Meiosis is came before by one interphase consist of of the G1, S, and G2 phases, i m sorry are nearly identical come the phases coming before mitosis. The G1 phase, i m sorry is also called the very first gap phase, is the very first phase the the interphase and is concentrated on cabinet growth. The S phase is the 2nd phase of interphase, during which the DNA that the chromosomes is replicated. Finally, the G2 phase, also called the 2nd gap phase, is the 3rd and final phase of interphase; in this phase, the cabinet undergoes the last preparations for meiosis.
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During DNA duplication in the S phase, each chromosome is replicated to create two similar copies, referred to as sister chromatids, the are held together in ~ the centromere by cohesin proteins. Cohesin stop the chromatids with each other until anaphase II. The centrosomes, which are the structures that organize the microtubules the the meiotic spindle, also replicate. This prepares the cell to go into prophase I, the very first meiotic phase.
Early in prophase I, prior to the chromosomes deserve to be seen clearly microscopically, the homologous chromosomes are attached at your tips to the nuclear envelope by proteins. Together the atom envelope begins to rest down, the proteins connected with homologous chromosomes bring the pair nearby to every other. (Recall that, in mitosis, homologous chromosomes do not pair together. In mitosis, homologous chromosomes line up end-to-end so that as soon as they divide, every daughter cabinet receives a sister chromatid indigenous both members the the homologous pair.) The synaptonemal complex, a lattice that proteins between the homologous chromosomes, an initial forms at certain locations and also then diffusion to cover the whole length of the chromosomes. The tight pairing the the homologous chromosomes is called synapsis. In synapsis, the genes on the chromatids the the homologous chromosomes room aligned exactly with each other. The synaptonemal complex supports the exchange the chromosomal segments in between non-sister homologous chromatids, a process called crossing over. Cross over can be it was observed visually after ~ the exchange as chiasmata (singular = chiasma) (Figure 1).
Figure 1. At an early stage in prophase I, homologous chromosomes come with each other to form a synapse. The chromosomes are bound strict together and in perfect alignment by a protein lattice referred to as a synaptonemal complicated and by cohesin proteins at the centromere.
In species such together humans, also though the X and Y sex chromosomes are not homologous (most of their genes differ), they have actually a small region of homology that allows the X and Y chromosomes to pair up throughout prophase I. A partial synaptonemal complex develops only between the regions of homology.
Located in ~ intervals along the synaptonemal complex are big protein assemblies called recombination nodules. These assemblies note the point out of later chiasmata and also mediate the multistep process of crossover—or genetic recombination—between the non-sister chromatids. Near the recombination nodule on every chromatid, the double-stranded DNA is cleaved, the cut ends space modified, and also a new connection is made in between the non-sister chromatids. As prophase i progresses, the synaptonemal complex begins to break down and the chromosomes begin to condense. As soon as the synaptonemal complex is gone, the homologous chromosomes stay attached to each various other at the centromere and also at chiasmata. The chiasmata continue to be until anaphase I. The variety of chiasmata different according come the varieties and the length of the chromosome. There have to be at the very least one chiasma per chromosome for proper separation that homologous chromosomes during meiosis I, yet there might be as numerous as 25. Following crossover, the synaptonemal facility breaks down and the cohesin connection between homologous bag is likewise removed. At the end of prophase I, the pairs are organized together only at the chiasmata (Figure 2) and also are called tetrads due to the fact that the 4 sister chromatids of every pair the homologous chromosomes are now visible.
Figure 2. Crossover occurs in between non-sister chromatids the homologous chromosomes. The an outcome is an exchange of genetic material in between homologous chromosomes.
The crossover occasions are the very first source of genetic variation in the nuclei produced by meiosis. A single crossover event in between homologous non-sister chromatids leader to a mutual exchange of indistinguishable DNA in between a maternal chromosome and a head chromosome. Now, once that sister chromatid is moved right into a gamete cell it will bring some DNA indigenous one parent of the individual and some DNA from the other parent. The sisters recombinant chromatid has a combination of maternal and paternal gene that did not exist prior to the crossover. Many crossovers in an eight of the chromosome have the very same effect, trading segments that DNA to create recombinant chromosomes.
The key event in prometaphase ns is the attachment of the spindle fiber microtubules to the kinetochore protein at the centromeres. Kinetochore proteins room multiprotein complexes that bind the centromeres of a chromosome come the microtubules of the mitotic spindle. Microtubules flourish from centrosomes inserted at the opposite poles the the cell. The microtubules relocate toward the center of the cell and also attach to among the two fused homologous chromosomes. The microtubules attach at every chromosomes’ kinetochores. Through each member of the homologous pair attached come opposite poles the the cell, in the following 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 finish of prometaphase I, every tetrad is attached come microtubules from both poles, v one homologous chromosome dealing with each pole. The homologous chromosomes room still held together in ~ chiasmata. In addition, the atom membrane has damaged down entirely.
During metaphase I, the homologous chromosomes are arranged in the center of the cell through the kinetochores facing opposite poles. The homologous pairs orient us randomly in ~ the equator. For example, if the two homologous members of chromosome 1 room labeled a and b, climate the chromosomes can line up a-b, or b-a. This is crucial in determining the genes lugged by a gamete, together each will just receive among the 2 homologous chromosomes. Recall the homologous chromosomes space not identical. Lock contain slight differences in their genetic information, bring about each gamete to have actually a unique genetic makeup.
This randomness is the physical basis for the production of the second kind of hereditary variation in offspring. Take into consideration that the homologous chromosomes that a sexually reproducing organism are initially inherited together two different sets, one from every parent. Using humans as an example, one set of 23 chromosomes is existing in the egg donated through the mother. The father provides the other set of 23 chromosomes in the sperm that fertilizes the egg. Every cabinet of the multicellular offspring has copies of the original two to adjust of homologous chromosomes. In prophase i of meiosis, the homologous chromosomes form the tetrads. In metaphase I, this pairs heat up at the midway point between the 2 poles of the cabinet to form the metaphase plate. Due to the fact that there is one equal opportunity that a microtubule fiber will encounter a maternally or paternally inherited chromosome, the setup of the tetrads at the metaphase key is random. Any type of maternally inherited chromosome may challenge either pole. Any kind of paternally inherited chromosome may also face either pole. The orientation of each tetrad is live independence of the orientation that the various other 22 tetrads.
This event—the arbitrarily (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 plan of the tetrads is different. The number of variations is dependent on the number of chromosomes consisting of a set. There space two possibilities because that orientation in ~ the metaphase plate; the possible variety of alignments thus equals 2n, where n is the number of chromosomes per set. Humans have actually 23 chromosome pairs, which results in over eight million (223) feasible genetically-distinct gametes. This number go not encompass the variability that was previously created in the sister chromatids by crossover. Offered these two mechanisms, the is extremely unlikely that any two haploid cell resulting native meiosis will have actually the same genetic composition (Figure 3).
Figure 3. Random, independent assortment during metaphase I can be prove by considering a cell v a set of two chromosomes (n = 2). In this case, there room two feasible arrangements at the equatorial airplane in metaphase I. The complete possible variety of different gametes is 2n, wherein n amounts to the number of chromosomes in a set. In this example, there are four feasible genetic combinations because that the gametes. V n = 23 in human being cells, there room over 8 million possible combinations of paternal and maternal chromosomes.
To summarize the genetic consequences of meiosis I, the maternal and also paternal genes are recombined by crossover occasions that occur in between each homologous pair during prophase I. In addition, the arbitrarily assortment of tetrads top top the metaphase key produces a unique combination of maternal and paternal chromosomes that will make their method into the gametes.
In anaphase I, the microtubules pull the linked chromosomes apart. The sisters chromatids continue to be tightly bound together at the centromere. The chiasmata are damaged in anaphase I as the microtubules attached come the fused kinetochores traction the homologous chromosomes apart (Figure 4).
Figure 4. The process of chromosome alignment differs between meiosis I and meiosis II. In prometaphase I, microtubules attach to the fused kinetochores the homologous chromosomes, and the homologous chromosomes room arranged in ~ the midpoint of the cabinet in metaphase I. In anaphase I, the homologous chromosomes room separated. In prometaphase II, microtubules affix to the kinetochores of sister chromatids, and the sister chromatids room arranged at the midpoint of the cell in metaphase II. In anaphase II, the sister chromatids room separated.
Telophase I and also Cytokinesis
In telophase, the separated chromosomes come at the opposite poles. The remainder of the common telophase occasions may or might not occur, relying on the species. In part organisms, the chromosomes decondense and nuclear envelopes type around the chromatids in telophase I. In various other organisms, cytokinesis—the physical separation the the cytoplasmic materials into two daughter cells—occurs without improvement of the nuclei. In nearly all species of animals and also some fungi, cytokinesis separates the cell components via a cleavage furrow (constriction of the actin ring that leads come cytoplasmic division). In plants, a cabinet plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. This cell plate will at some point lead to the formation of cell wall surfaces that different the two daughter cells.
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Two haploid cells are the end result of the first meiotic division. The cells room haploid because at each pole, there is simply one of every pair of the homologous chromosomes. Therefore, just one full collection of the chromosomes is present. This is why the cell are taken into consideration haploid—there is just one chromosome set, even though each homolog still consists of 2 sister chromatids. Recall that sister chromatids are merely duplicates of among the 2 homologous chromosomes (except for alters that developed during cross over). In meiosis II, these 2 sister chromatids will certainly separate, developing four haploid daughter cells.