In biology or life science, meiosis (pronounced my-oh-sis or mee-oh-sis) is a process of reduction division in which the number of chromosomes per cell is cut in half. Foundations of modern biology There are five unifying principles In animals, meiosis always results in the formation of gametes. A gamete (from Ancient Greek γαμέτης; translated gamete = wife gametes = husband is a cell that fuses with another gamete The word "meiosis" comes from the Greek verb meioun, meaning "to make small," since it results in a reduction in chromosome number in the gamete cell.
Meiosis is essential for sexual reproduction and therefore occurs in all eukaryotes (including single-celled organisms) that reproduce sexually. The Evolution of sexual reproduction is a major puzzle The first Fossilized evidence of sexually reproducing Organisms is from Eukaryotes of the Stenian Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex A few eukaryotes, notably the Bdelloid rotifers, have lost the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis. The rotifers make up a Phylum of microscopic and near-microscopic pseudocoelomate Animals They were first described by Rev Parthenogenesis (from the Greek παρθένος parthenos, "virgin" + γένεσις genesis, "creation" is an asexual form Meiosis does not occur in archaea or bacteria, which reproduce via asexual processes such as mitosis or binary fission. The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei Binary fission is the form of Asexual reproduction and Cell division used by Prokaryotic organisms (such as Bacteria or Archea) Each cell has half the number of chromosomes as the parent cell.
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. In classical genetics the genome of a Diploid Organism including Eukarya refers to a full set of chromosomes or genes in a Gamete, thereby "Haplo" redirects here For the fictional character see The Death Gate Cycle. Germ cells are progenitors of the Gametes. These singled out cells move through the gut to the developing Gonads and undergo mitotic proliferation followed Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known A chromosome is an organized structure of DNA and Protein that is found in cells. "Haplo" redirects here For the fictional character see The Death Gate Cycle. Each of these cells contain one complete set of chromosomes, or half of the genetic content of the original cell. A chromosome is an organized structure of DNA and Protein that is found in cells. If meiosis produces gametes, these cells must fuse during fertilization to create a new diploid cell, or zygote before any new growth can occur. For soil improvement see Fertilization (soil. For other meanings see Zygote (disambiguation. A zygote (from Greek ζυγωτός zugōtos "joined" or "yoked" 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. Genetic recombination is the process by which a strand of genetic material (usually DNA; but can also be RNA) is broken and then joined to a different DNA molecule Together, meiosis and fertilization constitute sexuality in the eukaryotes, and generate genetically distinct individuals in populations.
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. Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei There are several features unique to meiosis, most importantly the pairing and genetic recombination between homologous chromosomes. Homologous chromosomes are Chromosomes in a Biological cell that pair ( synapse) during Meiosis, or alternatively non-identical chromosomes that
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). Sea urchins are small globular spiny sea cat animals composing most of class Echinoidea. In most Birds and Reptiles an egg ( Latin ovum) is the Zygote, resulting from Fertilization of the Ovum. Year 1876 ( MDCCCLXXVI) was a Leap year starting on Saturday (link will display the full calendar of the Gregorian Calendar (or a Leap year Oscar Hertwig ( April 21, 1849, Friedberg, Hesse - October 25, 1922, Berlin) was a German Zoologist It was described again in 1883, at the level of chromosomes, by Belgian zoologist Edouard Van Beneden (1846-1910), in Ascaris worms' eggs. Year 1883 ( MDCCCLXXXIII) was a Common year starting on Monday (link will display the full calendar of the Gregorian calendar (or a Common The Kingdom of Belgium is a Country in northwest Europe. It is a founding member of the European Union and hosts its headquarters as well as those Edouard Van Beneden ( 5 March, 1846, Leuven – 28 April, 1910, Liège) son of Pierre-Joseph Van Beneden, was Ascaris is a genus of parasitic Nematode worms One species A suum, typically infects Pigs, while another 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. Year 1890 ( MDCCCXC) was a Common year starting on Wednesday (link will display the full calendar of the Gregorian calendar (or a Common Germany, officially the Federal Republic of Germany ( ˈbʊndəsʁepuˌbliːk ˈdɔʏtʃlant is a Country in Central Europe. Friedrich Leopold August Weismann (Birth January 17, 1834 in Frankfurt am Main; Death In 1911 the American geneticist Thomas Hunt Morgan (1866-1945) observed crossover in Drosophila melanogaster meiosis and provided the first true genetics. Year 1911 ( MCMXI) was a Common year starting on Sunday (link will display the full calendar of the Gregorian calendar (or a Common year The United States of America —commonly referred to as the Thomas Hunt Morgan ( September 25, 1866 &ndash December 4, 1945) was an American geneticist and embryologist. Drosophila melanogaster (from the Greek for black-bellied dew-lover) is a two-winged insect that belongs to the Diptera, the order
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. Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex The excavates are a major assemblage of Protists often known as 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. The opisthokonts ( Greek: (opisthō- = "rear posterior" + (kontos = "pole" i The Amoebozoa are a major group of Amoeboid protozoa including the majority that move by means of internal Cytoplasmic flow The Rhizaria are a species-rich supergroup of Protists They vary considerably in form but for the most part they are Amoeboids with filose reticulose or microtubule-supported The Archaeplastida or Primoplantae are a major line of Eukaryotes comprising the land plants green and Red algae and a small Chromalveolata is a Eukaryote supergroup first proposed by Thomas Cavalier-Smith as a refinement of his kingdom Chromista, which was first Some excavata species do have meiosis which is consistent with the hypothesis that excavata is an ancient, paraphyletic grade. The excavates are a major assemblage of Protists often known as Excavata. The excavates are a major assemblage of Protists often known as Excavata. In Phylogenetics, a group of organisms is said to be paraphyletic if the group contains its most recent common ancestor but does not contain all An example of eukaryotic organism in which meiosis does not exist is euglenoid. The euglenids (or euglenoids) are one of the best-known groups of Flagellates commonly found in freshwater especially when it is rich in organic materials with a
Occurrence of meiosis in eukaryotic life cycles
Meiosis occur in eukaryotic life cycles involving sexual reproduction, comprising of the constant cyclical process of meiosis and fertilization. A life cycle is a period involving 1 Generation of an Organism through means of Reproduction, whether through Asexual reproduction or Sexual The Evolution of sexual reproduction is a major puzzle The first Fossilized evidence of sexually reproducing Organisms is from Eukaryotes of the Stenian This takes place alongside normal mitotic cell division. Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei 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. Germ cells are progenitors of the Gametes. These singled out cells move through the gut to the developing Gonads and undergo mitotic proliferation followed The rest of the cells, called somatic cells, function within the organism and will die with it. Somatic cells are any cells forming the body of an organism as opposed to Germline cells Death is the termination of the biological functions that define living Organisms It refers both to a specific
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. For other meanings see Zygote (disambiguation. A zygote (from Greek ζυγωτός zugōtos "joined" or "yoked" In the zygotic life cycle the species is haploid instead, spawned by the proliferation and differentiation of a single haploid cell called the gamete. A gamete (from Ancient Greek γαμέτης; translated gamete = wife gametes = husband is a cell that fuses with another 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. A spermatozoon or spermatozoan ( pl spermatozoa) from the Ancient Greek σπέρμα (seed and ζῷον (living being and more commonly known These gametes then fertilize in the Fallopian tubes of the female, producing a diploid zygote. The Fallopian tubes, also known as oviducts, uterine tubes, and salpinges ( singular salpinx) are two very fine tubes lined with ciliated 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. The blastocyst is the structure formed in early Embryogenesis, after the formation of the Blastocoel, but before Implantation. The uterus (from the Latin word for womb) is the major Female reproductive organ of most Mammals including Humans One end the The endometrium is the inner membrane of the Mammalian Uterus. An embryo (from Greek:, plural, lit "that which grows" from en- "in" + bryein "to swell be full" is a multicellular
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 the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei 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.
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. Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei Many fungi and many protozoa are members of the zygotic life cycle. A fungus (ˈfʌŋgəs is a eukaryotic Organism that is a member of the kingdom Fungi (ˈfʌndʒaɪ Protozoa (in Greek πρῶτον proton "first" and ζῷα zoia "animals" are unicellular Eukaryotes (singular
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 Alternation of phases (or generations) describes the life cycle of Plants Fungi and Protists A multicellular Diploid phase alternates 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.
Process
Because meiosis is a "one-way" process, it cannot be said to engage in a cell cycle as mitosis does. The cell cycle, or cell-division cycle, is the series of events that take place in a eukaryotic cell leading to its replication However, the preparatory steps that lead up to meiosis are identical in pattern and name to the interphase of the mitotic cell cycle.
Interphase is divided into three phases:
- Growth 1 (G1) phase: Immediately follows cytokinesis. The G1 phase is a period in the Cell cycle during Interphase, after Cytokinesis and before the S phase. 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 S phase, short for synthesis phase, is a period in the Cell cycle during Interphase, between G1 phase and the G2 phase. The cell is still diploid, however, because it still contains the same number of centromeres. A centromere is a region of DNA typically found near the middle of a Chromosome where two Sister chromatids come in contact 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. G2 phase is the third final and usually the shortest subphase during interphase within the Cell cycle in which the cell undergoes a period of rapid growth to prepare 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. Homologous chromosomes are Chromosomes in a Biological cell that pair ( synapse) during Meiosis, or alternatively non-identical chromosomes that 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. A chromatid is one of two identical copies of DNA making up a replicated Chromosome, which are joined at their Centromeres for the process of Cell division 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. Prophase is a stage of Mitosis in which the Chromatin condenses into a highly ordered structure called a Chromosome Metaphase from the Ancient Greek μετά (after and φάσις (stage is a stage of Mitosis in the eukaryotic Cell cycle in which Anaphase, from the Ancient Greek ἀνά (up and φάσις (stage is the stage of Mitosis when Chromosomes Telophase (sometimes spelled telephase) from the Ancient Greek "τελος" (end and "φασις" (stage is a stage in either Meiosis 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).
Meiosis generates genetic diversity in two ways: (1) independent assortment of chromosomes at both of the meiotic divisions allows genetic differences among gametes; and (2) physical exchange of chromosomal regions by homologous recombination during prophase I results in new genetic combinations within chromosomes.
Meiosis I
In meiosis I, the homologous pairs in a diploid cell separate, producing two haploid cells (46, N). The 46 chromosomes number is significant. A regular diploid cell contains 46 chromosomes and is considered 2N because it contains 23 pairs of homologous chromosomes. However, after meiosis I, although the cell contains 46 chromosomes it is only considered N because later in anaphase I the identical sister chromatids will remain together as the spindle pulls the pair toward the pole of the new cell. In meiosis II, a process similar to mitosis will occur whereby the sister chromatids are finally split, creating 2 haploid cells (23, N).
Prophase I
Homologous chromosomes pair and crossing over, or recombination, occurs--a step unique to meiosis. Chromosomes form structures called synapsis. 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.
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. A bivalent is sometimes referred to as a tetrad During Meiosis, bivalents are a pair of associated Homologous Chromosomes formed after replication 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. The synaptonemal complex is a Protein structure that forms between two homologous chromosomes during Meiosis and that is thought to mediate chromosome pairing Synapsis (also called syndesis) is the pairing of two Homologous chromosomes that occurs during 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. A sex-determination system is a biological system that determines the development of sexual characteristics in an Organism. ) Exchange takes place at sites where recombination nodules or chiasmata (singular: chiasma) have formed. A chiasma (plural chiasmata) in Genetics, is thought to be the point where two homologous chromatids exchange genetic material during Chromosomal crossover 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.
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 synaptonemal complex is a Protein structure that forms between two homologous chromosomes during Meiosis and that is thought to mediate chromosome pairing The chromosomes themselves uncoil a bit, allowing some transcription of DNA. Transcription is the synthesis of RNA under the direction of DNA However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred. A chiasma (plural chiasmata) in Genetics, is thought to be the point where two homologous chromatids exchange genetic material during Chromosomal crossover The chiasmata remain on the chromosomes until they are severed in Anaphase I.
In fetal oogenesis all developing oocytes develop to this stage and stop before birth. Oogenesis or rarely oögenesis is the creation of an Ovum (egg cell 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. A spermatogonium (plural spermatogonia) is an intermediary male Gametogonium (a kind of Germ cell) in the production of Spermatozoa.
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. Prometaphase is the phase of Mitosis following Prophase and preceding Metaphase, in eukaryotic Somatic cells The nuclear The nucleolus (also called nucleole) is a structure found within the nucleus in which Ribosomal RNA is transcribed. The nuclear envelope (NE(also known as the perinuclear envelope, nuclear membrane, nucleolemma or karyotheca) is a double lipid bilayer that
Synchronous processes
During these stages, centrosomes, each containing a pair of centrioles are migrating to the two poles of the cell. In Cell biology, the centrosome is an Organelle that serves as the main Microtubule organizing center (MTOC of the animal cell as well A Centriole is a barrel shaped Organelle found in most animal Eukaryotic cells though absent in Higher plants and Fungi. These centrosomes, which were duplicated during S-phase, function as microtubule organizing centers nucleating microtubules, essentially cellular ropes and poles, during crossing over. Microtubules are one of the components of the Cytoskeleton. They have a diameter of 25 nm and length varying from 200 nanometers to 25 micrometers They invade the nuclear membrane after it disintegrates, attaching to the chromosomes at the kinetochore. The kinetochore (pronounced kin et' o core is the protein structure on Chromosomes where the Spindle fibers attach during division to pull the chromosomes apart 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 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 plate.
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 meiotic 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 chromatids 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 (46,1N) produced in meiosis I.
Prophase II takes an inversely proportional time compared to telophase I. This article is about proportionality the mathematical relation 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. The nuclear envelope (NE(also known as the perinuclear envelope, nuclear membrane, nucleolemma or karyotheca) is a double lipid bilayer that Centrioles move to the polar regions and arrange spindle fibers for the second meiotic division.
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. "Haplo" redirects here For the fictional character see The Death Gate Cycle. 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 occurs in cells and Organisms when there are more than two homologous sets of Chromosomes. 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.
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. A trisomy is a form of Aneuploidy with the presence of three copies instead of the normal two of a particular Chromosome. Monosomy is a form of Aneuploidy with the presence of only one Chromosome (instead of the typical two in humans from a pair Nondisjunction can occur in the meiosis I or meiosis II, phases of cellular reproduction, or during mitosis. Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei
This is a cause of several medical conditions in humans:
- Down Syndrome - trisomy of chromosome 21
- Patau Syndrome - trisomy of chromosome 13
- Edward Syndrome - trisomy of chromosome 18
- Klinefelter Syndrome - extra X chromosomes in males - ie XXY, XXXY, XXXXY
- Turner Syndrome - atypical X chromosome dosage in females - ie XO, XXX, XXXX
- XYY Syndrome - an extra Y chromosome in males
Meiosis in humans
In females, meiosis occurs in cells known as oogonia (singular: oogonium). Down syndrome, Down's syndrome, or trisomy 21 is a Chromosomal 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 Trisomy 18 or Edwards Syndrome is a Genetic disorder caused by the presence of all or part of an extra 18th chromosome. Klinefelter's syndrome, 47XXY or XXY syndrome is a condition caused by a Chromosome Aneuploidy. Turner syndrome or Ullrich-Turner syndrome encompasses several conditions of which monosomy X is the most common XYY syndrome is an Aneuploidy of the Sex chromosomes in which a Human Male receives an extra Y chromosome, producing a 47XYY An oogonium (plural oogonia) is an Immature ovum. It is a female Gametogonium. Each oogonium that initiates meiosis will divide twice to form a single oocyte and three polar bodies. An oocyte, ovocyte, or rarely ocyte, is a female Gametocyte or Germ cell involved in reproduction. A polar body is a cell structure found inside an Ovum. Both Animal and Plant ova possess it 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. Ovarian follicle is the basic unit of female reproductive biology and is composed of a roughly spherical aggregations of cells found in the Ovary. Follicles begin growth at a steady pace in a process known as folliculogenesis, and a small number enter the menstrual cycle. In Biology, folliculogenesis is the maturation of the Ovarian follicle, a densely-packed shell of Somatic cells that contains an immature Oocyte The menstrual cycle is a recurring cycle of physiologic changes that occurs in reproductive-age Females Overt menstruation (where there is blood flow from the 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. Oogenesis or rarely oögenesis is the creation of an Ovum (egg cell The dictyate or dictyotene is a prolonged resting phase in Oogenesis. In Cell biology, the centrosome is an Organelle that serves as the main Microtubule organizing center (MTOC of the animal cell as well
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. Seminiferous tubules are located in the Testicles and are the specific location of Meiosis, and the subsequent creation of gametes, namely Spermatozoa The testicle (from Latin testiculus, diminutive of testis, meaning "witness" virility plural testes) is the male Sperm is produced at a steady pace. The process of meiosis in males occurs during spermatogenesis. Spermatogenesis is the process by which male Spermatogonia develop into mature Spermatozoa.
References
- ^ a b c d e Principles of Genetics, Fourth Edition, John Wiley and Sons, Inc. , 2006.
- ^ Raven, Peter H. ; Johnson, George B. ; Mason, Kenneth A. ; Losos, Jonathan & Singer, Susan. Biology, Eighth Edition, McGraw-Hill, 2007.
- ^ 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.
- ^ BIL 104 - Lecture 15
See also
External links
- Meiosis Flash Animation
- Meiosis at Kimball's Biology Pages
- CCO The Cell-Cycle Ontology
Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic