific chromosomes or chromosome sets exist within the nucleus. ... Mitosis divides genetic information during cell division. ...


This is a cause of several medical conditions in humans:

Down syndrome or trisomy 21 (usually Downs Syndrome in British English[1]) is a genetic disorder caused by the presence of all or part of an extra 21st chromosome. ... Patau syndrome, also known as trisomy 13, is a chromosomal abnormality, a syndrome in which a patient has an additional chromosome 13 due to a non-disjunction of chromosomes during meiosis. ... Trisomy 18 or Edwards Syndrome, is the second most common trisomy after Downs Syndrome. ... XXY karyotype Klinefelters syndrome is a condition caused by a chromosome abnormality in males (specifically, a nondisjunction); sufferers have a pair of X sex chromosomes instead of just one. ... Turner syndrome or Ullrich-Turner syndrome encompasses several chromosomal abnormalities, of which monosomy X is the most common. ... Not to be confused with XXY syndrome. ...

Meiosis in humans

In females, meiosis occurs in precursor cells known as oogonia. Each oogonia that initiates meiosis will divide twice to form a single oocyte and three polar bodies. However, before these divisions occur, these cells stop at the diplotene stage of meiosis I and lay dormant within a protective shell of somatic cells called the follicle. Follicles begin growth at a steady pace in a process known as folliculogenesis, and a small number enter the menstrual cycle. Menstruated oocytes continue meiosis I and arrest at meiosis II until fertilization. The process of meiosis in females occurs during oogenesis, and differs from the typical meiosis in that it features a long period of meiotic arrest known as the Dictyate stage and lacks the assistance of centrosomes. A cell that arises from a primordial germ cell and differentiates into an oocyte in the ovary. ... An oocyte or ovocyte is a female gametocyte or germ cell involved in reproduction. ... Polar body is a cell structure found inside an ovum. ... Ovarian follicles or Graafian follicles (after Regnier de Graaf) are the roughly spherical cell aggregations in the ovary containing an ovum and from which the egg is released during ovulation. ... In biology, folliculogenesis refers to the maturation of the ovarian follicle, a densely-packed shell of somatic cells that contains an immature oocyte. ... Menstrual cycle In the female reproductive system, the menstrual cycle is a recurring cycle of physiologic changes that occurs in reproductive age females of several mammals, including human beings and other apes. ... Oogenesis or rarely oögenesis is the creation of an ovum (egg cell). ... The Dictyate stage of meiotic prophase is the prolonged resting phase that is terminated shortly before ovulation. ... The centrosome is the main microtubule organizing center (MTOC) of the cell as well as a regulator of cell-cycle progression. ...


In males, meiosis occurs in precursor cells known as spermatogonia that divide twice to become sperm. These cells continuously divide without arrest in the seminiferous tubules of the testicles. Sperm is produced at a steady pace. The process of meiosis in males occurs during spermatogenesis. This article needs to be wikified. ... Human male anatomy The testicles, known medically as testes (singular testis), are the male generative glands in animals. ... Cross section of the epithelium of a seminiferous tubule showing various stages of spermatocyte development Spermatogenesis is the process by which male spermatogonia develop into mature spermatozoa. ...


References

  1. ^ a b c d e Principles of Genetics, Fourth Edition, John Wiley and Sons, Inc., 2006.
  2. ^ Raven, Peter H.; Johnson, George B.; Mason, Kenneth A.; Losos, Jonathan & Singer, Susan. Biology, Eighth Edition, McGraw-Hill, 2007.
  3. ^ a b Petronczki, Mark; Siomos, Maria F. & Nasmyth, Kim (2003-02-21). "Un Ménage à Quatre The Molecular Biology of Chromosome Segregation in Meiosis", Cell 112 (4): 423-40. doi:10.1016/S0092-8674(03)00083-7.
  4. ^ BIL 104 - Lecture 15

See also

Mitosis divides genetic information during cell division. ... Ploidy is the number of homologous sets of chromosomes in a biological cell. ... Cross section of the epithelium of a seminiferous tubule showing various stages of spermatocyte development Spermatogenesis is the process by which male spermatogonia develop into mature spermatozoa. ... Oogenesis or rarely oögenesis is the creation of an ovum (egg cell). ... Multigene families occur when several similar genes exist in multiple copies in the same region of DNA. They are very similar and may differ by a few nucleotides. ... An allele (pronounced , ) (from the Greek αλληλος, meaning each other) is one member of a pair or series of different forms of a gene. ...

External links


  Results from FactBites:
 
Meiosis (398 words)
Meiosis reduces the number of chromosomes in the cells.
Many of the mechanics of the processes of mitosis and meiosis are similar, but the outcomes are very different.
To reduce the number of chromosomes, Meiosis uses two separate cell divisions (cytokinesis), in contrast to the single division in mitosis.
Meiosis (1102 words)
Meiosis is a special type of cell division that produces gametes with half as many chromosomes.
It is because of the way in which genes recombine in meiosis and syngamy that we have the whole study of genetics.
These two division processes are referred to as Meiosis I and Meiosis II, and result in a total of four daughter cells, each with a 1n chromosome number.
  More results at FactBites »

 
 

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