Multiple issues wikify December 2010 orphan January 2010 cleanup December 2009 Haploid relative risk HRR is a family based method for determining gene allele association to a disease. Nuclear families with one affected child are sampled using the parental haplotypes not transmitted as a control. While similar to genotype relative risk RR , HRR provides a solution to the problem of population stratification by only sampling within family trios. HRR method was first proposed by Rubinstein in 1981 then detailed in 1987 by Rubinstein and Falk ref Falk CT, Rubinstein P 1987 Haplotype relative risks and easy reliable way to construct a proper control sample for risk calculations . Ann Hum Genet 51 227 233 ref and is an important tool in genetic association studies. The original method proposed by Falk and Rubinstien fell under scrutiny in 1989, when Ott showed the equivalence of HRR to the classical RR method ref Woolf B 1955 On estimating the relation between blood group and disease . Ann Human Genet 19 251 253 ref demonstrating the HRR holds only when there is zero chance of recombination between disease locus and its markers. ref Ott J 1989 statistical properties of the haplotype relative risk. Genetic Epidemiol 6 127 130 ref This led to the adoption of the transmission disequilibrium test as the more commonly used method for association linkage Yet, when the recombination factor for a gene marker and its locus are 0 there is no tendency for false positives seen. ref Knapp,Seuchter,Baur 1987 The haplotype relative risk HRR method for analysis of association in nuclear families. Am J Hum Genet 52 1085 10093,1993 ref While HRR method has been shown as an effective means of avoiding population stratification another family based associations test known as transmission disequilibrium test ref a b c Spielman RS, McGinnis RE, Ewens WJ Mar 1993 . Transmission test for linkage disequilibrium ... 1085 10093,1993 ref References references DEFAULTSORT Haploid Relative Risk Category Genetics ... more details
and haploid mode of existence. The mating of yeast only occurs between haploids, which can be either ... haploid and diploid cells. Through a form of genetic recombination , haploid yeast can switch mating ... yeast can stably exist as either a diploid or a haploid. Both haploid and diploid yeast cells reproduce by mitosis , with daughter cells budding off of mother cells. Haploid cells are capable of mating with other haploid cells of the opposite mating type an a cell can only mate with an ... conditions such as nutrient depletion, can undergo meiosis to produce four haploid spore s two a spores and two spores. Differences between a and cells Image Yeast mating scheme.svg thumb Two haploid ... of haploid cells only to the mating pheromones of the opposite mating type allows mating between a and ... the cell to be an cell. Differences between haploid and diploid cells Haploid cells are one of two mating types a or , and respond to the mating pheromone produced by haploid cells of the opposite mating type, and can mate with cells of the opposite mating type. Haploid cells cannot undergo meiosis ... undergo meiosis to produce four haploid cells. Like the differences between haploid a and cells, different ... between haploid and diploid cells. In addition to the specific a and transcriptional patterns, haploid cells of both mating types share a haploid transcriptional pattern which activates haploid ... cells activate diploid specific genes and repress haploid specific genes. The different gene expression patterns of haploids and diploids are again due to the MAT locus. Haploid cells only contain ... the haploid transcriptional program. The alleles present at the MAT locus are Necessary and sufficient ... engineering genetic manipulations , a MAT a allele can be added to a MAT haploid cell. Despite having a haploid complement of chromosomes, the cell now has both the MAT a and MAT alleles, and will behave ... complement of chromosomes to behave like a haploid cell. Mating type switching Image Yeast mating reasoning.svg ... more details
. This way, the organism ends its diploid phase and produces several haploid cells. These cells divide mitotic ally to form either larger, multicellular individuals, or more haploid cells. Two opposite ... cycle, zygotes are the only diploid cell mitosis occurs only in the haploid phase. The individuals ... meiosis In gametic meiosis , instead of immediately dividing meiotically to produce haploid cells ... diploid cells. Cells from the diploid individuals then undergo meiosis to produce haploid cells or gamete s. Haploid cells may divide again by mitosis to form more haploid cells, as in many yeasts, but the haploid phase is not the predominant life cycle phase. In most diplonts, mitosis occurs .... In the whole cycle, gametes are usually the only haploid cells, and mitosis usually occurs only in the diploid ... producing haploid individuals called gametophyte s . Gametophytes now produce gametes via mitosis ..., gametes are usually the only haploid cells, and mitosis usually occurs only in the diploid phase. Haplodiplonts ... more details
Arrhenotoky or arrhenotokous parthenogenesis is a form of parthenogenesis in which unfertilized ovum egg s develop into haploid males. This form is observed in some marine invertebrate s, beetle s, scorpion s, mite s, bee s, etc. For some invertebrates it is a form of sex determination males are produced parthenogenetically, while diploid females are produced biparentally from fertilized eggs. In the case of the honeybee, the queen stores sperm and can lay both fertilized and unfertilized eggs. The unfertilized eggs developed into haploid Drone bee drones by arrhenotoky. See also thelytoky apomixis Haplodiploid sex determination system References cite journal author Roger I. C. Hansell coauthors Margaret M. Mollison William L. Putman title A cytological demonstration of arrhenotoky in three mites of the family Phytoseiidae journal Chromosoma publisher Springer Berlin Heidelberg issn 0009 5915 volume 15 issue 5 year 1964 doi 10.1007 BF00319990 pages 562 7 month Jan Category Reproduction ... more details
A megaspore mother cell is a Ploidy diploid cell in which meiosis will occur, resulting in the production of four megaspore s. ref Solomon, Eldra P., 2005, Biology , Thomson, Brooks Cole, United States ref This megasporcyte is formed from two distinct processes megasporogenesis and megagametogenesis. ref name Raven Raven, Peter H., 2005, Biology of Plants , W.H. Freeman and Company Publishers, United States ref Within each sporangium , meiosis of a megaspore mother cell produces four Diploidy Haploid and monoploid haploid megaspores. ref name Raven In gymnosperms and angiosperms , only one of these four megaspores is functional and the other three soon degenerate. The megaspore that remains divides Mitosis mitotically and develops into the female gametophyte, which eventually produces Egg biology eggs . ref name Raven References references Category Plant cells ... more details
Gametogenesis is a biology biological process by which diploid or haploid precursor cell s undergo cell division and differentiation to form mature haploid gamete s. Depending on the biological life cycle of the organism , gametogenesis occurs by meiosis meiotic division of diploid gametocyte s into various gametes, or by mitosis mitotic division of haploid gametogenous cells. For example, plants produce gametes through mitosis in gametophytes. The gametophytes grow from haploid spores after sporic meiosis. The existence of a multicellular, haploid phase in the life cycle between meiosis and gametogenesis is also referred to as alternation of generations . In animals Image Gray7.png thumb 400px Scheme showing analogies in the process of maturation of the ovum and the development of the spermatids, following their individual pathways. The oocytes and spermatocytes are both gametocytes. Ova and spermatids are complete gametes. In reality, the first polar body typically dies without dividing. Animals produce gametes directly through meiosis in organs called gonad s. Male s and female s of a species that sexual reproduction reproduces sexually have different forms of gametogenesis spermatogenesis male oogenesis female Stages However, before turning into gametogonia, the embryonic development of gametes is the same in males and females. Common path Gametogonia are usually seen as the initial stage of gametogenesis. However, gametogonia are themselves successors of primordial germ cells . During early embryonic development, primordial germ cells PGCs from the dorsal endoderm of the yolk sac migrate along the hindgut to the gonadal ridge . They multiply by mitosis and once they have reached the gonadal ridge in the late embryonic stage, they are called gametogonia. Gametogonia ... haploid 23 2N gametidogenesis meiosis 2 gametid haploid 23 1N gamete haploid 23 1N In gametangia Fungi, algae and primitive plants form specialized haploid structures called gametangium gametangia ... more details
A zygospore is a diploid reproductive stage in the life cycle of many fungi and protist s. Zygospores are created by the nuclear fusion of haploid cells. In fungi, zygospores are termed chlamydospore s and are formed after the fusion of hyphae of different mating types. In many eukaryotic algae , including many species of the Chlorophyta , zygospores are formed by the fusion of unicellular gametes of different mating types. A zygospore remains dormant while it waits for environmental cues, such as light, moisture, heat, or chemicals secreted by plant s. When the environment is favorable, the zygospore germinates, meiosis occurs, and haploid vegetative cells are released. In fungi, a sporangium is produced at the end of a sporangiophore that sheds spore s. A fungus that forms zygospores is called a zygomycete , indicating that the class biology class is characterized by this evolutionary development. External links http www.emc.maricopa.edu faculty farabee BIOBK BioBookglossWXYZ.html Definition of zygospore http www.mushroomthejournal.com greatlakesdata Terms zygos581.html A more detailed description of a zygospore Fungus structure Category Fungal morphology and anatomy Category Mycology eo Zigosporo nl Zygospore pl Zygospora pt Zig sporo ... more details
File Gametophyte2.png thumb Several gametophytes growing in a terrarium . Image Physcomitrella Protonema.jpg thumb Protonema of the moss Physcomitrella patens A gametophyte is the haploid , multicellular phase of plant s and algae that undergo alternation of generations , with each of its cells containing only a single set of chromosomes . The gametophyte produces male or female gamete s or both , by a process of cell division called mitosis . Sexual reproduction of plants The fusion of male and female gametes produces a diploid zygote , which develops by repeated mitotic cell divisions into a multicellular sporophyte . Because the sporophyte is the product of the fusion of two haploid gametes , its cells are diploid , containing two sets of chromosomes. The mature sporophyte produces spore s by a process called meiosis , sometimes referred to as reduction division because the chromosome pairs are separated once again to form single sets. The spores are therefore once again haploid and develop into a haploid gametophyte. In moss es, Marchantiophyta liverworts and Anthocerotophyta hornworts bryophytes , the gametophyte is the commonly known phase of the plant. An early developmental stage in the gametophyte of mosses immediately following germination of the meiospore is called the protonema . The adult gametophyte of mosses is called the gametophore as it carries the gamete producing sex organs. ref Ralf Reski 1998 Development, genetics and molecular biology of mosses. In Botanica Acta 111, pp 1 15. ref In most other embryophyte land plants , the gametophyte is very small. In fern s the gametophyte is a free living organism called the prothallus , in contrast to angiosperms. ref C.Michael Hogan. 2010. http www.eoearth.org article Fern Fern . Encyclopedia of Earth. National council for Science and the Environment . Washington, DC ref In gymnosperm s and angiosperm s, the gametophyte are reduced to only a few cells in angiosperms the female gametophyte sometimes called ... more details
HRR may stand for The History of Rock and Roll , a radio documentary produced by Bill Drake . Henley Royal Regatta , a rowing event held in Henley on Thames annually. Heiliges R misches Reich , German name of the Holy Roman Empire Heart rate Heart rate reserve Heart Rate Reserve Historicorum Romanorum reliquiae , a collection of ancient fragmentary Latin history works. HRR 98.5FM High Resolution Radar . Haploid relative risk , a family based method for determining gene allele association to a disease. Hospital Referral Region , a regional market area for tertiary medical care. the Hirzebruch Riemann Roch theorem disambig fa HRR it HRR ... more details
Unreferenced stub auto yes date December 2009 Image Gray1150.png thumb 350px right Cross section of the epithelium of a seminiferous tubule showing various stages of spermatocyte development Image Gray7.png thumb 350px right Scheme showing analogies in the process of maturation of the ovum and the development of the spermatids young spermatozoa . Spermatidogenesis is the creation of spermatid s from spermatocytes secondary spermatocytes during spermatogenesis . Secondary spermatocytes produced earlier rapidly enter meiosis meiosis II and divide to produce haploid spermatid s. The brevity of this stage means that secondary spermatocytes are rarely seen in histological preparations. Male reproductive system Category Male reproductive system Cell biology stub ru ... more details
For a non technical introduction to the topic Introduction to genetics A doubled haploid DH is a genotype formed when haploid cells undergo chromosome doubling. Artificial production of doubled haploids is important in plant breeding . Haploid cells are produced from pollen or Egg biology egg cells or from ... haploid cell is produced, which can be grown into a doubled haploid plant. If the original plant was diploid , the haploid cells are ploidy monoploid , and the term doubled monoploid may be used for the doubled haploids. Haploid organisms derived from polyploid tetraploids are sometimes called Ploidy ..., and R. E. Veilleux. 1996. In vitro haploid production in higher plants . Dordrecht Kluwer Academic ... programs that involve diploid wild relatives of the crops. History The first report of the haploid ... of haploids in the laboratory. Haploid production by wide crossing was reported in barley ... responsive species for doubled haploid production. Doubled haploid methodologies have now been applied ... of Doubled Haploids Doubled haploids can be produced in vivo or in vitro . Haploid embryos are produced in vivo by parthenogenesis , pseudogamy , or chromosome elimination after wide crossing. The haploid ... stages of seed development the H. bulbosum chromosomes are eliminated leaving a haploid embryo ..., the probability of obtaining this genotype is higher in haploid method than in diploid method. If n loci are segregating, the probability of getting the desirable genotype is 1 2 n by the haploid method and 1 4 n by the diploid method. Hence the efficiency of haploid method is obviously high ... of interest can be selected and converted into homozygous doubled haploid genotype. ref name ... in which DHs are readily available. Doubled haploid populations are ideal for genetic mapping ... et al., 2003 Genetic studies Genetic ratios and mutation rates can be read directly from haploid populations. A small doubled haploid DH population was used to demonstrate that a dwarfing gene ... more details
otheruses Tetrad disambiguation The tetrad is the four spore s of a yeast or Chlamydomonas , that separate after mating. If the two parents have a mutation in two different genes, the tetrad can segregate as the parental ditype PD , the non parental ditype NPD or as the tetratype TT . Image saccharomycescerevisiaetetrad.png frame Saccharomyces cerevisiae tetrad Parental ditype A tetrad type containing two different genotypes , both of which are parental. A spore arrangement in Ascomycetes that contains only the two non recombinant type ascospores . Non parental ditype A non parental ditype NPD is a spore arrangement in that contains only the two recombinant type ascospores assuming two segregating loci . A tetrad type containing two different genotypes, both of which are recombinant. Tetratype A Tetratype is a tetrad containing four different genotypes, two parental and two recombinant. A spore arrangement in Ascomycetes that consists of two parental and two recombinant spores indicates a single crossover between two linked loci. Linkage analysis The ratio between the different types arising after the sporulation is a measure of the genetic linkage linkage between the two genes. Tetrad Dissection Tetrad Dissection has become a powerful tool of yeast geneticists, and is used in conjunction with the many established procedures utilizing the versatility of yeasts as model organism s. Use of modern microscopy and micromanipulator micromanipulation techniques allows the four haploid spores of a yeast tetrad to be separated and germinated individually to form isolated spore colonies. Uses Tetrad analysis can be used to confirm whether a phenotype is caused by a specific mutation, construction of strains, and for investigating gene interaction. General Procedure Crosses are performed between haploid MATa and MAT mating strains, then the resulting diploids are transferred to sporulation media to form a tetrad containing four haploid spores. Tetrads can then be prepared wit ... more details
wiktionary tetrad Tetrad may refer to Meiosis Meiosis I Tetrad chromosomal pairing , Bivalents or Tetrad of homologous chromosomes consisting of four synapsed chromatids that become visible during the Pachytene stage of meiotic prophase Sporogenesis Formation of reproductive spores Tetrad pollen formation , group of four haploid and immature pollen grains in tetrahedral fashion produced by meiotic microsporogenesis Tetrad genetics , the four spores of a yeast Tetrad general relativity , a mathematical frame of reference in general relativity Tetractys or Tetrad , Greek philosophy Tetrad index notation , a concept in differential geometry Tetrad music , a set of four notes Tetrad unit of area , an area 2km x 2km square Tetromino , a shape composed of four squares Tetrad of media effects , a pedagogical tool disambig ... more details
Reference By anas sulemani , and got reference through the NWFP first year biology book, page number 159 Plasmogamy is a stage in the sexual reproduction of fungi . In this stage, the cytoplasm of two parent mycelia fuse together without the fusion of Nucleus cell nuclei , as occurs in higher terrestrial fungi. After plasmogamy occurs, the secondary mycelium forms. The secondary mycelium consists of dikaryotic cells, one nucleus from each of the parent mycelia. It is the fusion of protoplasm between two motile or non motile gametes. It involves the union of two protoplast bringing two haploid nuclei close together in the same cell... External links http www.biology online.org dictionary Plasmogamy Plasmogamy definition at Biology Online Category Mycology Fungus stub de Plasmogamie id Plasmogami it Plasmogamia mk nl Plasmogamie pt Plasmogamia ... more details
Onesource date April 2009 Image Gray9.png thumb 300px First stages of segmentation of a mammalian embryo. Semidiagrammatic. z.p. Zona striata . p.gl. Polar bodies. a. Two cell stage. b. Four cell stage. c. Eight cell stage. d, e. Morula stage. A polar body is a cell biology cell structure found inside an ovum . Both animal and plant ova possess it. It is also known as a polar cell. Asymmetrical cell division cytokinesis leads to the production of polar bodies during oogenesis . To conserve nutrients, the majority of cytoplasm is segregated into the secondary oocyte during meiosis I, and the ovum during meiosis II. The remaining daughter cells generated from the meiotic events are small and contain relatively little cytoplasm and are referred to as polar bodies. Eventually, the polar bodies degenerate. There may be one or two polar bodies in the ovum. The first polar body is one of the two products in the first stage of meiosis and is considered haploid, with 23 chromosomes and 46 chromatids. The second polar body is also haploid, with 23 chromosomes and 23 chromatids. Sometimes the first polar body undergoes the second meiotic cell division. In flowering plant s, the pollen releases two male gametes sperm nuclei into the ovule upon fertilization . One fertilizes the haploid ovum and develops into a normal embryo, while the other fertilizes the two polar bodies within the center of the ovary, creating a structure called endosperm . Endosperm stores nutrients for initial growth of plant seed s. Cereal Cereal grain is an example of this, and is consumed for its nutritional value by many animals, including humans. Polar bodies are the by products of the egg s division during meiosis . As an egg matures, it goes through a two step division process, dividing once at the time when ovulation would occur and again at the time of fertilization. The three haploid polar bodies are the by products of this division, and are essentially discarded by the egg. By analyzing the polar ... more details
. The embryo sac, which is primarily haploid , originates from the diploid megaspore mother cell within the ovule. The megasporocyte undergoes a meiotic cell division, producing four haploid megaspores ... rounds of mitotic division, resulting in one large cell with eight haploid nuclei. Membranes then divide ... sac consists of the haploid egg cell positioned in the middle of two other haploid cells, called ... mother cells, or microsporocytes. Each microsporocyte undergoes meiosis, forming four haploid ... more details
Urediniospores are the thinner walled spore s of some fungi Rust fungus rusts and Smut fungus smuts , from which the basidium arises. Development Urediniospores develop in the uredium at the bottom of a leaf s surface. Morphology Urediniospores are usually consist of two Dikaryon dikaryote nuclei within one constricted cell resembling the number 8 without a septate septation . As the spores germinate, the Cell nucleus nuclei undergo karyogamy and thereafter meiosis , giving rise to four celled basidia with haploid basidiospore s. They don t have a thick capped telia as compared to teliospore s. Looks like a balloon pinched in the middle, like an 8 without septation See also Portal Fungi Chlamydospore Urediniomycetes Teliospore Ustilaginomycetes References C.J. Alexopolous, Charles W. Mims, M. Blackwell, Introductory Mycology, 4th ed. John Wiley and Sons, Hoboken NJ, 2004 ISBN 0 471 52229 5 Category Germ cells Category Fungal morphology and anatomy Category Mycology Basidiomycota stub ... more details
A macrocyst is an aggregate of cells of Dictyosteliida formed during sexual reproduction enclosed in a cellulose wall ref name Brock cite book author Madigan M, Martinko J editors title Brock Biology of Microorganisms edition 11th ed. publisher Prentice Hall year 2006 isbn 0 13 144329 1 ref rp 468 . If two amoebae of different mating types are present in a dark and wet environment, they can fuse during aggregation to form a giant cell. The giant cell will then engulf the other cells in the aggregate and encase the whole aggregate in a thick, cellulose wall to protect it. This is known as a macrocyst. Inside the macrocyst, the giant cell divides first through meiosis, then through mitosis to produce many haploid amoebae that will be released to feed as normal amoebae would. See also Dictyostelium discoideum Life cycle and reproduction Life cycle and reproduction of Dictyostelium discoideum References references Category Mycetozoa Category Slime molds ... more details
. the outcome of meiosis is four genetically unique haploid cells, compared with the two genetically ... four haploid cells containing one copy of each chromosome. Each of the resulting chromosomes ... the cell divides into two daughter cells, each containing a haploid number of chromosomes composed ... of spores haploid cells that can divide vegetatively without undergoing fertilization. Some eukaryotes ... to transform one diploid cell into four haploid cells if the number of chromosomes had to be maintained ... organisms, there is an intermediary step between the diploid and haploid transition where ... alternating haploid and diploid states. The organism phase of the life cycle can occur either during the diploid state gametic or diploid life cycle , during the haploid state zygotic or haploid life ..., one during the haploid state and the other during the diploid state . In this sense there are three ... line stem cells undergo meiosis to create haploid gametes the spermatozoa for males and ova for females ... the species is haploid instead, spawned by the proliferation and differentiation of a single haploid cell called the gamete . 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 ... organism alternates between haploid and diploid states. Consequently, this cycle is also known ... spores. The spores 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 ... , each made up of two sister chromatids, into two cells. One entire haploid content of chromosomes ... into haploid daughter cells. The two cells resulting from meiosis I divide during meiosis II, creating 4 haploid daughter cells. Meiosis I and II are each divided into prophase , metaphase , anaphase .... Meiosis I Meiosis I separates homologous chromosomes, producing two haploid cells N chromosomes ... more details
with a multicellular gametophyte , which is haploid with N unpaired chromosomes. A mature sporophyte .... However, an animal directly produces haploid gametes by meiosis. No haploid spores capable of dividing are produced, so neither is a haploid gametophyte. There is no alternation between diploid and haploid forms. Other organisms, such as fungi, can have life cycles in which different .... Citation needed date January 2011 Life cycles, such as those of plants, with alternating haploid ... a diploid phase are referred to as diplontic . Life cycles in which there is only a haploid phase are referred ... of a multicellular diploid form with a multicellular haploid form as the defining characteristic ... Kerp Hass 2005 ref In some species, such as the alga Ulva lactuca , the diploid and haploid forms ... into a haploid gametophyte. However, in other species, either the sporophyte or the gametophyte ... Alternation of generations.svg 360px right Alternation of generations Two single celled haploid ... celled haploid spores, each containing N unpaired chromosomes. The single celled haploid spore germinates ... is a multi cellular haploid organism, called the gametophyte because at maturity it produces ... which are the organs which produce haploid gametes. At least one kind of gamete possesses some ... cycle is thus between a diploid 2N generation of sporophytes and a haploid N generation of gametophytes ... process is that a diploid 2N individual directly produces haploid N gametes by meiosis. Spores i.e. haploid cells which are able to undergo mitosis are not produced, so neither is a haploid multi cellular organism. The single celled gametes are the only entities which are haploid. Variations ... sporophytes and haploid gametophytes which are externally indistinguishable. ref name Shyam 1980 ..., the dominant form is the haploid gametophyte. The diploid sporophyte is not capable of an independent .... The haploid gametophyte is much smaller and simpler in structure. In seed plants, the gametophyte ... more details
Infobox Anatomy Name Spermatogonium Latin GraySubject 258 GrayPage 1243 Image Germinal epithelium testicle.svg Caption Germinal epithelium of the testicle. 1 basal lamina , 2 spermatogonia , 3 spermatocyte 1st order, 4 spermatocyte 2nd order, 5 spermatid , 6 mature spermatid, 7 Sertoli cell , 8 tight junction blood testis barrier Image2 Testicle histology boar.jpg Caption2 Histological section through testicular parenchyma of a boar . 1 Lumen of Tubulus seminiferus contortus , 2 spermatids , 3 spermatocytes , 4 spermatogonia, 5 Sertoli cell , 6 Myofibroblasts , 7 Leydig cell s, 8 capillaries Width 325 System MeshName Spermatogonia MeshNumber A05.360.490.890.900 A spermatogonium plural spermatogonia is an intermediary male gametogonium ref cite web url http www.biology online.org dictionary Spermatogonium title Spermatogonium definition work Biology Online.org accessdate 21 December 2011 ref a kind of germ cell in the production of spermatozoa . There are three subtypes Type A d cells, with dark nuclei. These cells replicate to ensure a constant supply of spermatogonia to fuel spermatogenesis. Type A p cells, with pale nuclei. These cells divide by mitosis to produce Type B cells. Type B cells, which divide to give rise to primary spermatocytes . Each primary spermatocyte duplicates its DNA and subsequently undergoes meiosis I to produce two haploid secondary spermatocytes. Each of the two secondary spermatocytes further undergo meiosis II to produce two spermatids haploid . 1 primary spermatocyte 4 spermatids The spermatids then undergo spermiogenesis to produce spermatozoa . Additional images gallery Image Gray1150.png Transverse section of a tubule of the testis of a rat . X 250. Image Spermatocytogenesis.png Schematic diagram of Spermatocytogenesis gallery References Reflist Category Germ cells Male reproductive system Cell biology stub ca Espermatogoni de Spermatogonium es Espermatogonia fr Spermatogonie it Spermatogonio pt Espermatog nia ru ... more details
italic title Taxobox name Micrasterias image Micrasterias radiata.jpg image caption Micrasterias furcata name Micrasterias regnum Plantae divisio Charophyta classis Zygnemophyceae ordo Desmidiales familia Desmidiaceae genus Micrasterias genus authority Carl Adolph Agardh C.Agardh ex John Ralfs Ralfs subdivision ranks Species subdivision Micrasterias crux melitensis br Micrasterias fimbriata br Micrasterias rotata Micrasterias is a Cell biology unicellular green alga of the Desmid Desmidiales order. Micrasterias, commonly known as green alga, is a Eukaryotic, unicellular, and 0.35mm in size. Micrasterias displays a bilateral symmetry, with two mirror image semi cells joined by a narrow isthmus containing the nucleus of the organism. This dual semi cell structure is unique to the group of green algae to which Micrasterias belongs. Each semi cell contains a single large chloroplast, the site of photosynthesis for the Micrsterias. Chloroplasts within Micrasterias contain chlorophylls A and B and the enzymes required for photosynthesis. The sugar created is used to provide energy for the organism or, if not used, taken up by many small round pyrenoids which are embedded in the chloroplast. They convert the sugar to a starch for storage. Micrasterias can produce both asexually and sexually. Asexual reproduction occurs via mitosis. When this occurs the genetic material of Micrasterias is duplicated and two small semi cells grow between the original semi cells, gradually increasing in size. Sexual reproduction occurs through a process called conjugation whereby two organisms come together and fuse their haploid cells to form a diploid zygote. This zygote typically forms a thick protective wall which can allow the organism to remain dormant for many months to survive cold winters and long droughts. When good conditions resume, the zygospore will germinate, undergo meiosis, and produce new haploid algal cells. External links http protist.i.hosei.ac.jp PDB Images Chlorophyta M ... more details
Heterokaryosis from the Greek heteros , meaning other and karyon , meaning kernel is a term used in biology meaning to have two or more genetically different nuclei within the same mycelium of a fungus or other life form. This is a special type of syncytium . A heterokaryon is a cell with more than one nucleus of differing genetic origin. The term was first used for ciliate protozoa ns such as Tetrahymena . This has two types of cell nuclei, a large, somatic macronucleus and a small, germline micronucleus . Both exist in a single cell at the same time and carry out different functions with distinct cytological and biochemical properties. Many fungi notably the mycorrhiza arbuscular mycorrhizal fungi exhibit heterokaryosis. The haploid nuclei within a mycelium may differ from one another not merely by accumulating mutation s, but by the non sexual fusion of genetically distinct fungal hyphae . This can lead to individuals that have different nuclei in different parts of their mycelium. In this case, the notion of individual itself becomes vague since the rule of one genome one individual does not apply any more. ref King R.C., Stansfield W.D. & Mulligan P.K. 2006. A dictionary of genetics . 7th ed, Oxford. p204 ref Heterokaryosis is most common in fungi, and lichen , but also occurs in slime mold s. This happens because the nuclei in the plasmodium form are the products of many pairwise fusions between amoeboid haploid individuals. References Reflist Category Mycology Category Cell biology ru ... more details
File Crosiere of arcbishop Heinrich of Finstingen.jpg thumb Christian crosier 1260 1286 of the form that is the name sake of the fungal structure. A crozier is an anatomical feature of many fungi in the phylum Ascomycota that form at the base of ascus asci and look like hook topped shepherd s staffs or stylized religious crosier s. The term is also used in describing the unfurled frond of certain fern species. ref C.Michael Hogan. 2010. http www.eoearth.org article Fern Fern . Encyclopedia of Earth. National council for Science and the Environment . Washington, DC. ref During the ascus initial formation the crozier helps to maintain a dikaryotic state in the ascus initial and its side branch that will continue the spreading growth of the ascogenous hyphae in Ascomycota fruitbodies. The tips of developing asci on these ascogenous hyphae curl over. One haploid nucleus migrates into the curved tip while the other compatible haploid nucleus remains in the penultimate space below the hook. The ascus initial itself forms as a radiating spur branch at the top of the hook. Each Cell nucleus nucleus divides resulting in the formation of a pair of compatible nuclei, i.e. a dikaryon , in the ascus. Two sister nuclei remain, one in the basal cell and the other in the crozier. The tip of the crozier fuses with the penultimate cell while walling itself off from the ascus by the formation of a septum . The nucleus from the crozier migrates into the penultimate cell joining the other nucleus, thus maintaining a dikaryotic state. These nuclei migrate into a side branch growing from the base of the ascus that repeats the ascus crozier formation innumerable times. Mature croziers are detectable through microscopic examination of mature asci as small curved bridges at the basal septa. A minority of Ascomycota lack crosiers, hence the presence or absence of croziers is an important taxonomic character. Croziers resemble and function similarly to clamp connection s on the dikaryotic hyph ... more details
. The recombined haploid nuclei appear among vegetative cells, which differ genetically from those of the parent ... reproduction. Stages Diploidization Occasionally, two haploid nuclei fuse to form a diploid nucleus ... be very stable and divide to form further diploid nuclei, along with the normal haploid nuclei. Thus the heterokaryon consists of a mixture of the two original haploid nuclear types as well as diploid ... few 2n 1 . Such nuclei with incomplete multiples of the haploid number are termed aneuploid , as they do ... would be similar. Once a diploid nucleus has formed by fusion of two haploid nuclei from different ... more details
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