by the end of meiosis i of oogenesis an oogonium has become
Oogonium


eventually become an oocyte through the process of oogenesis, and one daughter cell that is an identical oogonium to the parent cell. This occurs during the 15th week to the 7th month of embryonic development. Most oogonia have either degenerated or differentiated into primary oocytes by birth. Primary oocytes will undergo oogenesis in which they enter meiosis. However, primary oocytes are arrested in prophase 1 of the first meiosis and remain in that arrested stage until puberty begins in the female adult. This is in contrast to male primordial germ cells which are
12859413
Oogenesis


is a separate sub-process that accompanies and supports all three oogenetic sub-processes. Oogonium —(Oocytogenesis)— Primary Oocyte —(Meiosis I)— First Polar Body (Discarded afterward) + Secondary oocyte —(Meiosis II)— Second Polar Body (Discarded afterward) + Ovum Oocyte meiosis, important to all animal life cycles yet unlike all other instances of animal cell division, occurs completely without the aid of spindle-coordinating centrosomes. The creation of oogonia traditionally doesn't belong to oogenesis proper, but,
14592582
Immature ovum


An immature ovum is a cell that goes through the process of oogenesis to become an ovum. It can be an oogonium, an oocyte, or an ootid. An oocyte, in turn, can be either primary or secondary, depending on how far it has come in its process of meiosis. Oogonia are the cells that turn into primary oocytes in oogenesis. They are diploid, i.e. containing pairs of homologous chromosomes. There are 23 chromosome pairs. Thus, there are 46 chromosomes. Each chromosome, however, hasn't
4475051
Oogonium


which is also responsible for ovary differentiation. Since RSPO1 is produced in somatic cells, this protein acts on germ cells in a paracrine mode. "Rspo1", however, is not the only factor in "Stra8" regulation. Many other factors are under scrutiny and this process is still being evaluated. Because it is theorized that oogonia either degenerate or differentiate into primary oocytes which enter oogenesis and are halted in prophase I of the first meiosis post partum, it is concluded that adult mammalian females lack a population of germ cells
12859416
Oogenesis


the primary oocyte has now developed into the secondary oocyte and the first polar body. Immediately after meiosis I, the haploid secondary oocyte initiates meiosis II. However, this process is also halted at the metaphase II stage until fertilization, if such should ever occur. If the egg is not fertilized, it is disintegrated and released (menstruation) and the secondary oocyte does not complete meiosis II (and doesn't become an ovum). When meiosis II has completed, an ootid and another polar body have now been created.
14592591
Immature ovum


that is formed by meiosis I in oogenesis. Thus, it has only one of each pair of homologous chromosomes. In other words, it is haploid. However, each chromosome still has two chromatids, making a total of 46 chromatids (1N but 2C). The secondary oocyte continues the second stage of meiosis (meiosis II), and the daughter cells are one ootid and one polar body. Secondary oocytes are the immature ovum shortly after ovulation, to fertilization, where it turns into an ootid. Thus, the
4475054
Ovarian follicle


into a corpus luteum. In a larger perspective, the whole folliculogenesis from primordial to preovulatory follicle is located in the stage of meiosis I of ootidogenesis in oogenesis. Embryonic development in males and females follows a common pathway before gametogenesis. Once gametogonia enter the gonadal ridge, however, they attempt to associate with these somatic cells. Development proceeds and the gametogonia turn into oogonia, which become fully surrounded by a layer of cells (pre-granulosa cells). The Oogonia multiply by dividing mitotically; this proliferation ends when the oogonia
4323486
Meiosis


secretion from the anterior pituitary stimulates a few follicles to mature in a process known as folliculogenesis. During this process, the maturing oocytes resume meiosis and continue until metaphase II of meiosis II, where they are again arrested just before ovulation. If these oocytes are fertilized by sperm, they will resume and complete meiosis. During folliculogenesis in humans, usually one follicle becomes dominant while the others undergo atresia. The process of meiosis in females occurs during oogenesis, and differs from the typical meiosis in that it features a long period of
19258445
Oogenesis


few do so every menstrual cycle. Meiosis I of ootidogenesis begins during embryonic development, but halts in the diplotene stage of prophase I until puberty. The mouse oocyte in the dictyate (prolonged diplotene) stage actively repairs DNA damage, whereas DNA repair is not detectable in the pre-dictyate (leptotene, zygotene and pachytene) stages of meiosis. For those primary oocytes that continue to develop in each menstrual cycle, however, synapsis occurs and tetrads form, enabling chromosomal crossover to occur. As a result of meiosis I,
14592590
Oogenesis


in oogonia. In the brown alga "Fucus", all four egg cells survive oogenesis, which is an exception to the rule that generally only one product of female meiosis survives to maturity. In plants, oogenesis occurs inside the female gametophyte via mitosis. In many plants such as bryophytes, ferns, and gymnosperms, egg cells are formed in archegonia. In flowering plants, the female gametophyte has been reduced to an eight-celled embryo sac within the ovule inside the ovary of the flower. Oogenesis occurs within the embryo
14592594
Nondisjunction


most of them are maternally derived. This raises the question: Why is female meiosis more error prone? The most obvious difference between female oogenesis and male spermatogenesis is the prolonged arrest of oocytes in late stages of prophase I for many years up to several decades. Male gametes on the other hand quickly go through all stages of meiosis I and II. Another important difference between male and female meiosis concerns the frequency of recombination between homologous chromosomes: In the male, almost all chromosome pairs are joined by at least one crossover,
14535369
Oogenesis


acquire receptors for luteinizing hormone, a pituitary hormone that reinitiates meiosis in the oocyte and causes ovulation of a fertilizable egg. Luteinizing hormone acts on receptors in the outer layers of granulosa cells of the follicle, causing a decrease in cyclic GMP in the granulosa cells . Because the granulosa cells and oocyte are connected by gap junctions, cyclic GMP also decreases in the oocyte, causing meiosis to resume . Meiosis then proceeds to second metaphase, where it pauses again until fertililization. Luteinizing hormone also stimulates gene expression leading to ovulation . Oogenesis
14592586
Oogenesis


Synchronously with ootidogenesis, the ovarian follicle surrounding the ootid has developed from a primordial follicle to a preovulatory one. Both polar bodies disintegrate at the end of Meiosis II, leaving only the ootid, which then eventually undergoes maturation into a mature ovum. The function of forming polar bodies is to discard the extra haploid sets of chromosomes that have resulted as a consequence of meiosis. "In vitro maturation" ("IVM") is the technique of letting ovarian follicles mature in vitro. It can potentially be performed before an IVF
14592592
Nibrin


cells and during meiosis. Orthologs of NBS1 have been studied in mice and the plant arabidopsis. NBS1 mutant mice display cellular radiation sensitivity and female mice are sterile due to oogenesis failure. Studies of NBS1 mutants in Arabidopsis revealed that NBS1 has a role in recombination during early stages of meiosis. NBS1 has a role in microhomology-mediated end joining (MMEJ) repair of double strand breaks. It is one of 6 enzymes required for this error prone DNA repair pathway. NBS1 is often over-expressed in prostate cancer, in
15514523
Meiosis


each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred. The chiasmata remain on the chromosomes until they are severed at the transition to anaphase I. 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 the "diakinesis" stage, from
19258427
Germline development


of cadherins and connexins. To differentiate into spermatogonia, the gonocytes must lose their junctions to Sertoli cells and become migratory once again. They migrate to the basement membrane of the seminiferous cord and differentiate. In the gonads, the germ cells undergo either spermatogenesis or oogenesis depending on whether the sex is male or female respectively. Mitotic germ stem cells, spermatogonia, divide by mitosis to produce spermatocytes committed to meiosis. The spermatocytes divide by meiosis to form spermatids. The post-meiotic spermatids differentiate through spermiogenesis to become mature and functional
2046060
Reproductive biology


Spermatids are now haploid and undergo differentiation into sperm cells. Later in reproduction the sperm will fuse with a female oocyte to form the zygote. Oogenesis is the formation of a cell who will produce one ovum and three polar bodies. Oogenesis begins in the female embryo with the production of oogonia from primordial germ cells. Like spermatogenesis, the primordial germ cell undergo mitotic division to form the cells that will later undergo meiosis, but will be halted at the prophase I stage. This is known as the primary oocyte. Human females
1272701
Oogenesis


of ootidogenesis occurs when the primary oocyte develops into an ootid. This is achieved by the process of meiosis. In fact, a primary oocyte is, by its biological definition, a cell whose primary function is to divide by the process of meiosis. However, although this process begins at prenatal age, it stops at prophase I. In late fetal life, all oocytes, still primary oocytes, have halted at this stage of development, called the dictyate. After menarche, these cells then continue to develop, although only a
14592589
Oogenesis


instead, to the common process of gametogenesis, which, in the female human, begins with the processes of folliculogenesis, oocytogenesis, and ootidogenesis.Oogonia enter meiosis during embryonic development, becoming oocytes. Meiosis begins with DNA replication and meiotic crossing over. It then stops in early prophase. Mammalian oocytes are maintained in meiotic prophase arrest for a very long time -- months in mice, years in humans. Initially the arrest is due to lack of sufficient cell cycle proteins to allow meiotic progression. However, as the oocyte grows
14592583
Oogonium


that are able to renew or regenerate. But instead, have a large population of primary oocytes that are arrested in the first meiosis until puberty. At puberty, one primary oocyte will complete meiosis to eventually form an ovum during each menstrual cycle. Because there is an absence of regenerating germ cells and oogonia in the human, the number of primary oocytes dwindles after each menstrual cycle until menopause, when the female no longer has a population of primary oocytes. Recent research, however, has identified that renewable oogonia may be present
12859417
XXXY syndrome


zygote. In the case where the oocyte is the genetic cause of 48, XXXY syndrome the oocyte would contain three X chromosome. This would be caused by two non-disjunction events during oogenesis. In meiosis I both sets of duplicated X chromosomes would have to be not separated. Then in meiosis II one set of X chromosomes would have to not separate and the other set would separate resulting in one oocyte with three X chromosomes. A normal sperm containing a Y chromosome would have to fertilize the XXX oocyte to make a
13706587
Oogonium


ovaries lacking "Rspo1" or "Wnt4" will exhibit sex reversal of the gonads, the formation of ovotestes and the differentiation of somatic sertoli cells, which aid in the development of sperm. After female (XX) germ cells collect in the undifferentiated gonads, the up-regulation of "Stra8" is required for germ cell differentiation into an oogonium and eventually enter meiosis. One major factor that contributes to the up-regulation of "Stra8", is the initiation of the β-Catenin signaling pathway via RSPO1,
12859415
Phytophthora


single culture. Heterothallic species have mating strains, designated as A1 and A2. When mated, antheridia introduce gametes into oogonia, either by the oogonium passing through the antheridium (amphigyny) or by the antheridium attaching to the proximal (lower) half of the oogonium (paragyny), and the union producing oospores. Like animals, but not like most true fungi, meiosis is gametic, and somatic nuclei are diploid. Asexual (mitotic) spore types are chlamydospores, and sporangia which produce zoospores. Chlamydospores are usually spherical and
5446302
Gamete


an individual, one ploidy of each type, and are created through meiosis. Oogenesis is the process of female gamete formation in animals. This process involves meiosis (including meiotic recombination) occurring in the diploid primary oocyte to produce the haploid ovum. Spermatogenesis is the process of male gamete formation in animals. This process also involves meiosis occurring in the diploid primary spermatocyte to produce the haploid spermatozoon. In contrast to a gamete, the diploid somatic cells of an individual contain one copy of the chromosome set from the sperm and one
11876509
Genetic linkage


recombination compared to males. It is theorised that there are unique selections acting or meiotic drivers which influence the difference in rates. The difference in rates may also reflect the vastly different environments and conditions of meiosis in oogenesis and spermatogenesis. With very large pedigrees or with very dense genetic marker data, such as from whole-genome sequencing, it is possible to precisely locate recombinations. With this type of genetic analysis, a meiosis indicator is assigned to each position of the genome for each meiosis in a pedigree. The indicator indicates
892035
Oogonium


. In many algae (e.g., "Chara"), the main plant is haploid; oogonia and antheridia form and produce haploid gametes. The only diploid part of the life cycle is the spore (fertilized egg cell), which undergoes meiosis to form haploid cells that develop into new plants. This is a haplontic life cycle (with zygotic meiosis). The oogonia of certain Thallophyte species are usually round or ovoid, with contents are divided into several uninucleate oospheres. This is in contrast to the male antheridia which are
12859422
Oogonium


if the union of the male (motile or non-motile) and the female gamete takes place within this structure. In "Oomycota" and some other organisms, the female oogonia, and the male equivalent antheridia, are a result of sexual sporulation, i.e. the development of structures within which meiosis will occur. The haploid nuclei (gametes) are formed by meiosis within the antheridia and oogonia, and when fertilization occurs, a diploid oospore is produced which will eventually germinate into the diploid somatic stage of the thallophyte life cycle
12859421
Oedogonium


gametes will exit the antheridia and are chemotactically attracted to oogonia. A single sperm cell will pass through a pore opening in the oogonial cell wall, allowing fertilization. Zygotes (oospores) are initially green but will gradually become an orange-red colour and develop a thick multilayered cell wall with species specific surface adornments. Meiosis occurs in the zygote prior to germination, producing four multi-flagellated cells after germination. Once freed from the oogonium, each daughter cell is only motile for a short period of time. All four cells
18389430
Oogenesis


, these proteins are synthesized, and meiotic arrest becomes dependent on cyclic AMP . The cyclic AMP is generated by the oocyte by adenylyl cyclase in the oocyte membrane. The adenylyl cyclase is kept active by a constitutively active G-protein-coupled receptor known as GPR3 and a G-protein, Gs, also present in the oocyte membrane . Maintenance of meiotic arrest also depends on the presence of a multilayered complex of cells, known as a follicle, that surrounds the oocyte. Removal of the oocyte from the follicle causes meiosis
14592584
Advanced maternal age


are present at an intermediate stage of meiosis, that is prophase I (see Oogenesis). Titus et al. also showed that expression of four key DNA repair genes that are necessary for homologous recombinational repair ("BRCA1", "MRE11", "Rad51" and "ATM") decline in oocytes with age. This age-related decline in ability to repair double-strand damages can account for the accumulation of these damages, which then likely contributes to the decline in ovarian reserve. Women with an inherited mutation
2106386
Menopause


termed prophase I (see Oogenesis). Expression of four key DNA repair genes that are necessary for homologous recombinational repair during meiosis (BRCA1, MRE11, Rad51, and ATM) decline with age in oocytes. This age-related decline in ability to repair DNA double-strand damages can account for the accumulation of these damages, that then likely contributes to the depletion of the ovarian reserve. Ways of assessing the impact on women of some of these menopause effects, include the Greene climacteric scale questionnaire, the Cervantes scale and
4930369
Laminaria


Sea and off Brazil). "Laminaria" form a habitat for many fish and invertebrates. The life cycle of "Laminaria" has heteromorphic alternation of generations which differs from "Fucus". At meiosis the male and female zoospores are produced separately, then germinate into male and female gametophytes. The female egg matures in the oogonium until the male sperm fertilizes it. Life-Cycle: The most apparent form of "Laminaria" is its sporophyte phase, a structure composed of the holdfast, the stipe, and the blades
4962018
Polar body


A polar body is a small haploid cell that is formed concomitantly as an egg cell during oogenesis, but which generally does not have the ability to be fertilized. When certain diploid cells in animals undergo cytokinesis after meiosis to produce egg cells, they sometimes divide unevenly. Most of the cytoplasm is segregated into one daughter cell, which becomes the egg or ovum, while the smaller "polar bodies" only get a small amount of cytoplasm. They frequently die and disintegrate (by apoptosis) , but in some cases they remain
10899670
Oogonium


arrested in the spermatogonial stage at birth and do not enter into spermatogenesis and meiosis to produce primary spermatocytes until puberty in the adult male. The regulation and differentiation of germ cells into primary gametocytes ultimately depends on the sex of the embryo and the differentiation of the gonads. In female mice, the protein RPSO1 is responsible for the differentiation of female (XX) gonads into ovaries. RSPO1 activates the β-catenin signaling pathway by up-regulating "Wnt4" which is an essential step in ovary differentiation. Research has shown that
12859414
CHEK2


is essential for DNA damage surveillance in female meiosis. The response of oocytes to DNA double-strand break damage involves a pathway hierarchy in which ATR kinase signals to CHEK2 which then activates p53 and p63 proteins. In the fruitfly "Drosophila", irradiation of germ line cells generates double-strand breaks that result in cell cycle arrest and apoptosis. The "Drosophila" CHEK2 ortholog mnk and the p53 ortholog dp53 are required for much of the cell death observed in early oogenesis when oocyte selection and meiotic recombination occur. CHEK2 has
22105101
Gametogonium


, upregulating genes including ‘‘Stra8’‘, ‘‘Dmc1’‘ and ‘‘Sycp3’‘, which all have a role in meiosis. The male germ cells are protected from external signalling, like retinoic acid from the mesonephros, by the Leydig and Sertoli cells. Spermatogonium are the stem cell population that reside in the testes and undergo spermatogenesis to form sperm. They remain within the testes throughout adulthood. Oogonium are the stem cells for oocytes, and were previously thought to be depleted during development during the production
12859407
Meiosis


may not undergo a second division. In meiosis II, division of the daughter cell produces a second polar body, and a single haploid cell, which enlarges to become an ovum. Therefore, in females each primary oocyte that undergoes meiosis results in one mature ovum and one or two polar bodies. Note that there are pauses during meiosis in females. Maturing oocytes are arrested in prophase I of meiosis I and lie dormant within a protective shell of somatic cells called the follicle. At the beginning of each menstrual cycle, FSH
19258444
Phytophthora sojae


is a definite sign of "Phytophthora sojae". Oospores, in general, measure around 20-45 micrometers in diameter and have very thick cellulose cell walls for overwintering. "Phytophthora sojae" overwinters in plant debris and soil as oospores. Oospores are made after the male gamete, antheridium, and female gamete, oogonium, undergo fertilization and then sexual recombination (meiosis). They possess thick cell walls with cellulose that enables them to survive harsh conditions in the soil without germinating for several years. They begin to germinate once
19054538
Brown algae


, and may be isogamous, oogamous, or anisogamous. Fertilization may take place in the water with eggs and motile sperm, or within the oogonium itself. Certain species of brown algae can also perform asexual reproduction through the production of motile diploid zoospores. These zoospores form in plurilocular sporangium, and can mature into the sporophyte phase immediately. In a representative species "Laminaria", there is a conspicuous diploid generation and smaller haploid generations. Meiosis takes place within several unilocular sporangium along the algae's blade, each one forming either
8424104
Parthenogenesis


the production of males by parthenogenesis is referred to as arrhenotoky (e.g., bees). When unfertilized eggs develop into both males and females, the phenomenon is called deuterotoky. Parthenogenesis can occur without meiosis through mitotic oogenesis. This is called "apomictic parthenogenesis". Mature egg cells are produced by mitotic divisions, and these cells directly develop into embryos. In flowering plants, cells of the gametophyte can undergo this process. The offspring produced by apomictic parthenogenesis are "full clones" of their mother. Examples include aphids. Parthenogenesis
13556355
Spermatogenesis


Meiosis II. The spermatids are transformed into spermatozoa(sperm) by the process of spermiogenesis. These develop into mature spermatozoa, also known as sperm cells. Thus, the primary spermatocyte gives rise to two cells, the secondary spermatocytes, and the two secondary spermatocytes by their subdivision produce four spermatozoa and four haploid cells. Spermatozoa are the mature male gametes in many sexually reproducing organisms. Thus, spermatogenesis is the male version of gametogenesis, of which the female equivalent is oogenesis. In mammals it occurs in the seminiferous tubules of the male
20573008
Centimorgan


different propensities towards crossover) and it also depends on if the meiosis where the crossing-over takes place is a part of oogenesis (formation of female gametes) or spermatogenesis (formation of male gametes). One centimorgan corresponds to about 1 million base pairs in humans on average. The relationship is only rough as the physical chromosomal distance corresponding to one centimorgan varies from place to place in the genome, and also varies between men and women since recombination during gamete formation in females is significantly more frequent than in males. Morton
8610310
Human reproductive system


they are not homologous but rather analogous structures as they arise from different fetal structures. Gametes are produced within the gonads through a process known as gametogenesis. This occurs when certain types of germ cells undergo meiosis to split the normal diploid number of chromosomes (n=46) into haploid cells containing only 23 chromosomes. In females, gametogenesis is known as oogenesis; this occurs in the ovarian follicles of the ovaries. This process does not produce mature ovum until puberty. In contrast with males, each of the original diploid germ cells or
9911138
Homologous chromosome


chromosomes are now randomly segregated into two daughter cells that will undergo meiosis II to produce four haploid daughter germ cells. After the tetrads of homologous chromosomes are separated in meiosis I, the sister chromatids from each pair are separated. The two haploid(because the chromosome no. has reduced to half. Earlier two sets of chromosomes were present, but now each set exists in two different daughter cells that have arisen from the single diploid parent cell by meiosis I) daughter cells resulting from meiosis I undergo another cell division in meiosis II but
12126235
Ovarian follicle


the human. For several primary oocytes that complete meiosis I each month, only one or a few functional oocyte, the dominant follicles, completes maturation and undergoes ovulation. The other follicles that begin to mature will regress and become atretic follicles, eventually deteriorating. The primary oocyte turns into a secondary oocyte in mature ovarian follicles. Unlike the sperm, the egg is arrested in the secondary stage of meiosis until fertilization. Upon fertilization by sperm, the secondary oocyte continues the second part of meiosis and becomes a zygote. Any ovarian
4323488
ANLN


/CYK-4/RacGAP50C. Anillins have also been shown to organize the actomyosin cytoskeleton into syncytial structures observed in Drosophila embryos or C. elegans gonads. ANI-1 and ANI-2 (proteins homologous to anillin) are essential for embryonic viability in both organisms. ANI-1 is required for cortical ruffling, pseudocleavage, and all contractile events that occur in embryos prior to mitosis. ANI-1 is also crucial for segregation of polar bodies during meiosis. ANI-2 functions in the maintenance of the structure of the central core of the cytoplasm, the rachis, during oogenesis. ANI-2
135991
AURKC


Aurora B in diploid human fibroblasts, with mRNA and protein concentrations peaking during the G2/M phase. Aurora C levels, however, peak after those of Aurora B later in the M phase. While Aurora A and B are expressed in mitotic somatic cells, Aurora C is more often expressed during meiosis (spermatogenesis and oogenesis). Aurora B kinase regulates kinetochore maturation, destabilization of improper kinetochore-microtubule attachments, and spindle assembly checkpoint (SAC), central spindle organization, and cytokinesis. Aneuploidy results from independent and simultaneous
1874299
In vitro maturation


By 1991, the first pregnancy was recorded using IVM followed by IVF, and in 1994 the first birth using IVM oocytes from polycystic ovarian syndrome patients was recorded highlighting that PCOS patient's oocytes are capable of maturation. Oogenesis takes place during fetal life, in which primordial germ cells undergo mitosis until a few weeks prior to birth, forming oogonia. These then begin meiosis to form the oocyte within the primordial follicle. This follicle consists of the oocyte surrounded by flattened pregranulosa cells. Babies are born with 1-2 million primordial
21587309
Insect physiology


yolk deposition. Most insects are oviparous, where the young hatch after the eggs have been laid. Insect sexual reproduction starts with sperm entry that stimulates oogenesis, meiosis occurs and the egg moves down the genital tract. Accessory glands of the female secrete an adhesive substance to attach eggs to an object and they also supply material that provides the eggs with a protective coating. Oviposition takes place via the female ovipositor. The male's main reproductive function is to produce and store spermatozoa and provide transport to the reproductive tract of the female
3296712
Germline


transversion mutations. Such mutations occur throughout the mouse chromosomes as well as during different stages of gametogenesis. The mutation frequencies for cells in different stages of gametogenesis are about 5 to 10-fold lower than in somatic cells both for spermatogenesis and oogenesis. The lower frequencies of mutation in germline cells compared to somatic cells appears to be due to more efficient DNA repair of DNA damages, particularly homologous recombinational repair, during germline meiosis. Among humans, about five percent of live-born offspring have a genetic disorder, and of these, about
6528502
Meiosis


zygote. The zygote undergoes repeated mitosis and differentiation to become a diploid organism again. The haplodiplontic life cycle can be considered a fusion of the diplontic and haplontic life cycles. Meiosis occurs in all animals and plants. The end 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
19258442
Meiosis


II. Meiosis I and II are each divided into prophase, metaphase, anaphase, and telophase stages, similar in purpose to their analogous subphases in 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). Meiosis generates gamete genetic diversity in two ways: (1) Law of Independent Assortment. The independent orientation of homologous chromosome pairs along the metaphase plate
19258417
Meiosis


telophase I, and is marked by decondensation and lengthening of 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 occurs in eukaryotic life cycles involving sexual reproduction, consisting of the constant cyclical process of meiosis and fertilization. This takes place alongside normal mitotic cell division. In multicellular organisms, there is an intermediary step
19258437
Meiosis


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 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
19258416
Aurora A kinase


meiosis I-meiosis II. Furthermore, Aurora A has been observed to have a biphasic pattern of activation during progression through meiosis. It has been suggested that the fluctuations, or phases, of Aurora A activation are dependent on a positive-feedback mechanism with a p13SUC1-associated protein kinase Aurora A is not only implicated with the translation of MOS during meiosis but also in the polyadenylation and subsequent translation of neural mRNAs whose protein products are associated with synaptic plasticity. Aurora A dysregulation has been associated with high occurrence of cancer. For example
7776540
Quantitative genetics


, which facilitates more detailed analysis of selection problems. Here, i and z have already been defined, ½ is the "meiosis determination (b) for f=0", and the remaining symbol is the heritability. These are discussed further in following sections. Also notice that, more generally, R = b h. If the general meiosis determination ( b ) is used, the results of prior inbreeding can be incorporated into the selection. The phenotypic variance equation then becomes: σ = σ [1 − i ( i-
6802839
Ovulation


out into the peritoneal cavity through the stigma, where it is caught by the fimbriae at the end of the fallopian tube. After entering the fallopian tube, the oocyte is pushed along by cilia, beginning its journey toward the uterus. By this time, the oocyte has completed meiosis I, yielding two cells: the larger secondary oocyte that contains all of the cytoplasmic material and a smaller, inactive first polar body. Meiosis II follows at once but will be arrested in the metaphase and will so remain until fertilization. The
2028241
Meiotic recombination checkpoint


. The second major transition occurs at the entry into metaphase I. The main purpose of this step is to make sure that DNA replication has completed without error so that spindle pole bodies can separate. This event is triggered by the activation of M-Cdk in late prophase I. Then the spindle assembly checkpoint examines the attachment of microtubules at kinetochores, followed by initiation of metaphase I by APC. The special chromosome separation in meiosis, homologous chromosomes separation in meiosis I and chromatids separation in meiosis II, requires special tension between homologous chromatids
8310646
Meiosis


the whole series of nuclear changes included in the two divisions that were designated as Heterotype and Homotype by Flemming". The term was linguistically corrected to "meiosis" by Koernicke (1905), and by Pantel and De Sinety (1906). Meiosis is divided into meiosis I and meiosis II which are further divided into Karyokinesis I and Cytokinesis I and 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
19258415
Interkinesis


Interkinesis or interphase II is a period of rest that cells of some species enter during meiosis, between meiosis I and meiosis II. No DNA replication occurs during interkinesis, however it does occur during the interphase I stage of meiosis (See meiosis I). During interkinesis the single spindle of the first meiotic division disassembles and the microtubules reassemble into two new spindles for the second meiotic division. Interkinesis follows telophase I and is where Many plants skip telophase I and interkinesis, going immediately into prophase II. Each chromosome still consists of
11595075
Nondisjunction


pairs of sister chromatids resulting from the separation of bivalents in meiosis I are further separated in anaphase of meiosis II. In oocytes, one sister chromatid is segregated into the second polar body, while the other stays inside the egg. During spermatogenesis, each meiotic division is symmetric such that each primary spermatocyte gives rise to 2 secondary spermatocytes after meiosis I, and eventually 4 spermatids after meiosis II. Meiosis II-nondisjunction may also result in aneuploidy syndromes, but only to a much smaller extent than do segregation failures in meiosis I.
14535365
Charophyta


naked eye, especially the bright orange or red antheridium. Many species are dioecious. In others the monoecious condition is complicated by the development of the antheridium before the formation of the oogonium, thus preventing fertilization by antherozoids of the same alga. In this case the two types of sex organs usually arise from different points on the lateral branches. All cells of the Charales are haploid except for the fertilized zygote, the large single cell in the interior of the oogonium, which becomes enclosed in a thickened hard wall to form an
3272893
Trichonympha


few hours after fertilization. During meiosis I the zygote’s chromosomes duplicate and the zygote divides. During meiosis I, the centromeres are not duplicated. After meiosis I, meiosis II occurs, during which the centromeres, but not the chromosomes, are duplicated, and the cell divides again. The overall result of meiosis is 4 haploid cells. There is not a lot of fossil history pertaining to "Trichonympha," but some fossils of termite gut symbionts have been found. The fossils of a kalotermitid termite provide evidence that the
5036771
Aphanomyces euteiches


homothallic the antheridium and oogonium arise from the same hypha and are self-compatible, meaning separate mating types are not needed for sexual reproduction. The antheridium fertilizes an oogonium, which then develops into a single oospore, which is 20-35 micrometers in diameter. When growing under optimal conditions, an infection of "A. euteiches" can result in symptoms within 10 days, and oospores can be formed between 7 and 14 days. Oospores become dormant after being formed, and can survive for more than 10 years. Aphanomyces is
14994338
Homologous chromosome


chromosomes are important in the processes of meiosis and mitosis. They allow for the recombination and random segregation of genetic material from the mother and father into new cells. Meiosis is a round of two cell divisions that results in four haploid daughter cells that each contain half the number of chromosomes as the parent cell. It reduces the chromosome number in a germ cell by half by first separating the homologous chromosomes in meiosis I and then the sister chromatids in meiosis II. The process of meiosis I is generally longer than meiosis II because
12126229
Nuclear DNA


STRs). Like mitosis, meiosis is a form of eukaryotic cell division. Meiosis gives rise to four unique daughter cells, each of which has half the number of chromosomes as the parent cell. Because meiosis creates cells that are destined to become gametes (or reproductive cells), this reduction in chromosome number is critical — without it, the union of two gametes during fertilization would result in offspring with twice the normal number of chromosomes. Meiosis creates new combinations of genetic material in each of the four daughter cells. These
20102966
Oocyte


has been put into action as a means of conserving traditional livestock. The formation of an oocyte is called oocytogenesis, which is a part of oogenesis. Oogenesis results in the formation of both primary oocytes during fetal period, and of secondary oocytes after it as part of ovulation. Oocytes are rich in cytoplasm, which contains yolk granules to nourish the cell early in development. During the primary oocyte stage of oogenesis, the nucleus is called a germinal vesicle. The only normal human type of secondary oocyte has the 23rd (sex
9996970
Synapsis


Synapsis (also called syndesis) is the pairing of two homologous chromosomes that occurs during meiosis. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover between them. Synapsis takes place during prophase I of meiosis. When homologous chromosomes synapse, their ends are first attached to the nuclear envelope. These end-membrane complexes then migrate, assisted by the extranuclear cytoskeleton, until matching ends have been paired. Then the intervening regions of the chromosome are brought together, and may be connected by a
14002018
Leptotene stage


The leptotene stage, also known as the leptonema, is the first of five substages of prophase I in meiosis. The term "leptonema" derives from Greek words meaning "thin threads". A cell destined to become a gamete enters the leptotene stage after its chromosomes are duplicated during interphase. During the leptotene stage those duplicated chromosomes—each consisting of two sister chromatids—condense from diffuse chromatin into long, thin strands that are more visible within the nucleoplasm. The next stage of prophase I in meiosis is the zygotene stage
4826912
Oogenesis


Oogenesis, ovogenesis, or oögenesis is the differentiation of the ovum (egg cell) into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated in the embryonic stage. In mammals, the first part of oogenesis starts in the germinal epithelium, which gives rise to the development of ovarian follicles, the functional unit of the ovary. Oogenesis consists of several sub-processes: oocytogenesis, ootidogenesis, and finally maturation to form an ovum (oogenesis proper). Folliculogenesis
14592581
Meiosis


are also pervasive translational controls (e.g. selective usage of preformed mRNA), regulating 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 the ploidy is reduced from diploid to haploid, meiosis I
19258419
Meiosis


(mesonephros) stimulates meiosis in embryonic ovarian oogonia and that tissues of the embryonic male testis suppress meiosis by degrading retinoic acid. However, genetic loss-of-function studies on retinoic acid-generating enzymes have shown that retinoic acid is not required for initiation of either female meiosis which occurs during embryogenesis or male meiosis which initiates postnatally. The normal separation of chromosomes in meiosis I or sister chromatids in meiosis II is termed "disjunction". When the segregation is not normal, it is called "nondisjunction". This results
19258448
Cdc14


of Cdc14 from nucleolus results in cdk1 inactivation and ultimately in disassembly of spindle during Anaphase of meiosis I. Cells deprived of Cdc14 or SLK19 and SPO12 have abnormal meiosis. They have only one division during meiosis. The chromosomes also segregate abnormally. The abnormality arises due to delay in dissembling of spindle during Anaphase I. However, the segregation of chromosomes continue and both the phases of meiotic segregations take place on prolonged meiosis I spindle. Cdc14 along with SPO12 and SLK19 play a critical role in ensuring that the two phases of chromosomal segregation take
19869883
Caenorhabditis elegans


"sid-2", which, when inserted as a transgene in other species, allows them to take up RNA for RNAi as "C. elegans" does. Research into meiosis has been considerably simplified since every germ cell nucleus is at the same given position as it moves down the gonad, so is at the same stage in meiosis. In an early phase of meiosis, the oocytes become extremely resistant to radiation and this resistance depends on expression of genes "rad51" and "atm" that have key roles in recombinational repair
12364678
Heterothallism


ascus. Meiosis is an essential part of the life cycle of all sexually reproducing organisms, and in its main features, meiosis in "N. crassa" seems typical of meiosis generally. As the above events are occurring, the mycelial sheath that had enveloped the ascogonium develops as the wall of the perithecium, becomes impregnated with melanin, and blackens. The mature perithecium has a flask-shaped structure. A mature perithecium may contain as many as 300 asci, each derived from identical fusion diploid nuclei. Ordinarily, in nature,
7666429
Meiosis


meiotic arrest known as the dictyate stage and lacks the assistance of centrosomes. In males, meiosis occurs during spermatogenesis in the seminiferous tubules of the testicles. Meiosis during spermatogenesis is specific to a type of cell called spermatocytes, which will later mature to become spermatozoa. Meiosis of primordial germ cells happens at the time of puberty, much later than in females. Tissues of the male testis suppress meiosis by degrading retinoic acid, proposed to be a stimulator of meiosis. This is overcome at puberty when cells within seminiferous tubules called Sertoli
19258446
Meiosis


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 typically the longest phase of meiosis. During prophase I, homologous chromosomes pair and exchange DNA (homologous recombination). This often results in chromosomal crossover. This process is critical for pairing between homologous chromosomes and hence for accurate segregation of the chromosomes at the first meiosis division. The new combinations of
19258420
Folliculogenesis


and others stating that it occurs when entering the "tertiary" stage. Until the preovulatory stage, the follicle contains a primary oocyte that is arrested in prophase of meiosis I. During the late preovulatory stage, the oocyte continues meiosis and becomes a secondary oocyte, arrested in metaphase II. At 18–22 weeks post-conception, the cortex of the female ovary (foetal female ovary) contains its peak number of follicles (about 4 to 5 million in the average case, but individual peak populations range from 6 to 7 million)
14950826
Spermatocyte


Type B cells will move on to the adluminal compartment (towards the inner region of tubule) and become primary spermatocytes; this process takes about 16 days to complete. The primary spermatocytes within the adluminal compartment will continue on to Meiosis I and divide into two daughters cells, known as secondary spermatocytes, a process which takes 24 days to complete. Each secondary spermatocyte will form two spermatids after Meiosis II. Although spermatocytes that divide mitotically and meiotically are sensitive to radiation and cancer, spermatogonial stem cells are not. Therefore, after
8589424
Oogonium


cell development that involves a nuclear transfer into a zygote. Using these functional oogonia may help to create patient-specific stem cells lines using this method. There is a significant controversy regarding existence of mammalian oogonial stem cells. The controversy lies in negative data that has originated from many laboratories in the United States. Multiple approaches to verify the existence of oogonial stem cells have yielded negative results, and no research group in United States has been able to reproduce initial findings. In phycology and mycology, oogonium refers to a female gametangium
12859420
Parthenogenesis in squamata


division cycles in the process of meiosis result in a diploid, rather than haploid, genome. Whereas homologous chromosomes pair and separate during meiosis I in sexual species, identical duplicate sister chromosomes, produced through premeiotic replication, pair and separate during meiosis I in true parthenotes. Pairing of identical sister chromosomes, in comparison to the alternative of pairing homologous chromosomes, maintains heterozygosity in obligate parthenotes. Meiosis II involves the separation of sister chromatids in both sexual and parthenogenetic species. This method of parthenogenesis is observed in obligate parthenotes, such as
17055096
Pronucleus


the tail of the sperm was to help move the sperm cell to the egg cell. The formation of the female egg is asymmetrical, while the formation of the male sperm is symmetrical. Typically in a female, meiosis starts with one diploid cell and becomes one ovum and three polar bodies. In a male, meiosis starts with one diploid cell and ends with four sperm. In mammals, the female pronucleus starts in the center of the egg before fertilization. When the male pronucleus is formed, after the sperm cell reaches
21753067
Germ cell


oogenesis and spermatogenesis) is different for each species but the general stages are similar. Oogenesis and spermatogenesis have many features in common, they both involve: Despite their homologies they also have major differences: After migration primordial germ cells will become oogonia in the forming gonad (ovary). The oogonia proliferate extensively by mitotic divisions, up to 5-7 million cells in humans. But then many of these oogonia die and about 50,000 remain. These cells differentiate into primary oocytes. In week 11-12 "post coitus"
888680
Nondisjunction


of Columbia University. In general, nondisjunction can occur in any form of cell division that involves ordered distribution of chromosomal material. Higher animals have three distinct forms of such cell divisions: Meiosis I and meiosis II are specialized forms of cell division occurring during generation of gametes (eggs and sperm) for sexual reproduction, mitosis is the form of cell division used by all other cells of the body. Ovulated eggs become arrested in metaphase II until fertilization triggers the second meiotic division. Similar to the segregation events of mitosis, the
14535364
Coolia


cells stop moving and the fertilization bridge disappears, allowing the two nuclei to join together. This indicates that karyogamy has occurred. The theca then begins to develop and the cell becomes motile. As the planozygote matures, it shrinks and becomes immobile, eventually developing into a cyst. The cyst further develops to contain a single nucleus that makes up much of the volume of the cell. At the end of the process, meiosis occurs. The sexual life cycle is thought to last approximately 2 months. Phylogenetic analysis of different regions
21510119
Cell (biology)


occurs during the S phase of the cell cycle. In meiosis, the DNA is replicated only once, while the cell divides twice. DNA replication only occurs before meiosis I. DNA replication does not occur when the cells divide the second time, in meiosis II. Replication, like all cellular activities, requires specialized proteins for carrying out the job. Between successive cell divisions, cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions: catabolism
3756013
Biological life cycle


The description of the complex life cycles of various organisms contributed to the disproof of the ideas of spontaneous generation in the 1840s and 1850s. A zygotic meiosis is a meiosis of a zygote immediately after karyogamy, which is the fusion of two cell nuclei. This way, the organism ends its diploid phase and produces several haploid cells. These cells divide mitotically to form either larger, multicellular individuals, or more haploid cells. Two opposite types of gametes (e.g., male and female) from these individuals or cells fuse to become
14303777
Meiosis


, 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. 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 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 end result
19258434
Meiosis


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 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
19258436
XMAP215-Dis1 family


spindles (Msps) is essential for maintaining the integrity of mitotic spindles, which are important for separating chromosomes during mitosis. Reducing Msps activity creates short microtubules, which describes the name of the gene. Msps is also important during oogenesis. When oocytes are depleted of Msps expression, bicoid mRNA localization is less efficient during early stages of oogenesis, but then completely dispersed later in development. Msps is not only responsible for transporting bicoid mRNA throughout the cell, but it also localizes mRNA to the anterior (head) end of the
12323691
Chromosome segregation


the mitosis diagram, mitosis is preceded by a round of DNA replication, so that each chromosome forms two copies called chromatids. These chromatids separate to opposite poles, a process facilitated by a protein complex referred to as cohesin. Upon proper segregation, a complete set of chromatids ends up in each of two nuclei, and when cell division is completed, each DNA copy previously referred to as a chromatid is now called a chromosome. Chromosome segregation occurs at two separate stages during meiosis called prophase I and prophase II (see meiosis
17429895
Homologous chromosome


meiosis I is crucial for sister chromatid separation in meiosis II. A failure to separate properly is known as nondisjunction. There are two main types of nondisjunction that occur: trisomy and monosomy. Trisomy is caused by the presence of one additional chromosome in the zygote as compared to the normal number, and monosomy is characterized by the presence of one fewer chromosome in the zygote as compared to the normal number. If this uneven division occurs in meiosis I, then none of the daughter cells will have proper chromosomal distribution and severe effects
12126239
Cohesin


no way of ensuring whether the spindle fiber attached on each sister chromatid is from a different pole. 2. It facilitates spindle attachment onto chromosomes. 3. It facilitates DNA repair by recombination. 4. Recently many novel functions of cohesin have been discovered in many different cellular processes. Cohesin has been shown to be responsible for transcription regulation, DNA double strand break repair, chromosome condensation, pairing of homologous chromosomes during meiosis I, mono-orientation of sister kinetochores during meiosis I, non-homologous centromere coupling, chromosome architecture
5730692
SGOL2


has been shown that disruption of the mouse SGOL2 does not cause any alteration in sister chromatid cohesion in embryonic cultured fibroblasts and adult somatic tissues. Moreover, although these mutant mice also develop normally and survive to adulthood without any apparent alteration, both male and female Sgol2-deficient mice from this line are infertile. By different approaches it was demonstrated that SGOL2 is necessary for protecting centromeric cohesion during mammalian meiosis I. In vivo, the loss of SGOL2 promotes a premature release of the meiosis-specific REC8 cohesin complexes from anaphase I centromeres. This
6409047
XXXY syndrome


where the sperm is the genetic cause of 48, XXXY syndrome, the sperm would have to contain two X chromosomes and one Y chromosome. This would be caused by two non-disjunction events in spermatogenesis, both meosis I and meiosis II. The duplicated X chromosome in the sperm would have to fail to separate in both meiosis I and meiosis II for a sperm as well as the X and Y chromosomes would have to be in the same sperm. Then the XXY sperm would fertilize a normal oocyte to make a XXXY
13706586
Chromosome segregation


. After the first chromosome segregation in meiosis I is complete, there is further chromosome segregation during the second equational division of meiosis II. Both proper initial segregation of chromosomes in prophase I and the next chromosome segregation during equational division in meiosis II are required to generate gametes with the correct number of chromosomes. CO recombinants are produced by a process involving the formation and resolution of Holliday junction intermediates. As indicated in the figure titled "A current model of meiotic recombination", the formation of meiotic crossovers can be initiated by a
17429901
Immature ovum


yet duplicated itself. As such, there is only one chromatid on each chromosome, making the total quantity of chromatids 46. This is twice the number of chromosome pairs (2N). Oogonia are created in early embryonic life. All have turned into primary oocytes at late fetal age. The primary oocyte is defined by its process of ootidogenesis, which is meiosis. It has duplicated its DNA, so that each chromosome has two chromatids, i.e. 92 chromatids all in all (4C). When meiosis I is completed,
4475052
Meiosis


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 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
19258405
Germline development


continuous. These primary oocytes begin meiosis but pause in diplotene of meiosis I while in the embryo. All of the oogonia and many primary oocytes die before birth. After puberty in primates, small groups of oocytes and follicles prepare for ovulation by advancing to metaphase II. Only after fertilization is meiosis completed. Meiosis is asymmetric producing polar bodies and oocytes with large amounts of material for embryonic development. The mutation frequency of female mouse germ line cells, like male germ line cells, is also lower than that of somatic cells.
2046062
Neurospora crassa


in its main features, meiosis in "N. crassa" seems typical of meiosis generally. As the above events are occurring, the mycelial sheath that had enveloped the ascogonium develops as the wall of the perithecium, becomes impregnated with melanin, and blackens. The mature perithecium has a flask-shaped structure. A mature perithecium may contain as many as 300 asci, each derived from identical fusion diploid nuclei. Ordinarily, in nature, when the perithecia mature the ascospores are ejected rather violently into the air. These ascospores are heat
15616970