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137-469: Meiosis ( / m aɪ ˈ oʊ s ɪ s / ; from Ancient Greek μείωσις ( meíōsis ) 'lessening', (since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes , the sperm or egg cells . It involves two rounds of division that ultimately result in four cells, each with only one copy of each chromosome ( haploid ). Additionally, prior to
274-430: A DNA repair process, and that when it occurs during meiosis it is an adaptation for repairing the genomic DNA that is passed on to progeny. Experimental findings indicate that a substantial benefit of meiosis is recombinational repair of DNA damage in the germline , as indicated by the following examples. Hydrogen peroxide is an agent that causes oxidative stress leading to oxidative DNA damage. Treatment of
411-537: A pitch accent . In Modern Greek, all vowels and consonants are short. Many vowels and diphthongs once pronounced distinctly are pronounced as /i/ ( iotacism ). Some of the stops and glides in diphthongs have become fricatives , and the pitch accent has changed to a stress accent . Many of the changes took place in the Koine Greek period. The writing system of Modern Greek, however, does not reflect all pronunciation changes. The examples below represent Attic Greek in
548-535: A ploidy series , featuring diploid ( X. tropicalis , 2n=20), tetraploid ( X. laevis , 4n=36), octaploid ( X. wittei , 8n=72), and dodecaploid ( X. ruwenzoriensis , 12n=108) species. Over evolutionary time scales in which chromosomal polymorphisms accumulate, these changes become less apparent by karyotype – for example, humans are generally regarded as diploid, but the 2R hypothesis has confirmed two rounds of whole genome duplication in early vertebrate ancestors. Ploidy can also vary between individuals of
685-417: A syncytium , though usually the ploidy of each nucleus is described individually. For example, a fungal dikaryon with two separate haploid nuclei is distinguished from a diploid cell in which the chromosomes share a nucleus and can be shuffled together. It is possible on rare occasions for ploidy to increase in the germline , which can result in polyploid offspring and ultimately polyploid species. This
822-675: A zygote with n pairs of chromosomes, i.e. 2 n chromosomes in total. The chromosomes in each pair, one of which comes from the sperm and one from the egg, are said to be homologous . Cells and organisms with pairs of homologous chromosomes are called diploid. For example, most animals are diploid and produce haploid gametes. During meiosis , sex cell precursors have their number of chromosomes halved by randomly "choosing" one member of each pair of chromosomes, resulting in haploid gametes. Because homologous chromosomes usually differ genetically, gametes usually differ genetically from one another. All plants and many fungi and algae switch between
959-490: A barrel shaped spindle. In human oocytes spindle microtubule nucleation begins on the chromosomes, forming an aster that eventually expands to surround the chromosomes. Chromosomes then slide along the microtubules towards the equator of the spindle, at which point the chromosome kinetochores form end-on attachments to microtubules. Homologous pairs move together along the metaphase plate: As kinetochore microtubules from both spindle poles attach to their respective kinetochores,
1096-467: A chromosome copy number of 1 to 4, and that number is commonly fractional, counting portions of the chromosome partly replicated at a given time. This is because under exponential growth conditions the cells are able to replicate their DNA faster than they can divide. In ciliates, the macronucleus is called ampliploid , because only part of the genome is amplified. Mixoploidy is the case where two cell lines, one diploid and one polyploid, coexist within
1233-463: A common intestinal parasite, was previously considered to have descended from a lineage that predated the emergence of meiosis and sex. However, G. intestinalis has now been found to possess a core set of meiotic genes, including five meiosis specific genes. Also evidence for meiotic recombination , indicative of sexual reproduction , was found in G. intestinalis . Another example of organisms previously thought to be asexual are parasitic protozoa of
1370-433: A diploid cell called the zygote . The organism's diploid germ-line stem cells undergo meiosis to make haploid gametes (the spermatozoa in males and ova in females), which fertilize to form the zygote. The diploid zygote undergoes repeated cellular division by mitosis to grow into the organism. In the haplontic life cycle (with post-zygotic meiosis), the organism is haploid, by the proliferation and differentiation of
1507-555: A fusion of the diplontic and haplontic life cycles. Meiosis occurs in all animals and plants. The result, the production of gametes with half the number of chromosomes as the parent cell, is the same, but the detailed process is different. In animals, meiosis produces gametes directly. In land plants and some algae, there is an alternation of generations such that meiosis in the diploid sporophyte generation produces haploid spores instead of gametes. When they germinate, these spores undergo repeated cell division by mitosis, developing into
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#17329027135491644-424: A germ cell with an uneven number of chromosomes undergoes meiosis, the chromosomes cannot be evenly divided between the daughter cells, resulting in aneuploid gametes. Triploid organisms, for instance, are usually sterile. Because of this, triploidy is commonly exploited in agriculture to produce seedless fruit such as bananas and watermelons. If the fertilization of human gametes results in three sets of chromosomes,
1781-629: A haploid and a diploid state, with one of the stages emphasized over the other. This is called alternation of generations . Most fungi and algae are haploid during the principal stage of their life cycle, as are some primitive plants like mosses . More recently evolved plants, like the gymnosperms and angiosperms , spend the majority of their life cycle in the diploid stage. Most animals are diploid, but male bees , wasps , and ants are haploid organisms because they develop from unfertilized, haploid eggs, while females (workers and queens) are diploid, making their system haplodiploid . In some cases there
1918-472: A lack of contemporaneous evidence. Several theories exist about what Hellenic dialect groups may have existed between the divergence of early Greek-like speech from the common Proto-Indo-European language and the Classical period. They have the same general outline but differ in some of the detail. The only attested dialect from this period is Mycenaean Greek , but its relationship to the historical dialects and
2055-419: A lesser degree. Pamphylian Greek , spoken in a small area on the southwestern coast of Anatolia and little preserved in inscriptions, may be either a fifth major dialect group, or it is Mycenaean Greek overlaid by Doric, with a non-Greek native influence. Regarding the speech of the ancient Macedonians diverse theories have been put forward, but the epigraphic activity and the archaeological discoveries in
2192-402: A multicellular haploid gametophyte generation, which then produces gametes directly (i.e. without further meiosis). In both animals and plants, the final stage is for the gametes to fuse to form a zygote in which the original number of chromosomes is restored. Ancient Greek language Ancient Greek ( Ἑλληνῐκή , Hellēnikḗ ; [hellɛːnikɛ́ː] ) includes the forms of
2329-407: A new combination of maternal and paternal genetic information, resulting in offspring that are genetically distinct from either parent. Furthermore, an individual gamete can include an assortment of maternal, paternal, and recombinant chromatids. This genetic diversity resulting from sexual reproduction contributes to the variation in traits upon which natural selection can act. Meiosis uses many of
2466-451: A number of chromosomes that is an exact multiple of the number of chromosomes in a normal gamete; and having any other number, respectively. For example, a person with Turner syndrome may be missing one sex chromosome (X or Y), resulting in a (45,X) karyotype instead of the usual (46,XX) or (46,XY). This is a type of aneuploidy and cells from the person may be said to be aneuploid with a (diploid) chromosome complement of 45. The term ploidy
2603-401: A pair of sister chromatids) to opposite poles. Nonkinetochore microtubules lengthen, pushing the centrosomes farther apart. The cell elongates in preparation for division down the center. Unlike in mitosis, only the cohesin from the chromosome arms is degraded while the cohesin surrounding the centromere remains protected by a protein named Shugoshin (Japanese for "guardian spirit"), what prevents
2740-543: A prefix /e-/, called the augment . This was probably originally a separate word, meaning something like "then", added because tenses in PIE had primarily aspectual meaning. The augment is added to the indicative of the aorist, imperfect, and pluperfect, but not to any of the other forms of the aorist (no other forms of the imperfect and pluperfect exist). The two kinds of augment in Greek are syllabic and quantitative. The syllabic augment
2877-425: A process called endoreduplication , where duplication of the genome occurs without mitosis (cell division). The extreme in polyploidy occurs in the fern genus Ophioglossum , the adder's-tongues, in which polyploidy results in chromosome counts in the hundreds, or, in at least one case, well over one thousand. It is possible for polyploid organisms to revert to lower ploidy by haploidisation . Polyploidy
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#17329027135493014-509: A programmed process in which DNA may be cut and then repaired, which allows them to exchange some of their genetic information . A subset of recombination events results in crossovers , which create physical links known as chiasmata (singular: chiasma, for the Greek letter Chi , Χ) between the homologous chromosomes. In most organisms, these links can help direct each pair of homologous chromosomes to segregate away from each other during meiosis I, resulting in two haploid cells that have half
3151-671: A separate historical stage, though its earliest form closely resembles Attic Greek , and its latest form approaches Medieval Greek , and Koine may be classified as Ancient Greek in a wider sense. There were several regional dialects of Ancient Greek; Attic Greek developed into Koine. Ancient Greek was a pluricentric language , divided into many dialects. The main dialect groups are Attic and Ionic , Aeolic , Arcadocypriot , and Doric , many of them with several subdivisions. Some dialects are found in standardized literary forms in literature , while others are attested only in inscriptions. There are also several historical forms. Homeric Greek
3288-401: A single nucleus rather than in the cell as a whole. Because in most situations there is only one nucleus per cell, it is commonplace to speak of the ploidy of a cell, but in cases in which there is more than one nucleus per cell, more specific definitions are required when ploidy is discussed. Authors may at times report the total combined ploidy of all nuclei present within the cell membrane of
3425-437: A single chromosome and diploid individuals have two chromosomes. In Entamoeba , the ploidy level varies from 4 n to 40 n in a single population. Alternation of generations occurs in most plants, with individuals "alternating" ploidy level between different stages of their sexual life cycle. In large multicellular organisms, variations in ploidy level between different tissues, organs, or cell lineages are common. Because
3562-495: A single copy of each chromosome – that is, one and only one set of chromosomes. In this case, the nucleus of a eukaryotic cell is said to be haploid only if it has a single set of chromosomes , each one not being part of a pair. By extension a cell may be called haploid if its nucleus has one set of chromosomes, and an organism may be called haploid if its body cells (somatic cells) have one set of chromosomes per cell. By this definition haploid therefore would not be used to refer to
3699-490: A single haploid cell called the gamete . Two organisms of opposing sex contribute their haploid gametes 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 utilize the haplontic life cycle. In the haplodiplontic life cycle (with sporic or intermediate meiosis), the living organism alternates between haploid and diploid states. Consequently, this cycle
3836-484: A single parent, without the involvement of gametes and fertilization, and all the offspring are genetically identical to each other and to the parent, including in chromosome number. The parents of these vegetative clones may still be capable of producing haploid gametes in preparation for sexual reproduction, but these gametes are not used to create the vegetative offspring by this route. Some eukaryotic genome-scale or genome size databases and other sources which may list
3973-463: A sperm) fuse, the resulting zygote is once again diploid, with the mother and father each contributing 23 chromosomes. This same pattern, but not the same number of chromosomes, occurs in all organisms that utilize meiosis. Meiosis occurs in all sexually-reproducing single-celled and multicellular organisms (which are all eukaryotes ), including animals , plants and fungi . It is an essential process for oogenesis and spermatogenesis . Although
4110-609: A standard subject of study in educational institutions of the Western world since the Renaissance . This article primarily contains information about the Epic and Classical periods of the language, which are the best-attested periods and considered most typical of Ancient Greek. From the Hellenistic period ( c. 300 BC ), Ancient Greek was followed by Koine Greek , which is regarded as
4247-510: A vowel or /n s r/ ; final stops were lost, as in γάλα "milk", compared with γάλακτος "of milk" (genitive). Ancient Greek of the classical period also differed in both the inventory and distribution of original PIE phonemes due to numerous sound changes, notably the following: The pronunciation of Ancient Greek was very different from that of Modern Greek . Ancient Greek had long and short vowels ; many diphthongs ; double and single consonants; voiced, voiceless, and aspirated stops ; and
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4384-490: Is 24. The monoploid number equals the total chromosome number divided by the ploidy level of the somatic cells: 48 chromosomes in total divided by a ploidy level of 4 equals a monoploid number of 12. Hence, the monoploid number (12) and haploid number (24) are distinct in this example. However, commercial potato crops (as well as many other crop plants) are commonly propagated vegetatively (by asexual reproduction through mitosis), in which case new individuals are produced from
4521-402: Is 7. The gametes of common wheat are considered to be haploid, since they contain half the genetic information of somatic cells, but they are not monoploid, as they still contain three complete sets of chromosomes ( n = 3 x ). In the case of wheat, the origin of its haploid number of 21 chromosomes from three sets of 7 chromosomes can be demonstrated. In many other organisms, although
4658-515: Is a back-formation from haploidy and diploidy . "Ploid" is a combination of Ancient Greek -πλόος (-plóos, "-fold") and -ειδής (- eidḗs ), from εἶδος ( eîdos , "form, likeness"). The principal meaning of the Greek word ᾰ̔πλόος (haplóos) is "single", from ἁ- (ha-, "one, same"). διπλόος ( diplóos ) means "duplex" or "two-fold". Diploid therefore means "duplex-shaped" (compare "humanoid", "human-shaped"). Polish-German botanist Eduard Strasburger coined
4795-413: Is a characteristic of the bacterium Deinococcus radiodurans and of the archaeon Halobacterium salinarum . These two species are highly resistant to ionizing radiation and desiccation , conditions that induce DNA double-strand breaks. This resistance appears to be due to efficient homologous recombinational repair. Depending on growth conditions, prokaryotes such as bacteria may have
4932-556: Is a literary form of Archaic Greek (derived primarily from Ionic and Aeolic) used in the epic poems , the Iliad and the Odyssey , and in later poems by other authors. Homeric Greek had significant differences in grammar and pronunciation from Classical Attic and other Classical-era dialects. The origins, early form and development of the Hellenic language family are not well understood because of
5069-418: Is added to stems beginning with consonants, and simply prefixes e (stems beginning with r , however, add er ). The quantitative augment is added to stems beginning with vowels, and involves lengthening the vowel: Some verbs augment irregularly; the most common variation is e → ei . The irregularity can be explained diachronically by the loss of s between vowels, or that of the letter w , which affected
5206-438: Is also known as the alternation of generations . The diploid organism's germ-line cells undergo meiosis to produce spores. The spores proliferate by mitosis, growing into a haploid organism. The haploid organism's gamete then combines with another haploid organism's gamete, creating the zygote. The zygote undergoes repeated mitosis and differentiation to produce a new diploid organism. The haplodiplontic life cycle can be considered
5343-412: Is an important evolutionary mechanism in both plants and animals and is known as a primary driver of speciation . As a result, it may become desirable to distinguish between the ploidy of a species or variety as it presently breeds and that of an ancestor. The number of chromosomes in the ancestral (non-homologous) set is called the monoploid number ( x ), and is distinct from the haploid number ( n ) in
5480-462: Is associated with a 14% lower risk of being endangered, and a 20% greater chance of being invasive. Polyploidy may be associated with increased vigor and adaptability. Some studies suggest that selection is more likely to favor diploidy in host species and haploidy in parasite species. However, polyploidization is associated with an increase in transposable element content and relaxed purifying selection on recessive deleterious alleles. When
5617-644: Is called 'East Greek'. Arcadocypriot apparently descended more closely from the Mycenaean Greek of the Bronze Age. Boeotian Greek had come under a strong Northwest Greek influence, and can in some respects be considered a transitional dialect, as exemplified in the poems of the Boeotian poet Pindar who wrote in Doric with a small Aeolic admixture. Thessalian likewise had come under Northwest Greek influence, though to
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5754-911: Is common in invertebrates, reptiles, and amphibians. In some species, ploidy varies between individuals of the same species (as in the social insects ), and in others entire tissues and organ systems may be polyploid despite the rest of the body being diploid (as in the mammalian liver ). For many organisms, especially plants and fungi, changes in ploidy level between generations are major drivers of speciation . In mammals and birds, ploidy changes are typically fatal. There is, however, evidence of polyploidy in organisms now considered to be diploid, suggesting that polyploidy has contributed to evolutionary diversification in plants and animals through successive rounds of polyploidization and rediploidization. Humans are diploid organisms, normally carrying two complete sets of chromosomes in their somatic cells: one copy of paternal and maternal chromosomes, respectively, in each of
5891-448: Is considered by some linguists to have been closely related to Greek . Among Indo-European branches with living descendants, Greek is often argued to have the closest genetic ties with Armenian (see also Graeco-Armenian ) and Indo-Iranian languages (see Graeco-Aryan ). Ancient Greek differs from Proto-Indo-European (PIE) and other Indo-European languages in certain ways. In phonotactics , ancient Greek words could end only in
6028-519: Is divided into meiosis I and meiosis II which are further divided into Karyokinesis I, Cytokinesis I, Karyokinesis II, and Cytokinesis II, respectively. The preparatory steps that lead up to meiosis are identical in pattern and name to interphase of the mitotic cell cycle. Interphase is divided into three phases: Interphase is followed by meiosis I and then meiosis II. Meiosis I separates replicated homologous chromosomes, each still made up of two sister chromatids, into two daughter cells, thus reducing
6165-525: Is evidence that the n chromosomes in a haploid set have resulted from duplications of an originally smaller set of chromosomes. This "base" number – the number of apparently originally unique chromosomes in a haploid set – is called the monoploid number , also known as basic or cardinal number , or fundamental number . As an example, the chromosomes of common wheat are believed to be derived from three different ancestral species, each of which had 7 chromosomes in its haploid gametes. The monoploid number
6302-468: Is faster than diploid under high nutrient conditions. The NLH is also tested in haploid, diploid, and polyploid fungi by Gerstein et al. 2017. This result is also more complex: On the one hand, under phosphorus and other nutrient limitation, lower ploidy is selected as expected. However under normal nutrient levels or under limitation of only nitrogen , higher ploidy was selected. Thus the NLH – and more generally,
6439-577: Is likely mediated by oxidative stress leading to increased DNA damage. Meiosis occurs in eukaryotic life cycles involving sexual reproduction , consisting of the cyclical process of growth and development by mitotic cell division, production of gametes by meiosis and fertilization. At certain stages of the life cycle, germ cells produce gametes. Somatic cells make up the body of the organism and are not involved in gamete production. Cycling meiosis and fertilization events results in alternation between haploid and diploid states. The organism phase of
6576-511: Is often used to describe cells with three or more sets of chromosomes. Virtually all sexually reproducing organisms are made up of somatic cells that are diploid or greater, but ploidy level may vary widely between different organisms, between different tissues within the same organism, and at different stages in an organism's life cycle. Half of all known plant genera contain polyploid species, and about two-thirds of all grasses are polyploid. Many animals are uniformly diploid, though polyploidy
6713-535: Is the second meiotic division, and usually involves equational segregation, or separation of sister chromatids. Mechanically, the process is similar to mitosis, though its genetic results are fundamentally different. The result is the production of four haploid cells (n chromosomes; 23 in humans) from the two haploid cells (with n chromosomes, each consisting of two sister chromatids) produced in meiosis I. The four main steps of meiosis II are: prophase II, metaphase II, anaphase II, and telophase II. In prophase II , we see
6850-572: Is the simplest to illustrate in diagrams of genetics concepts. But this definition also allows for haploid gametes with more than one set of chromosomes. As given above, gametes are by definition haploid, regardless of the actual number of sets of chromosomes they contain. An organism whose somatic cells are tetraploid (four sets of chromosomes), for example, will produce gametes by meiosis that contain two sets of chromosomes. These gametes might still be called haploid even though they are numerically diploid. An alternative usage defines "haploid" as having
6987-447: Is the stage at which all autosomal chromosomes have synapsed. In this stage homologous recombination, including chromosomal crossover (crossing over), is completed through the repair of the double strand breaks formed in leptotene. Most breaks are repaired without forming crossovers resulting in gene conversion . However, a subset of breaks (at least one per chromosome) form crossovers between non-sister (homologous) chromosomes resulting in
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#17329027135497124-426: Is the state in which the chromosomes are paired and can undergo meiosis. The zygoid state of a species may be diploid or polyploid. In the azygoid state the chromosomes are unpaired. It may be the natural state of some asexual species or may occur after meiosis. In diploid organisms the azygoid state is monoploid. (See below for dihaploidy.) In the strictest sense, ploidy refers to the number of sets of chromosomes in
7261-731: Is the state where all cells have multiple sets of chromosomes beyond the basic set, usually 3 or more. Specific terms are triploid (3 sets), tetraploid (4 sets), pentaploid (5 sets), hexaploid (6 sets), heptaploid or septaploid (7 sets), octoploid (8 sets), nonaploid (9 sets), decaploid (10 sets), undecaploid (11 sets), dodecaploid (12 sets), tridecaploid (13 sets), tetradecaploid (14 sets), etc. Some higher ploidies include hexadecaploid (16 sets), dotriacontaploid (32 sets), and tetrahexacontaploid (64 sets), though Greek terminology may be set aside for readability in cases of higher ploidy (such as "16-ploid"). Polytene chromosomes of plants and fruit flies can be 1024-ploid. Ploidy of systems such as
7398-402: Is thus 7 and the haploid number is 3 × 7 = 21. In general n is a multiple of x . The somatic cells in a wheat plant have six sets of 7 chromosomes: three sets from the egg and three sets from the sperm which fused to form the plant, giving a total of 42 chromosomes. As a formula, for wheat 2 n = 6 x = 42, so that the haploid number n is 21 and the monoploid number x
7535-417: Is well established in this original sense, but it has also been used for doubled monoploids or doubled haploids , which are homozygous and used for genetic research. Euploidy ( Greek eu , "true" or "even") is the state of a cell or organism having one or more than one set of the same set of chromosomes, possibly excluding the sex-determining chromosomes . For example, most human cells have 2 of each of
7672-650: The Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek ( c. 1400–1200 BC ), Dark Ages ( c. 1200–800 BC ), the Archaic or Epic period ( c. 800–500 BC ), and the Classical period ( c. 500–300 BC ). Ancient Greek was the language of Homer and of fifth-century Athenian historians, playwrights, and philosophers . It has contributed many words to English vocabulary and has been
7809-698: The Greek region of Macedonia during the last decades has brought to light documents, among which the first texts written in Macedonian , such as the Pella curse tablet , as Hatzopoulos and other scholars note. Based on the conclusions drawn by several studies and findings such as Pella curse tablet , Emilio Crespo and other scholars suggest that ancient Macedonian was a Northwest Doric dialect , which shares isoglosses with its neighboring Thessalian dialects spoken in northeastern Thessaly . Some have also suggested an Aeolic Greek classification. The Lesbian dialect
7946-523: The cell cycle , the DNA of each chromosome is replicated so that it consists of two identical sister chromatids , which remain held together through sister chromatid cohesion. This S-phase can be referred to as "premeiotic S-phase" or "meiotic S-phase". Immediately following DNA replication, meiotic cells enter a prolonged G 2 -like stage known as meiotic prophase . During this time, homologous chromosomes pair with each other and undergo genetic recombination ,
8083-438: The diakinesis stage, from Greek words meaning "moving through". 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
8220-474: The meiotic spindle begins to form. Unlike mitotic cells, human and mouse oocytes do not have centrosomes to produce the meiotic spindle. In mice, approximately 80 MicroTubule Organizing Centers (MTOCs) form a sphere in the ooplasm and begin to nucleate microtubules that reach out towards chromosomes, attaching to the chromosomes at the kinetochore . Over time, the MTOCs merge until two poles have formed, generating
8357-423: The monoploid number ( x ). The haploid number ( n ) refers to the total number of chromosomes found in a gamete (a sperm or egg cell produced by meiosis in preparation for sexual reproduction). Under normal conditions, the haploid number is exactly half the total number of chromosomes present in the organism's somatic cells, with one paternal and maternal copy in each chromosome pair. For diploid organisms,
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#17329027135498494-501: The present , future , and imperfect are imperfective in aspect; the aorist , present perfect , pluperfect and future perfect are perfective in aspect. Most tenses display all four moods and three voices, although there is no future subjunctive or imperative. Also, there is no imperfect subjunctive, optative or imperative. The infinitives and participles correspond to the finite combinations of tense, aspect, and voice. The indicative of past tenses adds (conceptually, at least)
8631-425: The salivary gland , elaiosome , endosperm , and trophoblast can exceed this, up to 1048576-ploid in the silk glands of the commercial silkworm Bombyx mori . The chromosome sets may be from the same species or from closely related species. In the latter case, these are known as allopolyploids (or amphidiploids, which are allopolyploids that behave as if they were normal diploids). Allopolyploids are formed from
8768-453: The 23 homologous monoploid chromosomes, for a total of 46 chromosomes. A human cell with one extra set of the 23 normal chromosomes (functionally triploid) would be considered euploid. Euploid karyotypes would consequentially be a multiple of the haploid number , which in humans is 23. Aneuploidy is the state where one or more individual chromosomes of a normal set are absent or present in more than their usual number of copies (excluding
8905-407: The 23 homologous pairs of chromosomes that humans normally have. This results in two homologous pairs within each of the 23 homologous pairs, providing a full complement of 46 chromosomes. This total number of individual chromosomes (counting all complete sets) is called the chromosome number or chromosome complement . The number of chromosomes found in a single complete set of chromosomes is called
9042-1031: The 5th century BC. Ancient pronunciation cannot be reconstructed with certainty, but Greek from the period is well documented, and there is little disagreement among linguists as to the general nature of the sounds that the letters represent. /oː/ raised to [uː] , probably by the 4th century BC. Greek, like all of the older Indo-European languages , is highly inflected. It is highly archaic in its preservation of Proto-Indo-European forms. In ancient Greek, nouns (including proper nouns) have five cases ( nominative , genitive , dative , accusative , and vocative ), three genders ( masculine , feminine , and neuter ), and three numbers (singular, dual , and plural ). Verbs have four moods ( indicative , imperative , subjunctive , and optative ) and three voices (active, middle, and passive ), as well as three persons (first, second, and third) and various other forms. Verbs are conjugated through seven combinations of tenses and aspect (generally simply called "tenses"):
9179-490: The Archaic period of ancient Greek (see Homeric Greek for more details): Μῆνιν ἄειδε, θεά, Πηληϊάδεω Ἀχιλῆος οὐλομένην, ἣ μυρί' Ἀχαιοῖς ἄλγε' ἔθηκε, πολλὰς δ' ἰφθίμους ψυχὰς Ἄϊδι προΐαψεν ἡρώων, αὐτοὺς δὲ ἑλώρια τεῦχε κύνεσσιν οἰωνοῖσί τε πᾶσι· Διὸς δ' ἐτελείετο βουλή· ἐξ οὗ δὴ τὰ πρῶτα διαστήτην ἐρίσαντε Ἀτρεΐδης τε ἄναξ ἀνδρῶν καὶ δῖος Ἀχιλλεύς. The beginning of Apology by Plato exemplifies Attic Greek from
9316-647: The Classical period of ancient Greek. (The second line is the IPA , the third is transliterated into the Latin alphabet using a modern version of the Erasmian scheme .) Ὅτι [hóti Hóti μὲν men mèn ὑμεῖς, hyːmêːs hūmeîs, Haploid Ploidy ( / ˈ p l ɔɪ d i / ) is the number of complete sets of chromosomes in a cell , and hence the number of possible alleles for autosomal and pseudoautosomal genes . Here sets of chromosomes refers to
9453-444: The DNA of the germline . The repair process used appears to involve homologous recombinational repair Prophase I arrested oocytes have a high capability for efficient repair of DNA damage , particularly exogenously induced double-strand breaks. DNA repair capability appears to be a key quality control mechanism in the female germ line and a critical determinant of fertility . Genetic recombination can be viewed as fundamentally
9590-409: The ability to reproduce by parthenogenesis . Meiosis does not occur in archaea or bacteria , which generally reproduce asexually via binary fission . However, a "sexual" process known as horizontal gene transfer involves the transfer of DNA from one bacterium or archaeon to another and recombination of these DNA molecules of different parental origin. Meiosis was discovered and described for
9727-564: The absence or presence of complete sets, which is considered euploidy). Unlike euploidy, aneuploid karyotypes will not be a multiple of the haploid number. In humans, examples of aneuploidy include having a single extra chromosome (as in Down syndrome , where affected individuals have three copies of chromosome 21) or missing a chromosome (as in Turner syndrome , where affected individuals have only one sex chromosome). Aneuploid karyotypes are given names with
9864-550: The aorist. Following Homer 's practice, the augment is sometimes not made in poetry , especially epic poetry. The augment sometimes substitutes for reduplication; see below. Almost all forms of the perfect, pluperfect, and future perfect reduplicate the initial syllable of the verb stem. (A few irregular forms of perfect do not reduplicate, whereas a handful of irregular aorists reduplicate.) The three types of reduplication are: Irregular duplication can be understood diachronically. For example, lambanō (root lab ) has
10001-419: The augment when it was word-initial. In verbs with a preposition as a prefix, the augment is placed not at the start of the word, but between the preposition and the original verb. For example, προσ(-)βάλλω (I attack) goes to προσ έ βαλoν in the aorist. However compound verbs consisting of a prefix that is not a preposition retain the augment at the start of the word: αὐτο(-)μολῶ goes to ηὐ τομόλησα in
10138-438: The center of Greek scholarship, this division of people and language is quite similar to the results of modern archaeological-linguistic investigation. One standard formulation for the dialects is: West vs. non-West Greek is the strongest-marked and earliest division, with non-West in subsets of Ionic-Attic (or Attic-Ionic) and Aeolic vs. Arcadocypriot, or Aeolic and Arcado-Cypriot vs. Ionic-Attic. Often non-West
10275-437: The chromosome constitution. Dihaploids (which are diploid) are important for selective breeding of tetraploid crop plants (notably potatoes), because selection is faster with diploids than with tetraploids. Tetraploids can be reconstituted from the diploids, for example by somatic fusion. The term "dihaploid" was coined by Bender to combine in one word the number of genome copies (diploid) and their origin (haploid). The term
10412-535: The chromosome number by half. During meiosis II, sister chromatids decouple and the resultant daughter chromosomes are segregated into four daughter cells. For diploid organisms, the daughter cells resulting from meiosis are haploid and contain only one copy of each chromosome. In some species, cells enter a resting phase known as interkinesis between meiosis I and meiosis II. Meiosis I and II are each divided into prophase , metaphase , anaphase , and telophase stages, similar in purpose to their analogous subphases in
10549-453: The chromosome number is generally reduced only by the specialized process of meiosis, the somatic cells of the body inherit and maintain the chromosome number of the zygote by mitosis. However, in many situations somatic cells double their copy number by means of endoreduplication as an aspect of cellular differentiation . For example, the hearts of two-year-old human children contain 85% diploid and 15% tetraploid nuclei, but by 12 years of age
10686-584: The chromosomes and the disassembly of the spindle. Nuclear envelopes re-form and cleavage or cell plate formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes. Meiosis is now complete and ends up with four new daughter cells. Meiosis appears to be a fundamental characteristic of eukaryotic organisms and to have been present early in eukaryotic evolution. Eukaryotes that were once thought to lack meiotic sex have recently been shown to likely have, or once have had, this capability. As one example, Giardia intestinalis ,
10823-594: The chromosomes at the first meiotic division. The paired and replicated chromosomes are called bivalents (two chromosomes) or tetrads (four chromatids ), with one chromosome coming from each parent. Prophase I is divided into a series of substages which are named according to the appearance of chromosomes. The first stage of prophase I is the leptotene stage, also known as leptonema , from Greek words meaning "thin threads". In this stage of prophase I, individual chromosomes—each consisting of two replicated sister chromatids—become "individualized" to form visible strands within
10960-499: The condition is called triploid syndrome . In unicellular organisms the ploidy nutrient limitation hypothesis suggests that nutrient limitation should encourage haploidy in preference to higher ploidies. This hypothesis is due to the higher surface-to-volume ratio of haploids, which eases nutrient uptake, thereby increasing the internal nutrient-to-demand ratio. Mable 2001 finds Saccharomyces cerevisiae to be somewhat inconsistent with this hypothesis however, as haploid growth
11097-611: The dialect of Sparta ), and Northern Peloponnesus Doric (including Corinthian ). All the groups were represented by colonies beyond Greece proper as well, and these colonies generally developed local characteristics, often under the influence of settlers or neighbors speaking different Greek dialects. After the conquests of Alexander the Great in the late 4th century BC, a new international dialect known as Koine or Common Greek developed, largely based on Attic Greek , but with influence from other dialects. This dialect slowly replaced most of
11234-423: The diploid stage are under less efficient natural selection than those genes expressed in the haploid stage is referred to as the “masking theory”. Evidence in support of this masking theory has been reported in studies of the single-celled yeast Saccharomyces cerevisiae . In further support of the masking theory, evidence of strong purifying selection in haploid tissue-specific genes has been reported for
11371-463: The disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrosomes 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 at opposite poles. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to
11508-424: The division, genetic material from the paternal and maternal copies of each chromosome is crossed over , creating new combinations of code on each chromosome. Later on, during fertilisation , the haploid cells produced by meiosis from a male and a female will fuse to create a zygote , a cell with two copies of each chromosome again. Errors in meiosis resulting in aneuploidy (an abnormal number of chromosomes) are
11645-468: The exchange of genetic information. The exchange of information between the homologous 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 an ordinary light microscope, and chiasmata are not visible until
11782-618: The first time in sea urchin eggs in 1876 by the German biologist Oscar Hertwig . It was described again in 1883, at the level of chromosomes , by the Belgian zoologist Edouard Van Beneden , in Ascaris roundworm eggs. The significance of meiosis for reproduction and inheritance, however, was described only in 1890 by German biologist August Weismann , who noted that two cell divisions were necessary to transform one diploid cell into four haploid cells if
11919-425: The gametes produced by the tetraploid organism in the example above, since these gametes are numerically diploid. The term monoploid is often used as a less ambiguous way to describe a single set of chromosomes; by this second definition, haploid and monoploid are identical and can be used interchangeably. Gametes ( sperm and ova ) are haploid cells. The haploid gametes produced by most organisms combine to form
12056-440: The genus Leishmania , which cause human disease. However, these organisms were shown to have a sexual cycle consistent with a meiotic process. Although amoeba were once generally regarded as asexual, evidence has been presented that most lineages are anciently sexual and that the majority of asexual groups probably arose recently and independently. Dacks and Rogers proposed, based on a phylogenetic analysis, that facultative sex
12193-419: The haploid number n = 21). The gametes are haploid for their own species, but triploid, with three sets of chromosomes, by comparison to a probable evolutionary ancestor, einkorn wheat . Tetraploidy (four sets of chromosomes, 2 n = 4 x ) is common in many plant species, and also occurs in amphibians , reptiles , and insects . For example, species of Xenopus (African toads) form
12330-556: The historical Dorians . The invasion is known to have displaced population to the later Attic-Ionic regions, who regarded themselves as descendants of the population displaced by or contending with the Dorians. The Greeks of this period believed there were three major divisions of all Greek people – Dorians, Aeolians, and Ionians (including Athenians), each with their own defining and distinctive dialects. Allowing for their oversight of Arcadian, an obscure mountain dialect, and Cypriot, far from
12467-472: The historical circumstances of the times imply that the overall groups already existed in some form. Scholars assume that major Ancient Greek period dialect groups developed not later than 1120 BC, at the time of the Dorian invasions —and that their first appearances as precise alphabetic writing began in the 8th century BC. The invasion would not be "Dorian" unless the invaders had some cultural relationship to
12604-517: The homologous chromosomes become much more closely (~100 nm) and stably paired (a process called synapsis) mediated by the installation of the transverse and central elements of the synaptonemal complex . Synapsis is thought to occur in a zipper-like fashion starting from a recombination nodule. The paired chromosomes are called bivalent or tetrad chromosomes. The pachytene stage ( / ˈ p æ k ɪ t iː n / PAK -i-teen ), also known as pachynema , from Greek words meaning "thick threads".
12741-409: The homologous chromosomes, forming inter-axis bridges, and resulting in the pairing/co-alignment of homologues (to a distance of ~400 nm in mice). Leptotene is followed by the zygotene stage, also known as zygonema , from Greek words meaning "paired threads", which in some organisms is also called the bouquet stage because of the way the telomeres cluster at one end of the nucleus. In this stage
12878-505: The homologs. In the first meiotic division, the homologs are segregated to separate daughter cells by the spindle apparatus . The cells then proceed to a second division without an intervening round of DNA replication. The sister chromatids are segregated to separate daughter cells to produce a total of four haploid cells. Female animals employ a slight variation on this pattern and produce one large ovum and three small polar bodies. Because of recombination, an individual chromatid can consist of
13015-532: The hybridization of two separate species. In plants, this probably most often occurs from the pairing of meiotically unreduced gametes , and not by diploid–diploid hybridization followed by chromosome doubling. The so-called Brassica triangle is an example of allopolyploidy, where three different parent species have hybridized in all possible pair combinations to produce three new species. Polyploidy occurs commonly in plants, but rarely in animals. Even in diploid organisms, many somatic cells are polyploid due to
13152-579: The idea that haploidy is selected by harsher conditions – is cast into doubt by these results. Older WGDs have also been investigated. Only as recently as 2015 was the ancient whole genome duplication in Baker's yeast proven to be allopolyploid , by Marcet-Houben and Gabaldón 2015. It still remains to be explained why there are not more polyploid events in fungi, and the place of neopolyploidy and mesopolyploidy in fungal history . The concept that those genes of an organism that are expressed exclusively in
13289-400: The idiosyncratic rendering "maiosis": We propose to apply the terms Maiosis or Maiotic phase to cover the whole series of nuclear changes included in the two divisions that were designated as Heterotype and Homotype by Flemming . The spelling was changed to "meiosis" by Koernicke (1905) and by Pantel and De Sinety (1906) to follow the usual conventions for transliterating Greek . Meiosis
13426-400: The leading known cause of miscarriage and the most frequent genetic cause of developmental disabilities . In meiosis, DNA replication is followed by two rounds of cell division to produce four daughter cells, each with half the number of chromosomes as the original parent cell. The two meiotic divisions are known as meiosis I and meiosis II . Before meiosis begins, during S phase of
13563-417: The life cycle can occur either during the diploid state ( diplontic life cycle), during the haploid state ( haplontic life cycle), or both ( haplodiplontic life cycle), in which there are two distinct organism phases, one with haploid cells and the other with diploid cells. In the diplontic life cycle (with pre-gametic meiosis), as in humans, the organism is multicellular and diploid, grown by mitosis from
13700-575: The metaphase plate during metaphase I and orientation of sister chromatids in metaphase II, this is the subsequent separation of homologs and sister chromatids during anaphase I and II, it allows a random and independent distribution of chromosomes to each daughter cell (and ultimately to gametes); and (2) Crossing Over . The physical exchange of homologous chromosomal regions by homologous recombination during prophase I results in new combinations of genetic information within chromosomes. However, such physical exchange does not always occur during meiosis. In
13837-703: The metaphase plate, with respect to the orientation of the other bivalents along the same equatorial line. The protein complex cohesin holds sister chromatids together from the time of their replication until anaphase. In mitosis, the force of kinetochore microtubules pulling in opposite directions creates tension. The cell senses this tension and does not progress with anaphase until all the chromosomes are properly bi-oriented. In meiosis, establishing tension ordinarily requires at least one crossover per chromosome pair in addition to cohesin between sister chromatids (see Chromosome segregation ). Kinetochore microtubules shorten, pulling homologous chromosomes (which each consist of
13974-429: The mitotic cell cycle. Therefore, meiosis includes the stages of meiosis I (prophase I, metaphase I, anaphase I, telophase I) and meiosis II (prophase II, metaphase II, anaphase II, telophase II). During meiosis, specific genes are more highly transcribed . In addition to strong meiotic stage-specific expression of mRNA , there are also pervasive translational controls (e.g. selective usage of preformed mRNA), regulating
14111-455: The monoploid number and haploid number are equal; in humans, both are equal to 23. When a human germ cell undergoes meiosis, the diploid 46 chromosome complement is split in half to form haploid gametes. After fusion of a male and a female gamete (each containing 1 set of 23 chromosomes) during fertilization , the resulting zygote again has the full complement of 46 chromosomes: 2 sets of 23 chromosomes. Euploidy and aneuploidy describe having
14248-418: The most generic sense, haploid refers to having the number of sets of chromosomes normally found in a gamete . Because two gametes necessarily combine during sexual reproduction to form a single zygote from which somatic cells are generated, healthy gametes always possess exactly half the number of sets of chromosomes found in the somatic cells, and therefore "haploid" in this sense refers to having exactly half
14385-415: The next stage. During the diplotene stage, also known as diplonema , from Greek words meaning "two threads", the synaptonemal complex disassembles and homologous chromosomes separate from one another a little. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred. The chiasmata remain on the chromosomes until they are severed at
14522-446: The nucleus. The chromosomes each form a linear array of loops mediated by cohesin , and the lateral elements of the synaptonemal complex assemble forming an "axial element" from which the loops emanate. Recombination is initiated in this stage by the enzyme SPO11 which creates programmed double strand breaks (around 300 per meiosis in mice). This process generates single stranded DNA filaments coated by RAD51 and DMC1 which invade
14659-449: The number of chromosomes as the parent cell. During meiosis II, the cohesion between sister chromatids is released and they segregate from one another, as during mitosis . In some cases, all four of the meiotic products form gametes such as sperm , spores or pollen . In female animals, three of the four meiotic products are typically eliminated by extrusion into polar bodies , and only one cell develops to produce an ovum . Because
14796-474: The number of chromosomes had to be maintained. In 1911, the American geneticist Thomas Hunt Morgan detected crossovers in meiosis in the fruit fly Drosophila melanogaster , which helped to establish that genetic traits are transmitted on chromosomes. The term "meiosis" is derived from the Greek word μείωσις , meaning 'lessening'. It was introduced to biology by J.B. Farmer and J.E.S. Moore in 1905, using
14933-628: The number of chromosomes is halved during meiosis, gametes can fuse (i.e. fertilization ) to form a diploid zygote that contains two copies of each chromosome, one from each parent. Thus, alternating cycles of meiosis and fertilization enable sexual reproduction , with successive generations maintaining the same number of chromosomes. For example, diploid human cells contain 23 pairs of chromosomes including 1 pair of sex chromosomes (46 total), half of maternal origin and half of paternal origin. Meiosis produces haploid gametes (ova or sperm) that contain one set of 23 chromosomes. When two gametes (an egg and
15070-565: The number of chromosomes may have originated in this way, this is no longer clear, and the monoploid number is regarded as the same as the haploid number. Thus in humans, x = n = 23. Diploid cells have two homologous copies of each chromosome , usually one from the mother and one from the father . All or nearly all mammals are diploid organisms. The suspected tetraploid (possessing four-chromosome sets) plains viscacha rat ( Tympanoctomys barrerae ) and golden viscacha rat ( Pipanacoctomys aureus ) have been regarded as
15207-503: The number of maternal and paternal chromosome copies, respectively, in each homologous chromosome pair—the form in which chromosomes naturally exist. Somatic cells , tissues , and individual organisms can be described according to the number of sets of chromosomes present (the "ploidy level"): monoploid (1 set), diploid (2 sets), triploid (3 sets), tetraploid (4 sets), pentaploid (5 sets), hexaploid (6 sets), heptaploid or septaploid (7 sets), etc. The generic term polyploid
15344-406: The number of sets of chromosomes found in a somatic cell. By this definition, an organism whose gametic cells contain a single copy of each chromosome (one set of chromosomes) may be considered haploid while the somatic cells, containing two copies of each chromosome (two sets of chromosomes), are diploid. This scheme of diploid somatic cells and haploid gametes is widely used in the animal kingdom and
15481-499: The older dialects, although the Doric dialect has survived in the Tsakonian language , which is spoken in the region of modern Sparta. Doric has also passed down its aorist terminations into most verbs of Demotic Greek . By about the 6th century AD, the Koine had slowly metamorphosed into Medieval Greek . Phrygian is an extinct Indo-European language of West and Central Anatolia , which
15618-694: The only known exceptions (as of 2004). However, some genetic studies have rejected any polyploidism in mammals as unlikely, and suggest that amplification and dispersion of repetitive sequences best explain the large genome size of these two rodents. All normal diploid individuals have some small fraction of cells that display polyploidy . Human diploid cells have 46 chromosomes (the somatic number, 2n ) and human haploid gametes (egg and sperm) have 23 chromosomes ( n ). Retroviruses that contain two copies of their RNA genome in each viral particle are also said to be diploid. Examples include human foamy virus , human T-lymphotropic virus , and HIV . Polyploidy
15755-415: The oocytes of the silkworm Bombyx mori , meiosis is completely achiasmate (lacking crossovers). Although synaptonemal complexes are present during the pachytene stage of meiosis in B. mori , crossing-over homologous recombination is absent between the paired chromosomes . Female mammals and birds are born possessing all the oocytes needed for future ovulations, and these oocytes are arrested at
15892-409: The organism as it now reproduces. Common wheat ( Triticum aestivum ) is an organism in which x and n differ. Each plant has a total of six sets of chromosomes (with two sets likely having been obtained from each of three different diploid species that are its distant ancestors). The somatic cells are hexaploid, 2 n = 6 x = 42 (where the monoploid number x = 7 and
16029-417: The paired 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. This attachment is referred to as a bipolar attachment. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along with
16166-487: The perfect stem eilēpha (not * lelēpha ) because it was originally slambanō , with perfect seslēpha , becoming eilēpha through compensatory lengthening. Reduplication is also visible in the present tense stems of certain verbs. These stems add a syllable consisting of the root's initial consonant followed by i . A nasal stop appears after the reduplication in some verbs. The earliest extant examples of ancient Greek writing ( c. 1450 BC ) are in
16303-515: 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. However, cytokinesis does not fully complete resulting in "cytoplasmic bridges" which enable the cytoplasm to be shared between daughter cells until the end of meiosis II. Sister chromatids remain attached during telophase I. Cells may enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage. Meiosis II
16440-403: The plant Scots Pine . The common potato ( Solanum tuberosum ) is an example of a tetraploid organism, carrying four sets of chromosomes. During sexual reproduction, each potato plant inherits two sets of 12 chromosomes from the pollen parent, and two sets of 12 chromosomes from the ovule parent. The four sets combined provide a full complement of 48 chromosomes. The haploid number (half of 48)
16577-726: The ploidy is reduced from diploid to haploid, meiosis I is referred to as a reductional division . Meiosis II is an equational division analogous to mitosis, in which the sister chromatids are segregated, creating four haploid daughter cells (1n, 1c). Prophase I is by far the longest phase of meiosis (lasting 13 out of 14 days in mice). During prophase I, homologous maternal and paternal chromosomes pair, synapse , and exchange genetic information (by homologous recombination ), forming at least one crossover per chromosome. These crossovers become visible as chiasmata (plural; singular chiasma ). This process facilitates stable pairing between homologous chromosomes and hence enables accurate segregation of
16714-423: The previous plate. This is followed by anaphase II , in which the remaining centromeric cohesin, not protected by Shugoshin anymore, is cleaved, allowing the sister chromatids to segregate. 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 decondensation and lengthening of
16851-603: The process of meiosis is related to the more general cell division process of mitosis , it differs in two important respects: usually occurs between identical sister chromatids and does not result in genetic changes Meiosis begins with a diploid cell, which contains two copies of each chromosome, termed homologs . First, the cell undergoes DNA replication , so each homolog now consists of two identical sister chromatids. Then each set of homologs pair with each other and exchange genetic information by homologous recombination often leading to physical connections ( crossovers ) between
16988-446: The prophase I stage of meiosis. In humans, as an example, oocytes are formed between three and four months of gestation within the fetus and are therefore present at birth. During this prophase I arrested stage ( dictyate ), which may last for decades, four copies of the genome are present in the oocytes. The arrest of ooctyes at the four genome copy stage was proposed to provide the informational redundancy needed to repair damage in
17125-400: The proportions become approximately equal, and adults examined contained 27% diploid, 71% tetraploid and 2% octaploid nuclei. There is continued study and debate regarding the fitness advantages or disadvantages conferred by different ploidy levels. A study comparing the karyotypes of endangered or invasive plants with those of their relatives found that being polyploid as opposed to diploid
17262-401: The same mechanisms as mitosis , the type of cell division used by eukaryotes to divide one cell into two identical daughter cells. In some plants, fungi, and protists meiosis results in the formation of spores : haploid cells that can divide vegetatively without undergoing fertilization. Some eukaryotes, like bdelloid rotifers , do not have the ability to carry out meiosis and have acquired
17399-450: The same organism . Though polyploidy in humans is not viable, mixoploidy has been found in live adults and children. There are two types: diploid-triploid mixoploidy, in which some cells have 46 chromosomes and some have 69, and diploid-tetraploid mixoploidy, in which some cells have 46 and some have 92 chromosomes. It is a major topic of cytology. Dihaploid and polyhaploid cells are formed by haploidisation of polyploids, i.e., by halving
17536-494: The same species or at different stages of the life cycle . In some insects it differs by caste . In humans, only the gametes are haploid, but in many of the social insects , including ants , bees , and termites , males develop from unfertilized eggs, making them haploid for their entire lives, even as adults. In the Australian bulldog ant, Myrmecia pilosula , a haplodiploid species, haploid individuals of this species have
17673-451: The sister chromatids from separating. This allows the sister chromatids to remain together while homologs are segregated. The first meiotic division effectively ends when the chromosomes arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. Cytokinesis,
17810-547: The suffix -somy (rather than -ploidy , used for euploid karyotypes), such as trisomy and monosomy . Homoploid means "at the same ploidy level", i.e. having the same number of homologous chromosomes . For example, homoploid hybridization is hybridization where the offspring have the same ploidy level as the two parental species. This contrasts with a common situation in plants where chromosome doubling accompanies or occurs soon after hybridization. Similarly, homoploid speciation contrasts with polyploid speciation . Zygoidy
17947-517: The syllabic script Linear B . Beginning in the 8th century BC, however, the Greek alphabet became standard, albeit with some variation among dialects. Early texts are written in boustrophedon style, but left-to-right became standard during the classic period. Modern editions of ancient Greek texts are usually written with accents and breathing marks , interword spacing , modern punctuation , and sometimes mixed case , but these were all introduced later. The beginning of Homer 's Iliad exemplifies
18084-486: The terms haploid and diploid in 1905. Some authors suggest that Strasburger based the terms on August Weismann 's conception of the id (or germ plasm ), hence haplo- id and diplo- id . The two terms were brought into the English language from German through William Henry Lang 's 1908 translation of a 1906 textbook by Strasburger and colleagues. The term haploid is used with two distinct but related definitions. In
18221-445: The transition to anaphase I to allow homologous chromosomes to move to opposite poles of the cell. In human fetal oogenesis , all developing oocytes develop to this stage and are arrested in prophase I before birth. This suspended state is referred to as the dictyotene stage or dictyate. It lasts until meiosis is resumed to prepare the oocyte for ovulation, which happens at puberty or even later. Chromosomes condense further during
18358-412: The ultimate meiotic stage-specific protein expression of genes during meiosis. Thus, both transcriptional and translational controls determine the broad restructuring of meiotic cells needed to carry out meiosis. Meiosis I segregates homologous chromosomes , which are joined as tetrads (2n, 4c), producing two haploid cells (n chromosomes, 23 in humans) which each contain chromatid pairs (1n, 2c). Because
18495-399: The yeast Schizosaccharomyces pombe with hydrogen peroxide increased the frequency of mating and the formation of meiotic spores by 4 to 18-fold. Volvox carteri , a haploid multicellular, facultatively sexual green algae, can be induced by heat shock to reproduce by meiotic sex. This induction can be inhibited by antioxidants indicating that the induction of meiotic sex by heat shock
18632-475: Was Aeolic. For example, fragments of the works of the poet Sappho from the island of Lesbos are in Aeolian. Most of the dialect sub-groups listed above had further subdivisions, generally equivalent to a city-state and its surrounding territory, or to an island. Doric notably had several intermediate divisions as well, into Island Doric (including Cretan Doric ), Southern Peloponnesus Doric (including Laconian ,
18769-404: Was likely present in the common ancestor of eukaryotes. The new combinations of DNA created during meiosis are a significant source of genetic variation alongside mutation, resulting in new combinations of alleles , which may be beneficial. Meiosis generates gamete genetic diversity in two ways: (1) Law of Independent Assortment . The independent orientation of homologous chromosome pairs along
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