Meiosis

Meiosis Ancient Greek  'lessening', since it is for a reductional division is the special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such(a) as sperm or egg cells. It involves two rounds of division that ultimately a thing that is caused or produced by something else in four cells with only one copy of regarded and target separately. chromosome haploid. Additionally, prior to the division, genetic fabric from the paternal as well as maternal copies of used to refer to every one of two or more people or matters chromosome is crossed over, creating new combinations of code on each chromosome. Later on, during fertilisation, the haploid cells presents by meiosis from a male & female will fuse to shit a cell with two copies of each chromosome again, the zygote.

Errors in meiosis resulting in aneuploidy an abnormal number of chromosomes are the main known cause of miscarriage and the nearly frequent genetic earn of developmental disabilities.

In meiosis, DNA replication is followed by two rounds of cell division to produce four daughter cells, each with half the number of chromosomes as the original parent cell. The two meiotic divisions are invited as meiosis I and meiosis II. previously meiosis begins, during S phase of the cell cycle, the DNA of each chromosome is replicated so that it consists of two identical sister chromatids, which stay on held together through sister chromatid cohesion. This S-phase can be refers to as "premeiotic S-phase" or "meiotic S-phase". Immediately following DNA replication, meiotic cells enter a prolonged G₂-like stage asked as meiotic prophase. During this time, homologous chromosomes pair with each other and undergo genetic recombination, a programmed process in which DNA may be design and then repaired, which permits them to exchange some of their genetic information. A subset of recombination events results in crossovers, which create physical links known as chiasmata singular: chiasma, for the Greek letter Chi Χ between the homologous chromosomes. In near organisms, these links can support direct each pair of homologous chromosomes to segregate away from each other during Meiosis I, resulting in two haploid cells that have half the number of chromosomes as the parent cell.

During meiosis II, the cohesion between sister chromatids is released and they segregate from one another, as during mitosis. In some cases, any four of the meiotic products form gametes such(a) as sperm, spores or pollen. In female animals, three of the four meiotic products are typically eliminated by extrusion into polar bodies, and only one cell develops to produce an ovum. Because the number of chromosomes is halved during meiosis, gametes can fuse i.e. fertilization to form a diploid zygote that contains two copies of each chromosome, one from each parent. Thus, alternating cycles of meiosis and fertilization lets sexual reproduction, with successive generations maintaining the same number of chromosomes. For example, diploid human cells contain 23 pairs of chromosomes including 1 pair of sex chromosomes 46 total, half of maternal origin and half of paternal origin. Meiosis produces haploid gametes ova or sperm that contain one brand of 23 chromosomes. When two gametes an egg and a sperm fuse, the resulting zygote is one time again diploid, with the mother and father each contributing 23 chromosomes. This same pattern, but non the same number of chromosomes, occurs in any organisms that utilize meiosis.

Meiosis occurs in all sexually-reproducing single-celled and multicellular organisms which are all eukaryotes, including animals, plants and fungi. it is for an essential process for oogenesis and spermatogenesis.

Phases

Meiosis is divided into meiosis I and meiosis II which are further divided into Karyokinesis I and Cytokinesis I and Karyokinesis II and Cytokinesis II respectively. The preparatory steps that lead up to meiosis are identical in pattern and name to interphase of the mitotic cell cycle. Interphase is divided into three phases:

Interphase is followed by meiosis I and then meiosis II. Meiosis I separates replicated homologous chromosomes, each still presented up of two sister chromatids, into two daughter cells, thus reducing the chromosome number by half. During meiosis II, sister chromatids decouple and the resultant daughter chromosomes are segregated into four daughter cells. For diploid organisms, the daughter cells resulting from meiosis are haploid and contain only one copy of each chromosome. In some species, cells enter a resting phase known as interkinesis between meiosis I and meiosis II.

Meiosis I and II are each divided into prophase, metaphase, anaphase, and telophase stages, similar in aim 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.

During meiosis, particular genes are more highly transcribed. In addition to strong meiotic stage-specific expression of mRNA, there are also pervasive translational a body or process by which power to direct or setting or a particular component enters a system. e.g. selective ownership of preformed mRNA, regulating themeiotic stage-specific protein expression of genes during meiosis. Thus, both transcriptional and translational controls build the broad restructuring of meiotic cells needed to carry out meiosis.

Meiosis I segregates homologous chromosomes, which are joined as tetrads 2n, 4c, producing two haploid cells n chromosomes, 23 in humans which each contain chromatid pairs 1n, 2c. Because the ploidy is reduced from diploid to haploid, meiosis I is referred to as a reductional division. Meiosis II is an equational division analogous to mitosis, in which the sister chromatids are segregated, devloping four haploid daughter cells 1n, 1c.

Prophase I is by far the longest phase of meiosis lasting 13 out of 14 days in mice. During prophase I, homologous maternal and paternal chromosomes pair, synapse, and exchange genetic information by homologous recombination, forming at least one crossover per chromosome. These crossovers become visible as chiasmata plural; singular chiasma. This process facilitatespairing between homologous chromosomes and hence enables accurate segregation of the chromosomes at the first meiotic division. The paired and replicated chromosomes are called bivalents two chromosomes or tetrads four chromatids, with one chromosome coming from each parent. Prophase I is divided into a series of substages which are named according to the formation of chromosomes.

The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads".: 27  In this stage of prophase I, individual chromosomes—each consisting of two replicated sister chromatids—become "individualized" to form visible strands within the nucleus.: 27 : 353  The chromosomes each form a linear array of loops mediated by cohesin, and the lateral elements of the synaptonemal complex assemble forming an "axial element" from which the loops emanate. Recombination is initiated in this stage by the enzyme SPO11 which creates programmed double strand breaks around 300 per meiosis in mice. This process generates single stranded DNA filaments coated by RAD51 and DMC1 which invade the homologous chromosomes, forming inter-axis bridges, and resulting in the pairing/co-alignment of homologues to a distance of ~400 nm in mice.

Leptotene is followed by the zygotene stage, also known as zygonema, from Greek words meaning "paired threads",: 27  which in some organisms is also called the bouquet stage because of the way the telomeres cluster at one end of the nucleus. In this stage the homologous chromosomes become much more closely ~100 nm and stably paired a process called synapsis mediated by the installation of the transverse and central elements of the synaptonemal complex. Synapsis is thought to arise in a zipper-like fashion starting from a recombination nodule. The paired chromosomes are called bivalent or tetrad chromosomes.

The pachytene stage , also known as pachynema, from Greek words meaning "thick threads".: 27  is the stage at which all autosomal chromosomes have synapsed. In this stage homologous recombination, including chromosomal crossover crossing over, is completed through the repair of the double strand breaks formed in leptotene. Most breaks are repaired without forming crossovers resulting in gene conversion. However, a subset of breaks at least one per chromosome form crossovers between non-sister homologous chromosomes resulting in the exchange of genetic information. Sex chromosomes, however, are not wholly identical, and only exchange information over a small region of homology called the pseudoautosomal region. The exchange of information between the homologous chromatids results in a recombination of information; each chromosome has the complete line of information it had before, and there are no gaps formed as a result of the process. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through an ordinary light microscope, and chiasmata are not visible until the next stage.

During the diplotene stage, also known as diplonema, from Greek words meaning "two threads",: 30  the synaptonemal complex disassembles and homologous chromosomes separate from one another a little. However, the homologous chromosomes of each bivalent progress tightly bound at chiasmata, the regions where crossing-over occurred. The chiasmata remain on the chromosomes until they are severed at the transition to anaphase I to let homologous chromosomes to move to opposite poles of the cell.

In human fetal oogenesis, all development oocytes develop to this stage and are arrested in prophase I ago birth. This suspended state is referred to as the dictyotene stage or dictyate. It lasts until meiosis is resumed to ready the oocyte for ovulation, which happens at puberty or even later.

Chromosomes condense further during the diakinesis stage, from Greek words meaning "moving through".: 30  This is the first ingredient 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.

Unlike mitotic cells, human and mouse oocytes do not have centrosomes to produce the meiotic spindle. In mice, approximately 80 MicroTubule Organizing Centers MTOCs form a sphere in the ooplasm and begin to nucleate microtubules thatout towards chromosomes, attaching to the chromosomes at the kinetochore. Over time the MTOCs merge until two poles have formed, generating a barrel shaped spindle. In human oocytes spindle microtubule nucleation begins on the chromosomes, forming an aster that eventually expands to surround the chromosomes. Chromosomes then slide along the microtubules towards the equator of the spindle, at which section the chromosome kinetochores form end-on attachments to microtubules.

Homologous pairs move together along the metaphase plate: As kinetochore microtubules from both spindle poles attach to their respective kinetochores, the paired homologous chromosomes align along an equatorial plane that bisects the spindle, due to continual counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes. This attachment is referred to as a bipolar attachment. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along with the metaphase plate, with respect to the orientation of the other bivalents along the same equatorial line. The protein complex cohesin holds sister chromatids together from the time of their replication until anaphase. In mitosis, the force of kinetochore microtubules pulling in opposite directions creates tension. The cell senses this tension and does not progress with anaphase until all the chromosomes are properly bi-oriented. In meiosis, establishing tension ordinarily requires at least one crossover per chromosome pair in addition to cohesin between sister chromatids see Chromosome segregation.

Kinetochore microtubules shorten, pulling homologous chromosomes which each consist of a pair of sister chromatids to opposite poles. Nonkinetochore microtubules lengthen, pushing the centrosomes farther apart. The cell elongates in preparation for division down the center. Unlike in mitosis, only the cohesin from the chromosome arms is degraded while the cohesin surrounding the centromere supports protected by a protein named Shugoshin Japanese for "guardian spirit", what prevents the sister chromatids from separating. This allows the sister chromatids to remain together while homologs are segregated.

The first meiotic division effectively ends when the chromosomesat the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. The microtubules that constitute the spindle network disappear, and a new nuclear membrane surrounds each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells. However, cytokinesis does not fully prepare resulting in "cytoplasmic bridges" which enable the cytoplasm to be shared between daughter cells until the end of meiosis II. Sister chromatids remain attached during telophase I.

Cells may enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage.

Meiosis II is themeiotic division, and commonly involves equational segregation, or separation of sister chromatids. Mechanically, the process is similar to mitosis, though its genetic results are fundamentally different. The end result 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 leading 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 alive as the shortening and thickening of the chromatids. Centrosomes move to the polar regions and arrange spindle fibers for the second meiotic division.

In metaphase II, the centromeres contain two kinetochores that attach to spindle fibers from the centrosomes at opposite poles. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the preceding plate.

This is followed by anaphase II, in which the remaining centromeric cohesin, not protected by Shugoshin anymore, is cleaved, allowing the sister chromatids to segregate. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles.

The process ends with telophase II,which is similar to telophase I, and is marked by decondensation and lengthening of the chromosomes and the disassembly of the spindle. Nuclear envelopes re-form and cleavage or cell plate formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes.