Meiosis is preceded by an interphase consisting of the G1, S, and G2 phases, i beg your pardon are almost identical come the phases coming before mitosis. The G1 phase, i beg your pardon is also called the an initial gap phase, is the first phase that the interphase and also is concentrated on cabinet growth. The S phase is the 2nd phase that interphase, throughout which the DNA of the chromosomes is replicated. Finally, the G2 phase, likewise called the second gap phase, is the third and final phase the interphase; in this phase, the cabinet undergoes the last preparations because that meiosis.
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During DNA duplication in the S phase, every chromosome is replicated to develop two the same copies, referred to as sister chromatids, that are held together at the centromere by cohesin proteins. Cohesin holds the chromatids with each other until anaphase II. The centrosomes, which space the frameworks that to organize the microtubules that the meiotic spindle, likewise replicate. This prepares the cabinet to enter prophase I, the first meiotic phase.
Early in prophase I, prior to the chromosomes deserve to be seen plainly microscopically, the homologous chromosomes are attached at their tips to the nuclear envelope through proteins. As the atom envelope starts to rest down, the proteins linked with homologous chromosomes bring the pair near to every other. (Recall that, in mitosis, homologous chromosomes do not pair together. In mitosis, homologous chromosomes heat up end-to-end so that when they divide, each daughter cabinet receives a sister chromatid indigenous both members of the homologous pair.) The synaptonemal complex, a lattice the proteins between the homologous chromosomes, first forms at particular locations and then spreads to covering the whole length the the chromosomes. The chop pairing the the homologous chromosomes is called synapsis. In synapsis, the gene on the chromatids that the homologous chromosomes are aligned precisely with each other. The synaptonemal complex supports the exchange of chromosomal segments between non-sister homologous chromatids, a process called crossing over. Crossing over can be observed visually after the exchange together chiasmata (singular = chiasma) (Figure 1).
Figure 1. At an early stage in prophase I, homologous chromosomes come with each other to type a synapse. The chromosomes room bound strictly together and in perfect alignment by a protein lattice referred to as a synaptonemal complex and by cohesin protein at the centromere.
In types such as humans, even though the X and also Y sex chromosomes room not homologous (most the their gene differ), they have actually a small region of homology that allows the X and also Y chromosomes come pair up during prophase I. A partial synaptonemal complex develops only in between the areas of homology.
Located in ~ intervals along the synaptonemal complex are large protein assemblies called recombination nodules. These assemblies note the points of later chiasmata and also mediate the multistep procedure of crossover—or hereditary recombination—between the non-sister chromatids. Close to the recombination nodule on every chromatid, the double-stranded DNA is cleaved, the cut ends space modified, and also a brand-new connection is made between the non-sister chromatids. As prophase i progresses, the synaptonemal facility begins to failure and the chromosomes start to condense. As soon as the synaptonemal complex is gone, the homologous chromosomes continue to be attached to each other at the centromere and also at chiasmata. The chiasmata stay until anaphase I. The variety of chiasmata different according to the species and the length of the chromosome. There have to be at least one chiasma per chromosome for proper separation that homologous chromosomes during meiosis I, yet there may be as many as 25. Adhering to crossover, the synaptonemal complex breaks down and the cohesin connection in between homologous bag is likewise removed. In ~ the finish of prophase I, the pairs are hosted together just at the chiasmata (Figure 2) and also are called tetrads because the four sister chromatids of every pair of homologous chromosomes are currently visible.
Figure 2. Crossover occurs in between non-sister chromatids that homologous chromosomes. The result is an exchange of genetic material in between homologous chromosomes.
The crossover occasions are the first source of hereditary variation in the nuclei produced by meiosis. A single crossover event between homologous non-sister chromatids leader to a reciprocal exchange of indistinguishable DNA between a maternal chromosome and a head chromosome. Now, when that sister chromatid is moved right into a gamete cell it will bring some DNA from one parental of the individual and some DNA from the various other parent. The sister recombinant chromatid has a mix of maternal and also paternal gene that did not exist prior to the crossover. Multiple crossovers in an eight of the chromosome have actually the very same effect, exchanging segments of DNA to create recombinant chromosomes.
The vital event in prometaphase ns is the attachments of the spindle fiber microtubules to the kinetochore proteins at the centromeres. Kinetochore proteins space multiprotein complexes that bind the centromeres of a chromosome to the microtubules of the mitotic spindle. Microtubules flourish from centrosomes inserted at the opposite poles the the cell. The microtubules move toward the center of the cell and attach to one of the two fused homologous chromosomes. The microtubules connect at each chromosomes’ kinetochores. Through each member that the homologous pair attached come opposite poles the the cell, in the next phase, the microtubules have the right to pull the homologous pair apart. A spindle fiber that has actually attached to a kinetochore is dubbed a kinetochore microtubule. In ~ the end of prometaphase I, every tetrad is attached to microtubules from both poles, v one homologous chromosome dealing with each pole. The homologous chromosomes room still organized together in ~ chiasmata. In addition, the atom membrane has broken down entirely.
During metaphase I, the homologous chromosomes room arranged in the center of the cell v the kinetochores dealing with opposite poles. The homologous bag orient themselves randomly at the equator. Because that example, if the 2 homologous members that chromosome 1 room labeled a and b, climate the chromosomes might line increase a-b, or b-a. This is necessary in determining the genes lugged by a gamete, as each will just receive among the two homologous chromosomes. Recall the homologous chromosomes space not identical. Castle contain slight distinctions in their hereditary information, bring about each gamete to have a distinct genetic makeup.
This randomness is the physical basis because that the creation of the second form of genetic variation in offspring. Think about that the homologous chromosomes that a sexually reproducing biology are initially inherited together two separate sets, one from every parent. Using human beings as one example, one set of 23 chromosomes is existing in the egg donated through the mother. The father provides the other collection of 23 chromosomes in the sperm that fertilizes the egg. Every cabinet of the multicell offspring has copies of the initial two set of homologous chromosomes. In prophase i of meiosis, the homologous chromosomes form the tetrads. In metaphase I, this pairs line up in ~ the midway allude between the 2 poles that the cabinet to kind the metaphase plate. Because there is one equal possibility that a microtubule fiber will encounter a maternally or paternally inherited chromosome, the plan of the tetrads at the metaphase key is random. Any maternally inherited chromosome may challenge either pole. Any paternally inherited chromosome may also face either pole. The orientation of every tetrad is independent of the orientation of the other 22 tetrads.
This event—the random (or independent) assortment the homologous chromosomes in ~ the metaphase plate—is the second mechanism the introduces variation into the gametes or spores. In each cell the undergoes meiosis, the arrangement of the tetrads is different. The variety of variations is dependence on the variety of chromosomes comprising a set. There space two possibilities for orientation at the metaphase plate; the possible number of alignments as such equals 2n, whereby n is the number of chromosomes per set. Humans have actually 23 chromosome pairs, which results in end eight million (223) feasible genetically-distinct gametes. This number does not include the variability the was previously developed in the sister chromatids through crossover. Offered these 2 mechanisms, the is very unlikely that any kind of two haploid cells resulting from meiosis will have the same hereditary composition (Figure 3).
Figure 3. Random, elevation assortment during metaphase I can be demonstrated by considering a cell with a set of two chromosomes (n = 2). In this case, there space two feasible arrangements at the equatorial plane in metaphase I. The full possible number of different gametes is 2n, where n equates to the number of chromosomes in a set. In this example, there space four feasible genetic combinations for the gametes. With n = 23 in human being cells, there are over 8 million possible combinations the paternal and maternal chromosomes.
To summarize the genetic after-effects of meiosis I, the maternal and paternal genes space recombined by crossover occasions that occur in between each homologous pair throughout prophase I. In addition, the random assortment the tetrads top top the metaphase key produces a unique combination of maternal and also paternal chromosomes that will make their method into the gametes.
In anaphase I, the microtubules traction the connected chromosomes apart. The sister chromatids stay tightly bound together at the centromere. The chiasmata are broken in anaphase I together the microtubules attached come the fused kinetochores pull the homologous chromosomes personally (Figure 4).
Figure 4. The procedure of chromosome alignment differs in between meiosis I and meiosis II. In prometaphase I, microtubules affix to the fused kinetochores the homologous chromosomes, and also the homologous chromosomes are arranged at the midpoint of the cabinet in metaphase I. In anaphase I, the homologous chromosomes space separated. In prometaphase II, microtubules connect to the kinetochores of sister chromatids, and the sisters chromatids space arranged at the midpoint that the cells in metaphase II. In anaphase II, the sister chromatids room separated.
Telophase I and Cytokinesis
In telophase, the be separated chromosomes come at the opposite poles. The remainder of the common telophase events may or may not occur, depending on the species. In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I. In other organisms, cytokinesis—the physical separation of the cytoplasmic contents into 2 daughter cells—occurs without reformation of the nuclei. In almost all species of animals and some fungi, cytokinesis off the cell components via a cleavage furrow (constriction of the actin ring the leads to cytoplasmic division). In plants, a cell plate is formed throughout cell cytokinesis through Golgi vesicles fusing in ~ the metaphase plate. This cabinet plate will eventually lead to the development of cell walls that separate the 2 daughter cells.
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Two haploid cells space the end an outcome of the an initial meiotic division. The cells space haploid since at each pole, there is just one of every pair that the homologous chromosomes. Therefore, only one full collection of the chromosomes is present. This is why the cell are considered haploid—there is only one chromosome set, even though each homolog still is composed of two sister chromatids. Recall that sister chromatids are simply duplicates of among the 2 homologous chromosomes (except for changes that occurred during cross over). In meiosis II, these 2 sister chromatids will certainly separate, creating four haploid daughter cells.