meiosis ii does not take place in the oocyte until
fertilization
2942305
occurs and tetrads form, enabling chromosomal crossover to occur. As a result of meiosis I, 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.
Oogenesis
2942305
occurs and tetrads form, enabling chromosomal crossover to occur. As a result of meiosis I, 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.
Oogenesis
3377438
anywhere up to fifty years in 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 follicle that is larger than about
Ovarian follicle
13599679
of meiosis II. This secondary oocyte is then ovulated. The follicle and the ovary's wall rupture, allowing the secondary oocyte to escape. The secondary oocyte is caught by the fimbriated end and travels to the ampulla of the uterine tube where typically the sperm are met and fertilization occurs; meiosis II is promptly completed. The fertilized ovum, now a zygote, travels towards the uterus aided by activity of tubal cilia and activity of the tubal muscle. The early embryo requires critical development in the fallopian tube. After about five days the new embryo enters the uterine cavity and on about
Fallopian tube
2882637
cumulus-oocyte complex contains layers of tightly packed cumulus cells surrounding the oocyte in the Graafian follicle. The oocyte is arrested in Meiosis II at the stage of metaphase II and is considered a secondary oocyte. Before ovulation, the cumulus complex goes through a structural change known as cumulus expansion. The granulosa cells transform from tightly compacted to an expanded mucoid matrix. Many studies show that cumulus expansion is critical for the maturation of the oocyte because the cumulus complex is the oocyte’s direct communication with the developing follicle environment. It also plays a significant role in fertilization, though the mechanisms
Oocyte
9213700
divide by mitosis to produce primary oocytes committed to meiosis. Unlike sperm production, oocyte production is not 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
Germline development
2228344
and a large mature egg, both being haploid cells. The polar bodies degenerate. Oocyte maturation stands by at metaphase II in most vertebrates. During ovulation, the arrested secondary oocyte leaves the ovary and matures rapidly into an egg ready for fertilization. Fertilization will cause the egg to complete meiosis II. In human females there is proliferation of the oogonia in the fetus, meiosis starts then before birth and stands by at meiotic division I up to 50 years, ovulation begins at puberty. A 10 - 20 μm large somatic cell generally needs 24 hours to double its mass for mitosis.
Germ cell
10269719
these small antral follicles, 1 will become dominant and ovulate (in monoovulatory species). During ovulation, the primary oocyte will resume meiosis in response to signals, arresting in metaphase meiosis II, ready for fertilization. The dominant follicle contains the mature oocyte. Follicular development is directly under gonadotropins control, LH and FSH. These use cAMP as an intracellular second messenger, with growth factors and cytokines also influencing their development in vivo. Through in vitro maturation, folliculogenesis and latter parts of oogenesis are being mimicked outside of the ovaries- trying to recreate the conditions for these processes. If a follicle has reached the
In vitro maturation
10590088
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 time as a secondary oocyte is measured in days. The secondary oocyte is the largest cell in the body, and in humans is just visible to the naked eye. An "ootid" is the haploid result of ootidogenesis. In oogenesis, it doesn't really have any significance in itself, since it is very similar to the ovum. However, it fills the purpose as the female counterpart
Immature ovum
9637887
the beginning of the process of parthenogenesis in the asexual development of viable offspring. During oocyte development, high metaphase promoting factor (MPF) activity causes mammalian oocytes to arrest at the metaphase II stage until fertilization by a sperm. The fertilization event causes intracellular calcium oscillations, and targeted degradation of cyclin B, a regulatory subunit of MPF, thus permitting the MII-arrested oocyte to proceed through meiosis. To initiate parthenogenesis of swine oocytes, various methods exist to induce an artificial activation that mimics sperm entry, such as calcium ionophore treatment, microinjection of calcium ions, or electrical stimulation. Treatment with cycloheximide, a non-specific
Parthenogenesis
11318761
defects. Co-expression of this mutation with the 677C>T MTHFR polymorphism furthers the risk of spina bifida compared to an independent acting 66A>G mutation. Trisomy 21 or Down syndrome is the most common human chromosomal anomaly arising from abnormal chromosomal segregation in meiosis. The condition can occur during anaphase in meiosis(I) marking oocyte maturation before ovulation and/or during anaphase in meiosis (II) signifying fertilization. Metabolic impact during these stages is furthered by low vitamin B12. Methylation of homocysteine to methionine is affected, primarily by the (MTRR):c.66A>G polymorphism. Chronic homocysteine elevation increases s-adenosyl-L-homocysteine levels, consequently inhibiting methyltransferase activity and promoting DNA hypomethylation.
MTRR (gene)
5472896
meiosis II) only upon fertilization. The oocyte will now travel down one of the fallopian tubes to eventually be discharged through menstruation in the case that it is unfertilized or if it is not successfully implanted in the uterus (if previously fertilized). The ruptured follicle will undergo a dramatic transformation into the corpus luteum, a steroidiogenic cluster of cells that maintains the endometrium of the uterus by the secretion of large amounts of progesterone and minor amounts of estrogen. These two steps, while not part of folliculogenesis, are included for completeness. They are discussed in their entirety by their respective
Folliculogenesis
20840684
oocyte accumulates zinc by over 50% during the 12-16 hours required for maturation of the oocyte to its terminal stage of development before fertilization (MII stage). Second, they showed that this massive increase in the zinc quota during meiotic maturation is necessary to drive meiosis I exit and to establish MII arrest in the mouse egg. Drs. Woodruff and O’Halloran demonstrated this in a number of ways, including induction of zinc insufficiency in maturing oocytes via small molecule chelators. This treatment prevents maturation and results in premature meiotic arrest at telophase I. They next showed the zinc transporters Zip6 and
Teresa Woodruff
14780046
Oocyte selection Oocyte selection is a procedure that is performed prior to in vitro fertilization, in order to use oocytes with maximal chances of resulting in pregnancy. In contrast, embryo selection takes place "after" fertilization. Chromosomal evaluation may be performed. Embryos from rescued in vitro-matured metaphase II (IVM-MII) oocytes show significantly higher fertilization rates and more blastomeres per embryo compared with those from arrested metaphase I (MI) oocytes (58.5% vs. 43.9% and 5.7 vs. 5.0, respectively). Also, morphological features of the oocyte that can be obtained by standard light or polarized light microscopy. However, there is no clear tendency in
Oocyte selection
2678825
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 pairs of sister chromatids resulting from the separation of bivalents in meiosis I are further separated in anaphase of meiosis
Nondisjunction
1351922
is directed into the inner part of the oocyte (cytoplasm). The sperm is then released into the oocyte. The pictured oocyte has an extruded polar body at about 12 o'clock indicating its maturity. The polar body is positioned at the 12 or 6 o'clock position, to ensure that the inserted micropipette does not disrupt the spindle inside the egg. After the procedure, the oocyte will be placed into cell culture and checked on the following day for signs of fertilization. In contrast, in natural fertilization sperm compete and when the first sperm penetrates the oolemma, the oolemma hardens to block
Intracytoplasmic sperm injection
251324
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 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
Meiosis
14780047
recent publications to a general increase in predictive value of morphological features. Suggested techniques include "zona pellucida imaging", which can detect differences in birefringence between eggs, which is a predictor of compaction, blastulation and pregnancy. Potentially, polar body biopsy may be used for molecular analysis, and can be used for preimplantation genetic screening. Oocyte selection Oocyte selection is a procedure that is performed prior to in vitro fertilization, in order to use oocytes with maximal chances of resulting in pregnancy. In contrast, embryo selection takes place "after" fertilization. Chromosomal evaluation may be performed. Embryos from rescued in vitro-matured metaphase II
Oocyte selection
10157073
that has resulted in the most babies born from frozen oocytes worldwide. Ultra-rapid freezing or vitrification represents a potential alternative freezing method. In the fall of 2009, The American Society for Reproductive Medicine (ASRM) issued an opinion on oocyte cryopreservation concluding that the science holds “great promise for applications in oocyte donation and fertility preservation” because recent laboratory modifications have resulted in improved oocyte survival, fertilization, and pregnancy rates from frozen-thawed oocytes in IVF. The ASRM noted that from the limited research performed to date, there does not appear to be an increase in chromosomal abnormalities, birth defects, or developmental
Oocyte cryopreservation
9637849
of anaphase II are joined together. The criterion for "sexuality" varies from all cases of restitutional meiosis, to those where the nuclei fuse or to only those where gametes are mature at the time of fusion. Those cases of automixis that are classified as sexual reproduction are compared to self-fertilization in their mechanism and consequences. The genetic composition of the offspring depends on what type of apomixis takes place. When endomitosis occurs before meiosis or when "central fusion" occurs (restitutional meiosis of anaphase I or the fusion of its products), the offspring get all to more than half of the
Parthenogenesis
6713622
as "Science", "Nature", "Lancet", "Nature Medicine", "Nature Cell Biology", "Journal of Cell Biology", "Genetics", "Development", "Developmental Biology", "Biology of Reproduction", "Human Reproduction", and the "Journal of Law, Medicine and Ethics". Schatten’s work on fertilization examines the differential inheritance of cellular components contributed by the sperm and egg, respectively, as well as the program of oocyte activation and cell division during meiosis and mitosis. His group has demonstrated the importance of the sperm centrosome-centriole complex during mammalian fertilization (including humans), with the unexpected exception of rodents in which the centrosome is of maternal origin (see Selected Publications). Schatten also made contributions
Gerald Schatten
7489087
of the mature sperm (without flagellum), and ii) once released, it acts as hormone on the female germ cells, where it triggers oocyte maturation and stimulates the oviduct wall to contract to bring the oocytes into position for fertilization. MSP has first been identified in "Caenorhabditis elegans". Molecular structures of MSP from "Ascaris suum" and "Caenorhabditis elegans" have been determined by X-ray crystallography and NMR spectroscopy. MSP molecules from these species share 83% sequence identity and their structures are highly similar. MSP does not harbor any known conserved domain. It is made of a seven-stranded β sandwich, having opposing three-stranded
Major sperm protein
251350
by mitosis, developing into the haploid gametophyte generation, which then gives rise to gametes directly (i.e. without further meiosis). In both animals and plants, the final stage is for the gametes to fuse, restoring the original number of chromosomes. In females, meiosis occurs in cells known as oocytes (singular: oocyte). Each primary oocyte divides twice in meiosis, unequally in each case. The first division produces a daughter cell, and a much smaller polar body which may or may not undergo a second division. In meiosis II, division of the daughter cell produces a second polar body, and a single haploid
Meiosis
5472885
follicle, belongs to the stage of ootidogenesis of oogenesis. In addition, follicles that have formed an antrum are called antral follicles or Graafian follicles. Definitions differ in where this shift occurs in the staging given above, with some stating that it occurs when entering the "secondary" stage, 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
Folliculogenesis
16674446
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 XXXY zygote. The additional X chromosomes that are characteristic of this condition are associated with an
XXXY syndrome
251351
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 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,
Meiosis
16674445
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 zygote. In the case where the oocyte is
XXXY syndrome
1406161
tissue at the site of the blister, forming a hole called the "stigma". The secondary oocyte leaves the ruptured follicle and moves 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
Ovulation
2942308
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 sac and leads to the formation of a single egg cell per ovule. In "ascaris", the oocyte does not even begin meiosis until the sperm touches it, in contrast to mammals, where meiosis is completed in the estrus cycle. Oogenesis Oogenesis, ovogenesis, or oögenesis is the
Oogenesis
2942301
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, instead, to the common process of gametogenesis, which, in the female human, begins with the processes of folliculogenesis, oocytogenesis, and ootidogenesis. Oogenesis starts with the process of developing primary oocytes, which occurs via the transformation of oogonia into primary oocytes, a process called oocytogenesis. Oocytogenesis is complete
Oogenesis
10269717
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 follicles, and by puberty have around 300,000. Of these primordial follicles, only around 400 mature oocytes are released and could be potentially fertilised,
In vitro maturation
5472895
time between selection of a tertiary follicle and its subsequent growth into a preovulatory follicle.The actual time for development of a follicle varies. " By the end of the follicular (or proliferative) phase of the thirteenth day of the menstrual cycle, the cumulus oophorus layer of the preovulatory follicle will develop an opening, or "stigma", and excrete the oocyte with a complement of cumulus cells in a process called ovulation. The oocyte is technically still a secondary oocyte, suspended in the metaphase II of meiosis. It will develop into an ootid, and rapidly thereafter into an ovum (via completion of
Folliculogenesis
10269718
with the rest undergoing atresia. 'Maturation' of an oocyte is the process by which an 'oocyte attains the competence to be fertilised and undergo embryogenesis'. Folliculogenesis is the mechanism by which the ovarian follicles mature. This can take many months in vivo and involves primordial follicle growth and differentiation. Primordial follicles containing the primary oocyte, arrested at prophase of meiosis I, develop into primary follicle containing cuboidal granulosa cells. A secondary follicle is formed with a few granulosa cell layers, as well as a theca layer. Finally before ovulation, a tertiary follicle is formed containing a follicular-fluid filled antrum. Of
In vitro maturation
17753617
artificially increased. Increased calcium is also thought to activate actin-depolymerizing proteins such as gelsolin and scinderin. In mammals, these actin-depolymerizing proteins serve to disassemble cortical actin thereby allowing space for cortical granule translocation toward the plasma membrane. An oocyte acquires the ability to complete cortical granule exocytosis by the time the oocyte has reached late maturity. More specifically, in mice, for example, the ability to undergo cortical granule exocytosis arises some time between metaphase I and metaphase II of meiosis, which is also 5 hours before ovulation occurs. The oocyte has been shown to obtain maximum proficiency for releasing calcium
Cortical granule
2942303
cells (originating from bone marrow and peripheral blood) has been reported in the postnatal mouse ovary. In contrast, DNA clock measurements do not indicate ongoing oogenesis during human females' lifetimes. Thus, further experiments are required to determine the true dynamics of small follicle formation. The succeeding phase 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
Oogenesis
2942304
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 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
Oogenesis
10590087
specifically in prophase I of meiosis. The halt is called dictyate. Most degenerate by atresia, but a few go through ovulation, and that's the trigger to the next step. Thus, an immature ovum can spend up to ~55 years as a primary oocyte (the last ovulation before menopause). The secondary oocyte is the cell 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
Immature ovum
5099316
to an increase in Ca2+ levels that will activate CaMKII which will degrade MPF, leading to the resumption of meiosis. The increased Ca levels will induce the exocytosis of cortical granules that degrade ZP receptors, used by sperm to penetrate the oocyte, blocking polyspermy. Deregulation of these pathways will lead to several diseases like, oocyte maturation failure syndrome which results in infertility. Increasing our molecular knowledge of oocyte development mechanisms could improve the outcome of assisted reproduction procedures, facilitating conception. Spermatozoon is the male gamete. After ejaculation this cell is not mature, so it can't fertilize the oocyte. To have
Biochemical cascade
5735244
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 in the lining of the female ovaries of humans, primates and mice.
Oogonium
5368413
the asymmetry of meiosis in females: the driving allele ends up in the oocyte instead of in the polar bodies with a probability greater than one half. This is termed true meiotic drive, as it does not rely on a post-meiotic mechanism. The best-studied examples include the neocentromeres (knobs) of maize, as well as several chromosomal rearrangements in mammals. The general molecular evolution of centromeres is likely to involve such mechanisms. The Medea gene causes the death of progeny from a heterozygous mother that do not inherit it. It occurs in the flour beetle ("Tribolium castaneum"). Maternal-effect selfish genes have
Intragenomic conflict
5099314
KIT in the oocyte, leading to the activation of PI3K/Akt pathway, allowing oocyte survival and development. During embryogenesis, oocytes initiate meiosis and stop in prophase I. This arrest is maintained by elevated levels of cAMP within the oocyte. It was recently suggested that cGMP cooperates with cAMP to maintain the cell cycle arrest. During meitotic maturation, the LH peak that precedes ovulation activates MAPK pathway leading to gap junction disruption and breakdown of communication between the oocyte and the follicular cells. PDE3A is activated and degrades cAMP, leading to cell cycle progression and oocyte maturation. The LH surge also leads
Biochemical cascade
466568
(a haploid secondary oocyte with replicate chromosome copies) and a haploid sperm cell (male gamete)—combine to form a single 2n diploid cell called the zygote. Once the single sperm enters the oocyte, it completes the division of the second meiosis forming a haploid daughter with only 23 chromosomes, almost all of the cytoplasm, and the sperm in its own pronucleus. The other product of meiosis is the second polar body with only chromosomes but no ability to replicate or survive. In the fertilized daughter, DNA is then replicated in the two separate pronuclei derived from the sperm and ovum, making
Zygote
10590086
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, one secondary oocyte and one polar body is created. Primary oocytes have been created in late fetal life. This is the stage where immature ova spend most of their lifetime, more
Immature ovum
5022160
gives rise to four spermatids through meiosis. 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 are
Reproductive biology
2228347
small molecules, no macromolecules, but eventually their smaller precursor molecules, by gap junctions. The mutation frequency of female germline cells is about 5-fold lower than that of somatic cells. The mouse oocyte in the dictyate (prolonged diplotene) stage of meiosis actively repairs DNA damage, whereas DNA repair was not detected in the pre-dictyate (leptotene, zygotene and pachytene) stages of meiosis. The long period of meiotic arrest at the four chromatid dictyate stage of meiosis may facilitate recombinational repair of DNA damages. Mammalian spermatogenesis is representative for most animals. In human males, spermatogenesis begins at puberty in seminiferous tubules in the
Germ cell
3208480
Anovulation Anovulation is when the ovaries do not release an oocyte during a menstrual cycle. Therefore, ovulation does not take place. However, a woman who does not ovulate at each menstrual cycle is not necessarily going through menopause. Chronic anovulation is a common cause of infertility. In addition to the alteration of menstrual periods and infertility, chronic anovulation can cause or exacerbate other long term problems, such as hyperandrogenism or osteopenia. It plays a central role in the multiple imbalances and dysfunctions of polycystic ovary syndrome. During the first two years after menarche 50% of the menstrual cycles could be
Anovulation
3208505
In contrast, anovulation usually manifests itself as irregularity of menstrual periods, that is, unpredictable variability of intervals, duration, or bleeding. Anovulation can also cause cessation of periods (secondary amenorrhea) or excessive bleeding (dysfunctional uterine bleeding). Mittelschmerz and premenstrual symptoms tend to be absent or reduced when a woman is anovulatory. Anovulation Anovulation is when the ovaries do not release an oocyte during a menstrual cycle. Therefore, ovulation does not take place. However, a woman who does not ovulate at each menstrual cycle is not necessarily going through menopause. Chronic anovulation is a common cause of infertility. In addition to the
Anovulation
17753611
of translocation, cortical granules are evenly distributed throughout the cortex of the oocyte. However, it has been observed in rodents that some cortical granules are rearranged leaving a space amidst the remaining cortical granules. This space is called the cortical granule free domain (CGFD) and has been observed in both the cell's meiotic spindle regions during metaphase I and metaphase II of meiosis. CGFDs have not been observed in feline, equine, bovine, porcine, nor human oocytes. Studies with rodent oocytes suggest that certain cortical granules undergo redistribution and/or exocytosis throughout the meiotic cycle thus establishing the CGFDs. More specifically, evidence
Cortical granule
3042003
and thus control their ability to function. In 1971, two independent teams of researchers (Yoshio Masui and Clement Markert, as well as Dennis Smith and Robert Ecker) found that frog oocytes arrested in G could be induced to enter M phase by microinjection of cytoplasm from oocytes that had been hormonally stimulated with progesterone. Because the entry of oocytes into meiosis is frequently referred to as oocyte maturation, this cytoplasmic factor was called maturation promoting factor (MPF). Further studies showed, however, that the activity of MPF is not restricted to the entry of oocytes into meiosis. To the contrary, MPF
Maturation promoting factor
13599678
Wolffian ducts) develops in males into the epididymis and vas deferens. Normally, only one of the pairs of tubes will develop while the other regresses and disappears "in utero". The homologous organ in the male is the rudimentary appendix testis. The tube allows passage of the egg from the ovary to the uterus. When an oocyte is developing in an ovary, it is encapsulated in a spherical collection of cells known as an ovarian follicle. Just prior to ovulation the primary oocyte completes meiosis I to form the first polar body and a secondary oocyte which is arrested in metaphase
Fallopian tube
2214052
with a set of cells called nurse cells. Both the oocyte and the nurse cells are descended from a single germline stem cell, however cytokinesis is incomplete in these cell divisions, and the cytoplasm of the nurse cells and the oocyte is connected by structures known as ring canals. Only the oocyte undergoes meiosis and contributes DNA to the next generation. Many maternal effect "Drosophila" mutants have been found that affect the early steps in embryogenesis such as axis determination, including "bicoid, dorsal, gurken" and "oskar". For example, embryos from homozygous "bicoid" mothers fail to produce head and thorax structures.
Maternal effect
251337
meiosis is resumed to prepare the oocyte for ovulation, which happens at puberty or even later. Chromosomes condense further during 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 the meiotic spindle begins to form. During these stages, two centrosomes, containing a pair of centrioles in animal
Meiosis
304060
removing DNA damages, especially double-strand breaks, from germ line DNA. (see Meiosis and Origin and function of meiosis). Homologous recombinational repair is especially promoted during meiosis. Titus et al. also found that expression of 4 key genes necessary for homologous recombinational repair of DNA double-strand breaks (BRCA1, MRE11, RAD51 and ATM) decline with age in the oocytes of humans and mice. They hypothesized that DNA double-strand break repair is vital for the maintenance of oocyte reserve and that a decline in efficiency of repair with age plays a key role in ovarian aging. Ovarian diseases can be classified as endocrine
Ovary
2088524
the same in mitosis as they do in meiosis. Prior to every single mitotic division a cell undergoes, the chromosomes in the parent cell replicate themselves. The homologous chromosomes within the cell will ordinarily not pair up and undergo genetic recombination with each other. Instead, the replicants, or sister chromatids, will line up along the metaphase plate and then separate in the same way as meiosis II - by being pulled apart at their centromeres by nuclear mitotic spindles. If any crossing over does occur between sister chromatids during mitosis, it does not produce any new recombinant genotypes. Homologous pairing
Homologous chromosome
20456206
co-author, with Harvey Lodish, of the book "Model Organisms: Drosophila," published in 1997 by Academic Press. Terry Orr-Weaver Terry L. Orr-Weaver is an American molecular biologist in the MIT Department of Biology with a joint appointment to the Whitehead Institute. She does research on developmental biology, with a focus on "[c]oordination of cell growth and division with development, with particular focus on the oocyte-to-embryo transition, control of cell size, and regulation of metazoan DNA replication." Orr-Weaver and her collaborators have identified two proteins necessary for the proper sorting of chromosomes during meiosis with implications for cancer and birth defects. In
Terry Orr-Weaver
20456203
Terry Orr-Weaver Terry L. Orr-Weaver is an American molecular biologist in the MIT Department of Biology with a joint appointment to the Whitehead Institute. She does research on developmental biology, with a focus on "[c]oordination of cell growth and division with development, with particular focus on the oocyte-to-embryo transition, control of cell size, and regulation of metazoan DNA replication." Orr-Weaver and her collaborators have identified two proteins necessary for the proper sorting of chromosomes during meiosis with implications for cancer and birth defects. In 2006 she was elected to the National Academy of Sciences. Terry Orr-Weaver received her PhD in
Terry Orr-Weaver
43645
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, in which the cell breaks down complex molecules to produce energy and reducing power, and
Cell (biology)
14408705
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 place consecutively during meiosis. Cdc14 is widely distributed, being found in most eukaryote kingdoms. However, it is not found in all species based on searches of sequenced genomes. One or more Cdc14 genes are found in alveolates, animals, fungi, trypanosomes, and lower plants. However, Cdc14 genes
Cdc14
5022161
born with all the primary oocytes they will ever have. Starting at puberty the process of meiosis can complete resulting in the secondary oocyte and the first polar body. The secondary oocyte can later be fertilized with the male sperm. Reproductive biology Reproductive biology includes both sexual and asexual reproduction. Reproductive biology includes a wide number of fields: Human reproductive biology is primarily controlled through hormones, which send signals to the human reproductive structures to influence growth and maturation. These hormones are secreted by endocrine glands, and spread to different tissues in the human body. In humans, the pituitary gland
Reproductive biology
10590085
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 both pairs of homologous chromosomes. There are 23 chromosome pairs. Thus, there are 46 chromosomes. Each chromosome, however, hasn't yet duplicated itself. As such, there is only one chromatid on each chromosome, making
Immature ovum
5735233
al., 2015. The mechanisms for male and female differentiation are markedly different, since a population of sperm producing spermatogonia are retained throughout development and into adult life Reitzel et al., 2015, unlike oogonia which only produce oocyte in utero. When the germ cells reach the gonads, they undergo proliferation via mitosis and at 13.5 days of rat development they begin to undergo meiosis in the ovary but arrested at the mitotic stage in the testes. In the ovary, after mitosis, the gametogonium undergo meiosis, which is initiated by intrinsic competence factor DazL and extrinsic retinoic acid, excreted by the mesonephros.
Gametogonium
2942300
Oogenesis Oogenesis, ovogenesis, or oögenesis is the differentiation of the ovum (egg cell) into a cell competent to further development 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 is a separate sub-process that accompanies and supports all three oogenetic sub-processes. Oogonium —(Oocytogenesis)—> Primary Oocyte —(Meiosis I)—>
Oogenesis
6309956
the purging of the detrimental. Following synapsis, a type of recombination referred to as synthesis dependent strand annealing (SDSA) occurs frequently. SDSA recombination involves information exchange between paired non-sister homologous chromatids, but not physical exchange. SDSA recombination does not cause crossing-over. Both the non-crossover and crossover types of recombination function as processes for repairing DNA damage, particularly double-strand breaks (see Genetic recombination). The central function of synapsis is therefore the identification of homologues by pairing, an essential step for a successful meiosis. The processes of DNA repair and chiasma formation that take place following synapsis have consequences at many levels,
Synapsis
251354
of the male testis suppress meiosis by degrading retinoic acid, a stimulator of meiosis. This is overcome at puberty when cells within seminiferous tubules called Sertoli cells start making their own retinoic acid. 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 in the production of gametes which have either too many or too few of a particular chromosome, and is a common mechanism for trisomy or monosomy. Nondisjunction can occur in the meiosis I or meiosis II, phases of
Meiosis
6456199
Rather, one homologous segment "writes over" the other. The mechanism is presumed to be different from RecLOH events in autosomal chromosomes, since the target is the very same chromosome instead of the homologous one. During the mutation one of these copies overwrites the other. Thus the differences between the two are lost. Because differences are lost, heterozygosity is lost. Recombination on the Y-chromosome does not only take place during meiosis, but virtually at every mitosis when the Y chromosome condenses, because it doesn't require pairing between chromosomes. Recombination frequency even exceeds the frame shift mutation frequency (slipped strand mispairing) of
RecLOH
251328
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 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
Meiosis
10157075
was published. It compared vitrification (the newest technology) versus slow freezing (the oldest one). Key results of that review showed that the clinical pregnancy rate was almost 4 times higher in the oocyte vitrification group than in the slow freezing group, with moderate quality of evidence. Immature oocytes have been grown until maturation "in vitro" at a 10% survival rate, but no experiment has been performed to fertilize such oocytes. The cost of the egg freezing procedure (without embryo transfer) in the United States, the United Kingdom and other European countries varies in between $5,000 and $12,000. This does not
Oocyte cryopreservation
251340
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 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 requires at least one crossover per chromosome
Meiosis
1539878
germ cells. However, it was not until a few years later that maiosis was renamed "meiosis" to better embody the Greek definition, "to reduce" or "diminish". Meiosis typically works to diminish the importance of something or someone in order to simultaneously heighten something else in its place. Such a comparative approach is necessary for the effectiveness of the device. An example of this device occurs in the New Testament where Paul belittles himself in order to emphasize the power of God: For I am the least of all the apostles and do not even deserve to be called an apostle,
Meiosis (figure of speech)
157510
recombination) occurring in the diploid primary oocyte to produce the haploid ovum (gamete). Spermatogenesis is the process of male gamete formation in animals. This process also involves meiosis (including meiotic recombination) occurring in the diploid primary spermatocyte to produce the haploid spermatozoon (gamete). In contrast to a gamete, the diploid somatic cells of an individual contain one copy of the chromosome set from the sperm and one copy of the chromosome set from the egg cell; that is, the cells of the offspring have genes expressing characteristics of both the "father" and the "mother". A gamete's chromosomes are not exact
Gamete
14022041
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. Meiotic recombination checkpoint The meiotic recombination checkpoint monitors the meiotic recombination during meiosis, and blocks the entry into metaphase I if the recombination is not efficiently processed. Meiotic recombination contributes to the cells in three different ways. First, to achieve proper segregation, each pair of homologous chromosomes must be linked to each other to maintain a certain level of tension between
Meiotic recombination checkpoint
16959106
nonreciprocal genetic transfer takes place between two homologous sequences. Assembly of a nucleoprotein filament comprising single-stranded DNA (ssDNA) and the RecA homolog, Rad51, is a key step necessary for homology search during recombination. In the budding yeast "Saccharomyces cerevisiae", Srs2 translocase dismantles Rad51 filaments during meiosis. By directly interacting with Rad51, Srs2 dislodges Rad51 from nucleoprotein filaments thereby inhibiting Rad51-dependent formation of joint molecules and D-loop structures. This dismantling activity is specific for Rad51 since Srs2 does not dismantle DMC1 (a meiosis-specific Rad51 homolog), Rad52 (a Rad 51 mediator) or replication protein A (RPA, a single-stranded DNA binding protein). Srs2
Synthesis-dependent strand annealing
2088523
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 without another round of chromosomal replication. The sister chromatids in the two daughter cells are pulled apart during anaphase II by nuclear spindle fibers, resulting in four haploid daughter cells. Homologous chromosomes do not function
Homologous chromosome
251327
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 three phases: Interphase is followed by meiosis I and then meiosis II. Meiosis I separates 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
Meiosis
17753609
preimplantation. Once the cortical granules complete their functions, the oocyte does not replenish them. Cortical granule formation occurs during the early stages of oocyte growth. More specifically, in the human, monkey, hamster, and rabbit, cortical granules are established once the ovarian follicle is multilayered. In the rat and mouse, cortical granules have been observed earlier in follicle development when the ovarian follicle is only single layered. During the early stages of oocyte growth, the Golgi complex increases in size, proliferates, and produces small vesicles that migrate to the cell's subcortical region. These small vesicles will fuse with one another to
Cortical granule
5500368
and the chromosomes do not condense during mitosis. The basal body, unlike the centrosome of most eukaryotic cells, does not play a role in the organisation of the spindle and instead is involved in division of the kinetoplast. The events of reproduction are: In the 1980s, DNA analyses of the developmental stages of "T. brucei" started to indicate that the trypomastigote in the tsetse fly undergoes meiosis, i.e. a sexual reproduction stage. But it is not always necessary for a complete life cycle. The existence of meiosis-specific proteins was reported in 2011. The haploid gametes (daughter cells produced after meiosis)
Trypanosoma brucei
19419760
a Class II pericentric interstitial deletion, followed by a complex rearrangement. The neocentromere can either appear on the linear chromosome or on the ring chromosome, depending on which ever one lacks a centromere. For Class II, it is unclear when the rearrangement occurs. General postulations include the chromosome breaking twice, with the ends rejoining. The alternative argument is that during meiosis I, looping and homologous recombination within a sister chromatid can cause this rearrangement. A rough estimation approximates that neocentromere formation on inverted duplicated chromosomes happens every 70,000–200,000 live births. However, this statistic does not include Class II rearrangements. Human
Neocentromere
6331230
takes place, preventing other sperm from fertilizing the same egg. The oocyte now undergoes its second meiotic division producing the haploid ovum and releasing a polar body. The sperm nucleus then fuses with the ovum, enabling fusion of their genetic material. The fusion of cell membranes of the secondary oocyte and sperm takes place. In preparation for the fusion of their genetic material both the oocyte and the sperm undergo transformations as a reaction to the fusion of cell membranes. The oocyte completes its second meiotic division. This results in a mature ovum. The nucleus of the oocyte is called
Human fertilization
2999707
to mark the wild-type chromosome as above, GAL80 serves this purpose, so that when it is removed by mitotic recombination, GAL4 is allowed to function, and GFP turns on. This results in the cells of interest being marked brightly in a dark background. In 1929, Alfred Sturtevant studied mosaicism in "Drosophila". A few years later, In the 1930s, Curt Stern demonstrated that genetic recombination, normal in meiosis, can also take place in mitosis. When it does, it results in somatic (body) mosaics. These are organisms which contain two or more genetically distinct types of tissue. The term "somatic mosaicism" was
Mosaic (genetics)
11013184
metaphase I and anaphase I in the meiosis diagram. Different pairs of chromosomes segregate independently of each other, a process termed “independent assortment of non-homologous chromosomes”. This process results in each gamete usually containing a mixture of chromosomes from both original parents. Improper chromosome segregation can result in aneuploid gametes having either too few or too many chromosomes. The second stage at which segregation occurs during meiosis is prophase II (see meiosis diagram). During this stage, segregation occurs by a process similar to that during mitosis, except that in this case prophase II is not preceded by a round of
Chromosome segregation
2088518
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 it takes more time for the chromatin to replicate and for the homologous chromosomes to be properly oriented and segregated by the processes of pairing and synapsis in meiosis I. During meiosis, genetic recombination (by random segregation) and crossing over produces daughter cells that each contain different combinations of maternally and paternally coded genes. This recombination of genes allows for the introduction of new allele pairings and genetic variation. Genetic variation among organisms
Homologous chromosome
19755060
inconclusive as they did not have oocyte markers and thus could not say for sure that these cells were a part of the stem cell population. A number of scientists have since then used mathematical models to suggest that, without an oocyte stem cell (OSC) population, the female mammal will not have enough oocytes to complete their reproductive lives due to rate of atresia during the normal cycle is significant. However, in 2004, new research by Jonathan Tilly and colleagues came about to suggest that a new population of stem cells in female mammals does exist, which could possibly be
Oogonial stem cells
11622120
REC8 Meiotic recombination protein REC8 homolog is a protein that in humans is encoded by the "REC8" gene. Rec8 is a meiosis-specific component of the cohesin complex that binds sister chromatids in preparation for the two divisions of meiosis. Rec8 is sequentially removed from sister chromatids. It is removed from the arms of chromosomes in the first division - separating homologous chromosomes from each other. However, Rec8 is maintained at centromeres so that sister chromatids are kept joined until anaphase of meiosis II, at which point removal of remaining cohesin leads to the separation of sister chromatids. This gene encodes
REC8
11331240
viability patterns suggesting that segregation of non-exchange chromosomes occurred efficiently. Thus, in "S. cerevisiae", proper segregation apparently does not entirely depend on crossovers between homologous pairs. The "him-14" gene of the worm "Caenorhabditis elegans" encodes an ortholog of "MSH4". Formation of crossovers during "C. elegans" meiosis requires the "him-14(MSH4)" gene. Loss of him-14(MSH-4) function severely reduces crossing over, resulting in lack of chiasmata between homologs and consequent missegregation. Thus, in "C. elegans", segregation apparently does depend on crossovers between homologous pairs. Him-14(MSH4) functions during the pachytene stage of meiosis, indicating that it is not needed for establishing the preceding stages
MSH4
2228343
ribosomes, and the mRNA needed for protein synthesis during early embryonic growth. These intensive RNA biosynthese are mirrored in the structure of the chromosomes, which decondense and form lateral loops giving them a lampbrush appearance (see Lampbrush chromosome). Oocyte maturation is the following phase of oocyte development. It occurs at sexual maturity when hormones stimulate the oocyte to complete meiotic division I. The meiotic division I produces 2 cells differing in size: a small polar body and a large secondary oocyte. The secondary oocyte undergoes meiotic division II and that results in the formation of a second small polar body
Germ cell
2678826
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. Division of somatic cells through mitosis is preceded by replication of the genetic material in S phase. As a result, each chromosome consists of two sister chromatids held together at
Nondisjunction
19974016
Fusome The fusome is a membranous structure found in the developing germ cell cysts of many insect orders. Initial description of the fusome occurred in the 19th century and since then the fusome has been extensively studied in "Drosophila melanogaster" male and female germline development. This structure has roles in maintaining germline cysts, coordinating the number of mitotic divisions prior to meiosis, and oocyte determination by serving as a structure for intercellular communication. In "D. melanogaster", germline cysts form from four mitotic divisions with incomplete cytokinesis that originated from one germline stem cell. Incomplete cytokinesis results in intercellular bridges connecting
Fusome
5472901
are accurately repaired during meiosis by searching for, and building off of, the matching strand (termed “homologous recombinational repair”). Titus et al. (2013) found that, as humans (and mice) age, expression of four key DNA repair genes necessary for homologous recombinational repair declines in oocytes. They hypothesized that DNA double-strand break repair is vital for the maintenance of oocyte reserve, and that a decline in efficiency of repair with age plays a key role in the depletion of the ovarian reserve (ovarian aging). Folliculogenesis In biology, folliculogenesis is the maturation of the ovarian follicle, a densely packed shell of somatic
Folliculogenesis
4905873
its secondary oocyte arrested in second meiosis. Thecal shape is also important in graptolite taxonomy. In microbiology and planktology, a theca is a subcellular structural component out of which the frustules of diatoms and dinoflagellates are constructed. Theca A theca (plural thecae) refers to a sheath or a covering. In botany,the theca is related to plant's flower anatomy.The theca of an angiosperm consists of a pair of microsporangia that are adjacent to each other and share a common area of dehiscence called the stomium. Any part of a microsporophyll that bears microsporangia is called an anther. Most anthers are formed
Theca
19054648
eggs while the females are produced as diploids from fertilized eggs. According to this practice, "M. uniraptor" would normally produce both male and female offspring. However, in "M. uniraptor", "Wolbachia" causes thelytoky, a type of parthenogenesis in which females asexually reproduce only female offspring. "M. uniraptor" generally follows the process of automixis, or meiotic thelytoky, where meiosis occurs in the developing oocyte and diploidy is restored by fusing the meiotic or mitotic products yielding diploid females or haploid males. However, the bacterium, "Wolbachia", has evolved with this species of wasp through vertical transfer to induce irreversible thelytokous reproduction of only
Muscidifurax uniraptor
3367086
thus attempts at meiosis in the plant's ovules cannot produce a viable gamete. Only rarely does the first reduction division in meiosis in the plants' flowers tidily fail completely, resulting in a euploid triploid ovule, which can be fertilized by normal haploid pollen from a diploid banana variety; a whole stem of bananas would contain only a few seeds and sometimes none. As a result, the resulting new banana variety is tetraploid, and thus contains seeds; the market for bananas is not accustomed to bananas with seeds. Experience showed that where both meiosis steps failed, causing a heptaploid seedling, or
Panama disease
7594152
the fertilising sperm allows the oocyte to resume meiosis and the polar body is extruded. Pronuclear embryo. Two separate haploid components are present - the maternal and paternal pronuclei. The pronunclei move towards each other and eventually compress their envelopes where they lie adjacent near the centre of the wall. Syngamic embryo. The last phase of fertilisation. The pronuclear envelopes disappear and the parental chromosomes come together in a process called syngamy. Carnegie stage 2 begins when the zygote undergoes its first cell division, and ends when the blastocyst forms a cavity and comprises more than 16 cells. At this
Carnegie stages
15827891
very short or non-existent. During the remaining months of the year it gradually lengthens once more. These changes take place over several generations as each individual rotifer has a lifespan of just a few days or weeks. Periodically sexual reproduction takes place. It is not known why this occurs at some times and not others. The female undergoes meiosis and produces eggs with half the usual number of chromosomes. Some of these develop into male rotifers. Each of these is able to inseminate another of these eggs to restore the full complement of chromosomes in the offspring. Keratella cochlearis Keratella
Keratella cochlearis
251343
of sister chromatids. Mechanically, the process is similar to mitosis, though its genetic results are fundamentally different. The end result is 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 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
Meiosis
765892
Rad 51 protein levels. These findings suggest that ciliates such as "T. thermophila" utilize a Rad51-dependent recombinational pathway to repair damaged DNA. The Rad51 recombinase of "T. thermophila" is a homolog of the "Escherichia coli" RecA recombinase. In "T. thermophila", Rad51 participates in homologous recombination during mitosis, meiosis and in the repair of double-strand breaks. During conjugation, Rad51 is necessary for completion of meiosis. Meiosis in "T. thermophila" appears to employ a Mus81-dependent pathway that does not use a synaptonemal complex and is considered secondary in most other model eukaryotes. This pathway includes the Mus81 resolvase and the Sgs1 helicase.
Tetrahymena
7514233
they allow the X and Y chromosomes to pair and properly segregate during meiosis in males. Pseudoautosomal genes are found in two different locations: PAR1 and PAR2. These are believed to have evolved independently. in mice, some PAR1 genes have transferred to autosomes. Pairing (synapsis) of the X and Y chromosomes and crossing over (recombination) between their pseudoautosomal regions appear to be necessary for the normal progression of male meiosis. Thus, those cells in which X-Y recombination does not occur will fail to complete meiosis. Structural and/or genetic dissimilarity (due to hybridization or mutation) between the pseudoautosomal regions of the
Pseudoautosomal region
11328549
(BUB1/BUB3 complex) to inhibit the anaphase-promoting complex or cyclosome (APC/C) as soon as the spindle-assembly checkpoint is activated. BUB3 also phosphorylates: Another function of BUB3 is to promote correct kinetochore-microtubule (K-MT) attachments when the spindle-assembly checkpoint is active. It plays a role in the localization of kinetochore of BUB1. BUB3 serves in oocyte meiosis as the regulator of chromosome segregation. Defects in BUB3 in the cell cycle can contribute to the following diseases: BUB3 Mitotic checkpoint protein BUB3 is a protein that in humans is encoded by the "BUB3" gene. Bub3 is a protein involved with the regulation of the
BUB3
7546412
humans. In addition to BRCA1, the MCM8-MCM9 protein complex also plays a crucial role in the recombinational repair of DNA double-strand breaks. In humans, an MCM8 mutation can give rise to premature ovarian failure, as well as chromosomal instability. MCM9, as well as MCM8, mutations are also associated with ovarian failure and chromosomal instability. The MCM8-MCM9 complex is likely required for the homologous recombinational repair of DNA double-strand breaks that are present during the pachytene stage of meiosis I. In women homozygous for MCM8 or MCM9 mutations, failure to repair breaks apparently leads to oocyte death and small or absent
Premature ovarian failure
14022040
CHEK2 regulates cell cycle progression and spindle assembly during mouse oocyte maturation and early embryo development. Although CHEK2 is a down stream effector of the ATM kinase that responds primarily to double-strand breaks it can also be activated by ATR (ataxia-telangiectasia and Rad3 related) kinase that responds primarily to single-strand breaks. In mouse, 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
Meiotic recombination checkpoint
8101230
becomes a key plot point. In Act II of the opera, Frederic promises to claim his bride on his 21st birthday, which will not take place until 1940. Lely thus died one "birthday" after that. In 2007, Simon Moss revealed a newly discovered pamphlet called "Mr Durward Lely, The Eminent Tenor". The 20-page booklet describes Lely's career. It was first published in three installments in March and April 1894 in "The Blairgowrie Advertiser" and then issued in booklet form. Although the pamphlet does not credit an author, it is patent that it is self-written. Lely praises Gilbert as a director.
Durward Lely
6309953
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 protein-RNA complex called the synaptonemal complex. Autosomes undergo synapsis during meiosis, and are held together
Synapsis