Meiosis ( Ã, ( listen ) ; from the Greek ???????, meiosis , which means subtract) is a special type of cell division that reduces the number of half chromosomes, creates four haploid cells, each genetically different from the stem cells that gave rise to them. This process occurs in all sexual reproduction of single-celled and multicellular eukaryotes, including animals, plants, and fungi. The errors in meiosis that result in aneuploidy are the major causes of known miscarriage and the most frequent genetic cause of developmental defects.
In meiosis, DNA replication is followed by two rounds of cell division to produce four child cells, each with half the number of chromosomes as the original stem cell. Two meiotic divisions are known as Meiosis I and Meiosis II. Before the meiosis begins, during phase S of the cell cycle, each DNA of the chromosome is replicated so that it consists of two identical sister chromatids, which remain united through the sister chromatid cohesion. S-phase can be referred to as "S-phase premeiotik" or "phase meiotik S". Immediately after DNA replication, meiotic cells enter the G2 stage as it is known as meiotic prophase. During this time, homologous chromosomes pair up with each other and undergo genetic recombination, a preprogrammed process in which DNA is cut and then repaired, allowing them to exchange some of their genetic information. A subset of recombination events produces a crossover, which creates a physical link known as chiasmata (singular: chiasma, for the Greek letter Chi (X)) between the homologous chromosomes. In most organisms, this relationship is important for directing each pair of homologous chromosomes to separate from each other during Meiosis I, producing two haploid cells that have half the number of chromosomes as stem cells. During Meiosis II, cohesion between chromatids is released and they separate from each other, such as during mitosis. In some cases, the four meiotic products form gametes such as sperm, spores, or pollen. In female animals, three of the four meiotic products are usually eliminated by extrusion into the polar body, and only one cell develops to produce an egg.
Since the number of chromosomes is halved during meiosis, the gametes can melt (ie fertilization) to form a diploid zygote containing two copies of each chromosome, one from each parent. Thus, the alternating cycle of meiosis and fertilization allows sexual reproduction, with successive generations maintaining the same number of chromosomes. For example, diploid human cells contain 23 pairs of chromosomes including 1 pair of sex chromosomes (46 total), half of maternal origin and half of paternal origin. Meiosis produces a haploid gamete (ovum or sperm) containing a set of 23 chromosomes. When two gametes (eggs and sperm) melt, the resulting zygote is once again diploid, with the mother and father each contributing 23 chromosomes. This same pattern, but not the same number of chromosomes, occurs in all organisms that utilize meiosis.
Video Meiosis
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Although the meiotic process is associated with a more common process of cell division than mitosis, it differs in two important ways:
Meiosis begins with a diploid cell, which contains two copies of each chromosome, called homologous. First, the cell undergoes DNA replication, so every homologue now consists of two identical sister chromatids. Then each pair of homologues pair up and exchange DNA with homologous recombination leading to a physical connection (cross) between homologs. In the first meiotic division, the homologues are separated to separate the child's cells by a roller. The cells then proceed to the second division without a round of DNA intervening replication. The sister chromatids are separated to separate the child's cells to produce a total of four haploid cells. Female animals use little variation on this pattern and produce one large ovum and two small polar bodies. Because of recombination, individual chromatids may consist of new combinations of maternal and paternal DNA, producing genetically different offspring from both parents. Furthermore, individual gametes may include various maternal, paternal, and recombinant chromatids. The genetic diversity resulting from this sexual reproduction contributes to the variation of the traits on which natural selection can act.
Meiosis uses many of the same mechanisms as mitosis, a type of cell division used by eukaryotes to divide one cell into two identical daughter cells. In some plants, fungi, and meiotic protists result in spore formation: haploid cells that can divide vegetatively without undergoing fertilization. Some eukaryotes, such as bdelloid rotifer, lack the ability to perform meiosis and have acquired the ability to reproduce with parthenogenesis.
Meiosis does not occur in archaea or bacteria, which generally reproduce asexually through binary division. However, a "sexual" process known as horizontal gene transfer involves transferring DNA from one bacterium or archaeon to another and recombinating these DNA molecules from different parent origins.
Maps Meiosis
History
Meiosis was discovered and described for the first time in sea urchin eggs in 1876 by German biologist Oscar Hertwig. It was described again in 1883, at the chromosome level, by the Belgian zoologist Edouard Van Beneden, in the Ascaris egg roundworm. The importance of meiosis for reproduction and inheritance, however, was only explained in 1890 by German biologist August Weismann, who noted that two cell divisions are needed to convert a diploid cell into four haploid cells if the number of chromosomes must be preserved. In 1911, American geneticist Thomas Hunt Morgan detected a cross in meiosis in the Drosophila melanogaster fruit fly, which helped establish that genetic traits were transmitted on chromosomes.
The term "meiosis" (originally spelled "maiosis") comes from the Greek word ??????? , which means 'subtract'. It was introduced to biology by J.B. Farmer and J.E.S. Moore in 1905:
We propose to apply the term Maiosis or the Maiotik phase to encompass a whole series of nuclear changes that fall within the two divisions defined as Heterotype and Homotype by Flemming .
This term is linguistically corrected to "meiosis" by Koernicke (1905), and by Pantel and De Sinety (1906).
Phase
Meiosis is divided into meiosis I and meiosis II which is further divided into Karyokinesis I and Cytokinesis I and Karyokinesis II and Cytokinesis II respectively. The preparatory steps leading to meiosis are identical in patterns and names for the interphase of the cell cycle of mitosis. Interphase is divided into three phases:
- Growth phase 1 (G 1 ): In this highly active phase, the cells synthesize a wide range of proteins, including enzymes and structural proteins required for growth. In G 1 , each chromosome consists of a single molecule of DNA.
- Phase Synthesis (S): Reproduced genetic material; each cell chromosome is doubled into two identical twin chromatids attached to the centromere. This replication does not change the cell ploidies because the centromere number remains the same. The identical twin chromatids have not yet been condensed into packed solid chromosomes visible with a light microscope. This will last for prophase I on meiosis. Phase 2 (G 2 ) phase: G 2 as seen before mitosis is not present in meiosis. Meiotic prophase relates closest to the G 2 phase of the cell cycle of mitosis.
Interphase is followed by meiosis I and then meiosis II. Meiosis I separates the homologous chromosomes, each of which still consists of two chromatid sisters, into two daughter cells, thus reducing the number of half chromosomes. During meiosis II, the female sister chromatid separates and the resulting daughter chromosomes are separated into four child cells. For diploid organisms, the child cells resulting from meiosis are haploid and contain only one copy of each chromosome. In some species, cells enter the resting phase known as interkinesis between meiosis I and meiosis II.
Meiosis I and II are each divided into phases of prophase, metaphase, anaphase, and telophase, similar in purpose to their analog sub-phase in the cell cycle of mitosis. Therefore, meiosis includes meiosis I (profase I, metaphase I, anaphase I, telophase I) and meiosis II (prophase II, metaphase II, anaphase II, telophase II).
Meiosis produces genetic diversity of gametes in two ways: (1) Independent Assortment Law. Independent orientation of homologous chromosome pairs along the metaphase plate during metaphase I & amp; your chromatid orientation in metaphase II, this is your homologous and chromatid separation during anaphase I & amp; II, allowing random and independent distribution of chromosomes to every child cell (and finally to gametes); and (2) Crossing Over. The physical exchange of homologous chromosomal regions with homologous recombination during prophase I produces new combinations of DNA within the chromosomes.
During meiosis, more specific genes are transcribed. In addition to strong meiotic mRNA expression, there is also a pervasive translational control (eg the selective use of previously formed mRNAs), which regulate end-stage specific protein expression of geni during meiosis. Thus, both transcription and translation control determine the extensive restructuring of meiotic cells necessary to perform meiosis.
Meiosis I
Meiosis I separates the homologous chromosome, which joins as tetrad (2n, 4c), producing two haploid cells (n chromosome, 23 in humans) each containing a chromatid pair (1n, 2c). Since the ploidy is reduced from diploid to haploid, my meiosis is referred to as the reductional division . Meiosis II is an equational divide analogous to mitosis, in which sister chromatids are separated, creating four daughter cells of haploid (1n, 1c).
Profase I
Profase I is usually the longest meiotic phase. During prophase I, the pair of homologous chromosomes and DNA exchanges (homologous recombination). This often results in crossover chromosomes. This process is particularly important for couples between homologous chromosomes and hence for accurate chromosome segregation in the first meiotic division. The new DNA combinations created during crossovers are a significant source of genetic variation, and produce new allele combinations, which may be useful. Coupled and replicated chromosomes are called bivalen or tetrad, which has two chromosomes and four chromatids, with one chromosome derived from each parent. The process of pairing a homologous chromosome is called a synapses. At this stage, non-brother chromatids can cross at a point called chiasmata (plural, single chiasma). My prophylaxis has historically been divided into a series of substages named after the appearance of chromosomes.
Leptotene
The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin thread". In this stage of prophase I, individual chromosomes - each composed of two chromatid sisters - become "individual" to form the strands seen in the nucleus. Both chromatid sisters are closely related and visually indistinguishable from each other. During leptotene, the lateral elements of the complex assembly are synaptonemal. Leptotene has a very short duration and progressive condensation and chromosomal fiber rolling occurs.
Zygotene
Stage zygotene , also known as zygonema , from the Greek words meaning "paired threads", occurs when chromosomes are more or less parallel to each other into homologous chromosome pairs. In some organisms, this is called the bouquet stage because the way telomere clumps at one end of the nucleus. At this stage, the synapses (pair/come together) of the homologous chromosomes take place, facilitated by the assembly of the central elements of the sinaptone complex. Pairing is carried in a zipper-like fashion and can be initiated at the centromere (procentric), at the end of the chromosome (proterminal), or in other parts (medium). The individual pair is the same length and in the centromere position. So the pair is very specific and precise. Couple chromosomes are called bivalent or tetrad chromosomes.
Pachytene
The pachytene ( Diplotene
During the plotted stage, also known as diplonema , from the Greek word meaning "two threads", the degradation of the synaptonemal complex and the homologous chromosome are separated from each other slightly. The chromosome itself opens up a bit, allowing some DNA transcription. However, the homologous chromosomes of each bivalent remain closely bound to the chiasmata, the area in which the crossing occurs. Chiasmata remain on chromosomes until they are disconnected in transition to anaphase I.
In the oogenesis of the human fetus, all developing oocytes develop into this stage and are captured in prophase I before birth. This suspended status is referred to as dictyotene stage or dictyate. This lasts until meiosis is continued to prepare the oocyte for ovulation, which occurs at puberty or even later.
Diakinesis
Chromosomes further condense during the stage of deacineis , from the Greek words meaning "to move through". This is the first point in meiosis where four parts of the tetrad are really visible. Crossings involve together, effectively overlapping, making chiasmata visible. In addition to this observation, the rest of the stage resembles the prometafase of mitosis; nucleolus disappears, nuclear membrane crumbles into vesicles, and the meiotic spindle begins to form.
Sync process
During this stage, two centrosomes, containing a pair of centrioles in an animal cell, migrate to the two poles of the cell. This centrosome, which is duplicated during the S-phase, serves as a microtubule nucleation microtubule organizing center, which is basically a cell and pole strap. Microtubules attack the nuclear area after the nuclear envelope is destroyed, attached to the chromosomes in the kinetokor. The function of the kinetocor as a motor, pulls the chromosome along the microtubule attached to the centrosome that begins, like the train on the track. There are four kinetochors in each tetrad, but a pair of kinetocors in each chromatid sister fuse and serve as a unit during meiosis I.
The microtubules attached to the kinetokora are known as microtubules kinetokor . Other microtubules will interact with microtubules from the opposite centrosome: these are called microtubules nonkinetochore or polar microtubules . The third type of microtubule, microtubule daisies, radiates from the centrosome into the cytoplasm or the contact component of the membrane framework.
Metaphase I
The homologous couples move together along the metaphase plates: Like microtubules of the kinetocores of both centrosomes attached to their respective kinetocors, the paired homologous chromosomes align along the equatorial plane dividing the two axes, since the continuous balancing force given to the bivalent by microtubules derived from two homologous chromosome kinetokores. This appendix is ​​referred to as bipolar attachment. The physical basis of an independent set of chromosomes is the random orientation of each bivalent along the metaphase plate, with respect to the orientation of other bivalents along the equator. The cohesin complex protein holds the sister chromatids together from the time of their replication to anaphase. In mitosis, the pulling kretokor microtubule force in the opposite direction creates tension. Cells feel this tension and do not develop with anaphase until all the chromosomes are really bi-oriented. In meiosis, assigning tension requires at least one crossover per chromosome pair in addition to cohesin between chromatid brothers.
Anafase I
Kinetochore's microtubules shorten, pulling the homologous chromosome (which consists of a pair of sister chromatids) to the opposite pole. The non-metallocore microtubule extends, pushing the centrosome further apart. Cells extend in preparation for dividing in the middle. Unlike in mitosis, only cohesin from the chromosomal arm is degraded while the cohesin surrounding the centromere remains protected. This allows the sister chromatids to remain together while the homologues are separated.
Telophase I
The first meiotic division effectively ends when the chromosome arrives at the poles. Each child cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. The microtubules that form the spindle network disappear, and the new nuclear membrane surrounds each haploid. The chromosome reopens into chromatin. Cytokinesis, pinching cell membranes in animal cells or the formation of cell walls in plant cells, occur, completes the creation of two daughter cells. Sister chromatids remain attached during telophase I.
Cells can enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage.
Meiosis II
Meiosis II is a second meiotic division, and usually involves segregation of equations, or the separation of your chromatids. Mechanically, the process is similar to mitosis, although its genetic outcome is fundamentally different. The end result is the production of four haploid cells (chromosome n, 23 in humans) of two haploid cells (with n chromosomes, each composed of two chromatid brothers) produced in meiosis I. The four main steps of meiosis II are: prophase II, metaphase II, anaphase II, and telophase II.
In profase II we see the loss of nucleoli and nuclear envelope again as well as shortening and thickening of chromatids. Centrosomes moved into the polar regions and adjusted the spindle fibers for the second meiotic division.
In metaphase II , the centromere contains two kinetochors attached to the spindle fibers of centrosomes at opposite poles. The new equatorial metaphase plate is rotated 90 degrees when compared to meiosis I, perpendicular to the previous plate. {{}}
This is followed by anaphase II , in which the remaining centromeric cohesin is cleaved allowing the sister chromatid to separate. Sister chromatids by convention are now called sister chromosomes as they move toward opposite poles.
The process ends with telofase II , which is similar to telophase I, and is characterized by decondensation and chromosome elongation and spindle discharge. Reform of the nuclear envelope and the cleavage or formation of the cell plate eventually produce a total of four child cells, each with a haploid set of chromosomes.
Meiosis is now complete and ends with four new daughter cells.
Origin and function
the origin and function of meiosis is essential for understanding the evolution of sexual reproduction in eukaryotes. There is no current consensus among biologists about the question of how sex in Eukaryot appears in evolution, what is the basic function of sexual reproduction, and why it is preserved, given the doubling of the basic cost of sex. It is clear that it evolved more than 1.2 billion years ago, and that almost all species that are descendants of the original sexual reproduction species are still sexual reproduction, including plants, fungi, and animals.
Meiosis is a key event of the sexual cycle in Eukaryotes. This is the stage of the life cycle when a cell produces two haploid cells (gametes) each of which has half a chromosome. Two haploid gametes, emerging from different individual organisms, join the process of fertilization, thus completing the sexual cycle.
Meiosis is ubiquitous among eukaryotes. This occurs in single-celled organisms such as yeast, as well as multicellular organisms, such as humans. Eukaryotes emerged from prokaryotes more than 2.2 billion years ago and the earliest possible eukaryotes of single-celled organisms. To understand sex in eukaryotes, it is necessary to understand (1) how meiosis arises in single-celled eukaryotes, and (2) meiotic function.
Genesis
In the life cycle
Meiosis occurs in the eukaryotic life cycle that involves sexual reproduction, which consists of cyclic meiotic processes and constant fertilization. This occurs simultaneously with normal cell division of mitosis. In multicellular organisms, there is an intermediate step between the diploid and haploid transitions in which the organism grows. At certain stages of the life cycle, the seed cells produce gametes. Somatic cells form the body of the organism and are not involved in the production of gametes.
Meiotic cycling and fertilization events produce a series of alternating transitions between alternating haploid and diploid. The organism phase of the life cycle may occur either during the diploid state (the life cycle is copied ), during the haploid state (haplontic life cycle ), or both ( haplodiplontic life cycle, where there are two distinct phases of the organism, one during the haploid state and the other during the diploid state). In this sense there are three types of life cycles that utilize sexual reproduction, differentiated by the location of the organism phase.
In the diplomatic life cycle (with pre-gametic meiosis), in which the human being is a part, diploid organism, grows from a diploid cell called a zygote. Stem cells of diploid line organisms undergo meiosis to create haploid gametes (spermatozoa for males and eggs for females), which nourish to form a zygote. The diploid zygote undergoes repeated cellular division with mitosis to grow into an organism.
In the haplontic life cycle (with post-zygotic meiosis), these organisms are haploid instead, giving birth by the proliferation and differentiation of a haploid cell called a gamete. Two sex opponents donate their haploid gametes to form a diploid zygote. Zygote undergoes meiosis immediately, creating four haploid cells. These cells undergo mitosis to create organisms. Many mushrooms and many protozoa utilize the haplontik life cycle.
Finally, in the haplodiplontic life cycle (with sporik or intermediate meiosis), living organisms alternate between haploid and diploid conditions. As a result, this cycle is also known as a succession of generations. Germ-line cells of diploid organisms undergo meiosis to produce spores. Spores breed with mitosis, grown into haploid organisms. The haploid organism gametes then merge with other haploid organism gametes, creating a zygote. Zygote undergoes mitotic and repeated differentiation to become a diploid organism again. The haplodiplontic life cycle can be regarded as a combination of a diplontis and haplontik lifecycle.
In plants and animals
Meiosis occurs in all animals and plants. The end result, gamete production with half the number of chromosomes as stem cells, is the same, but the detail process is different. In animals, meiosis produces gametes directly. In plant soil and some algae, there is a turn of generations so that meiosis in the diploid sporophyte generation produces haploid spores. These spores multiply with mitosis, developing into a haploid gametophyte generation, which then induce gametes directly (ie without further meiosis). Both in animals and plants, the last stage is the gamete to converge, returning the original chromosome number.
In the mammals
In women, meiosis occurs in cells known as oocytes (single: oocyte). Each primary oocyte is divided twice in meiosis, not the same in each case. The first division produces a daughter cell, and a much smaller polar body that may or may not undergo the second division. In meiosis II, the division of a child's cell produces a second polar body, and a single haploid cell, which enlarges into an egg. Therefore, in women any primary oocyte undergoes meiotic results in one adult ovum and one or two polar bodies.
Note that there are pauses during meiosis in women. A mature oocyte is captured in prophase I of meiosis I and falls asleep inside a protective somatic cell shell called a follicle. At the beginning of every menstrual cycle, FSH secretion from the anterior pituitary stimulates some follicles to mature in a process known as follicologenesis. During this process, oocyte maturing continues meiosis and continues until metaphase II of meiosis II, where they are re-arrested just before ovulation. If the oocyte is fertilized by sperm, they will continue and complete meiosis. During follicologenesis in humans, usually one follicle becomes dominant while the other undergoes atresia. The process of meiosis in women occurs during oogenesis, and differs from a typical meiosis because it has a long period of meiotic arrest known as dictyate stage and does not have centrosomes.
In men, meiosis occurs during spermatogenesis in the testis seminiferous tubule. Meiosis during spermatogenesis is specific to a type of cell called spermatocytes, which later matures into spermatozoa. Meiosis of primordial germ cells occurs at puberty, much slower than in women. Men's testicular tissue suppresses meiosis by lowering retinoic acid, a meiotic stimulator. This is overcome at puberty when cells in a seminiferous tubule called Sertoli cells begin to create their own retinoic acids. Sensitivity to retinoic acid is also adapted to proteins called nanos and DAZL.
In female mammals, meiosis begins as soon as primordial germ cells migrate to the ovaries in the embryo. It is retinoic acid, derived from a primitive kidney (mesonefros) that stimulates meiosis in ovarian oogonia. Men's testicular tissue suppresses meiosis by lowering retinoic acid, a meiotic stimulator. This is overcome at puberty when cells in a seminiferous tubule called Sertoli cells begin to create their own retinoic acids.
Variations
Nondisjunction
The normal separation of chromosomes in meiosis I or chromatic sisters in meiosis II is called disjunction . When segregation is abnormal, this is called nondisjunction . This results in the production of gametes that have too many or too few specific chromosomes, and is a common mechanism for trisomy or monosomy. Nondisjunction can occur in meiosis I or meiosis II, cellular reproductive phase, or during mitosis.
Most human embryos monosomes and trisomy can not live, but some aneuploidies can be tolerated, such as trisomy for the smallest chromosome, chromosome 21. The phenotype of these aneuploidies ranges from severe to asymptomatic developmental disorders. Medical conditions include but are not limited to:
- Down syndrome - chromosome trisomy 21
- P syndrome - chromosome trisomy 13
- Edwards syndrome - chromosomal trisomy 18
- Klinefelter syndrome - extra X chromosomes in men - XXY, XXXY, XXXXY, etc.
- Turner syndrome - less one X chromosome in women - that is X0
- Triple X syndrome - extra X chromosome in women
- XYY syndrome - additional Y chromosomes in men.
The probability of nondisjunction in the human oocyte increases with age, perhaps due to cohesin loss over time.
More
Along with meiotic variations associated with the time when meiosis occurs in the life cycle, resulting in post-zygotic, pre-gametic and intermediate meiosis (see above), the number of nuclear divisions in meiosis also varies. The majority of eukaryotes have a two-divisional meiosis (though sometimes achiasmatic), but very rare forms, one-division meiosis, occur in some of the flagellates (parabasalids and oxymonads) of the intestine from the wood-eating cockroach Cryptocercus .
Comparison with mitosis
To understand meiosis, comparisons with mitosis are helpful. The table below shows the difference between meiosis and mitosis.
See also
References
Text cited
- Freeman, Scott (2005). Biological Sciences (3rd ed.). Upper Saddle River, NJ: Pearson Prentice Hall.
External links
- Meiosis Flash Animation
- Animation from U. Arizona University of Biology.
- Meiosis in the Kimball Biology Page
- Khan Academy, video lecture
- CCO Cell-Cycle Ontology
- Mayosis animation stage
- * "Abby Dernburg Seminar: Chromosome Dynamics During Meiosis"
Source of the article : Wikipedia
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