Meiosis is came before by one interphase consisting of the G1, S, and also G2 phases, which are nearly identical to the phases preceding mitosis. The G1 phase, i m sorry is also called the first gap phase, is the an initial phase that the interphase and is concentrated on cabinet growth. The S step is the second phase that interphase, throughout which the DNA of the chromosomes is replicated. Finally, the G2 phase, likewise called the 2nd gap phase, is the third and final phase of interphase; in this phase, the cell undergoes the final preparations for meiosis.
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During DNA duplication in the S phase, each chromosome is replicated to develop two the same copies, referred to as sister chromatids, the are organized together in ~ the centromere by cohesin proteins. Cohesin hold the chromatids with each other until anaphase II. The centrosomes, which room the frameworks that organize the microtubules of the meiotic spindle, likewise replicate. This prepares the cell to enter prophase I, the very first meiotic phase.
Early in prophase I, prior to the chromosomes can be seen plainly microscopically, the homologous chromosomes space attached at their tips to the atom envelope through proteins. As the atom envelope starts to rest down, the proteins associated with homologous chromosomes lug the pair near to each other. (Recall that, in mitosis, homologous chromosomes execute not pair together. In mitosis, homologous chromosomes heat up end-to-end therefore that once they divide, each daughter cabinet receives a sister chromatid from both members that the homologous pair.) The synaptonemal complex, a lattice the proteins between the homologous chromosomes, first forms at specific locations and then diffusion to cover the whole length the the chromosomes. The tight pairing that the homologous chromosomes is referred to as synapsis. In synapsis, the gene on the chromatids that the homologous chromosomes space aligned precisely with each other. The synaptonemal complicated supports the exchange the chromosomal segments in between non-sister homologous chromatids, a process called crossing over. Crossing over can be it was observed visually after the exchange together chiasmata (singular = chiasma) (Figure 1).
Figure 1. Beforehand in prophase I, homologous chromosomes come with each other to type a synapse. The chromosomes space bound strictly together and in perfect alignment by a protein lattice called a synaptonemal facility and by cohesin protein at the centromere.
In species such together humans, also though the X and Y sex chromosomes room not homologous (most the their genes differ), they have a small region of homology that allows the X and Y chromosomes come pair up during prophase I. A partial synaptonemal complicated develops only between the regions of homology.
Located at intervals follow me the synaptonemal complex are large protein assemblies called recombination nodules. This assemblies note the clues of later on chiasmata and also mediate the multistep process of crossover—or hereditary recombination—between the non-sister chromatids. Near the recombination nodule on each chromatid, the double-stranded DNA is cleaved, the reduced ends are modified, and a brand-new connection is made between the non-sister chromatids. As prophase ns progresses, the synaptonemal complex begins to failure and the chromosomes begin to condense. Once the synaptonemal complex is gone, the homologous chromosomes remain attached to each other at the centromere and also at chiasmata. The chiasmata stay until anaphase I. The number of chiasmata different according to the varieties and the length of the chromosome. There have to be at the very least one chiasma per chromosome for suitable separation that homologous chromosomes during meiosis I, yet there might be as numerous as 25. Adhering to crossover, the synaptonemal complicated breaks down and also the cohesin connection in between homologous bag is likewise removed. In ~ the end of prophase I, the bag are organized together only at the chiasmata (Figure 2) and are called tetrads because the four sister chromatids of each pair that homologous chromosomes are currently visible.
Figure 2. Crossover occurs in between non-sister chromatids the homologous chromosomes. The an outcome is one exchange of hereditary material in between homologous chromosomes.
The crossover occasions are the an initial source of hereditary variation in the nuclei produced by meiosis. A solitary crossover event in between homologous non-sister chromatids leads to a mutual exchange of indistinguishable DNA in between a maternal chromosome and also a paternal chromosome. Now, once that sister chromatid is moved into a gamete cell it will lug some DNA indigenous one parent of the individual and also some DNA native the various other parent. The sister recombinant chromatid has a mix of maternal and paternal gene that did no exist prior to the crossover. Multiple crossovers in an arm of the chromosome have the same effect, trading segments that DNA to produce recombinant chromosomes.
The key event in prometaphase i is the attachment of the spindle fiber microtubules to the kinetochore protein at the centromeres. Kinetochore proteins room multiprotein complexes that tie the centromeres the a chromosome come the microtubules the the mitotic spindle. Microtubules thrive from centrosomes put at the opposite poles that the cell. The microtubules move toward the center of the cell and also attach to among the 2 fused homologous chromosomes. The microtubules attach at each chromosomes’ kinetochores. With each member the the homologous pair attached to 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 attached come a kinetochore is dubbed a kinetochore microtubule. In ~ the finish of prometaphase I, every tetrad is attached come microtubules from both poles, through one homologous chromosome encountering each pole. The homologous chromosomes room still hosted together in ~ chiasmata. In addition, the atom membrane has broken down entirely.
During metaphase I, the homologous chromosomes space arranged in the center of the cell through the kinetochores encountering opposite poles. The homologous pairs orient us randomly at the equator. For example, if the 2 homologous members of chromosome 1 room labeled a and also b, climate the chromosomes could line increase a-b, or b-a. This is necessary in identify the genes brought by a gamete, together each will only receive one of the two homologous chromosomes. Recall that homologous chromosomes space not identical. Castle contain slight differences in their hereditary information, bring about each gamete to have a distinctive genetic makeup.
This randomness is the physical basis because that the production of the second type of genetic variation in offspring. Think about that the homologous chromosomes the a sexually reproducing biology are initially inherited as two separate sets, one from every parent. Using humans as one example, one collection of 23 chromosomes is existing in the egg donated by the mother. The father gives the other collection of 23 chromosomes in the sperm that fertilizes the egg. Every cabinet of the multicellular offspring has copies of the initial two sets of homologous chromosomes. In prophase i of meiosis, the homologous chromosomes kind the tetrads. In metaphase I, these pairs line up in ~ the midway allude between the two poles of the cell to kind the metaphase plate. Since there is one equal opportunity that a microtubule fiber will certainly encounter a maternally or paternally inherited chromosome, the plan of the tetrads at the metaphase bowl is random. Any kind of maternally inherited chromosome may confront either pole. Any paternally inherited chromosome may likewise face one of two people pole. The orientation of each tetrad is independent of the orientation the the various other 22 tetrads.
This event—the arbitrarily (or independent) assortment the homologous chromosomes at the metaphase plate—is the second mechanism the introduces variation into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations is dependence on the number of chromosomes making up a set. There room two possibilities for orientation in ~ the metaphase plate; the possible variety of alignments thus equals 2n, wherein n is the variety of chromosomes every set. Humans have actually 23 chromosome pairs, which outcomes in over eight million (223) possible genetically-distinct gametes. This number go not include the variability the was previously developed in the sister chromatids by crossover. Given these 2 mechanisms, the is extremely unlikely that any two haploid cell resulting from meiosis will have actually the same hereditary composition (Figure 3).
Figure 3. Random, elevation assortment throughout metaphase I have the right to be demonstrated by considering a cell v a set of 2 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 amounts to the number of chromosomes in a set. In this example, there are four possible genetic combinations for the gametes. V n = 23 in person cells, there space over 8 million possible combinations of paternal and also maternal chromosomes.
To summary the genetic after-effects of meiosis I, the maternal and paternal genes room recombined by crossover occasions that occur between each homologous pair during prophase I. In addition, the arbitrarily assortment the tetrads ~ above the metaphase key produces a unique mix of maternal and also paternal chromosomes that will make their method into the gametes.
In anaphase I, the microtubules pull the connected chromosomes apart. The sister chromatids remain tightly bound with each other at the centromere. The chiasmata are damaged in anaphase I as the microtubules attached to the fused kinetochores traction the homologous chromosomes apart (Figure 4).
Figure 4. The procedure of chromosome alignment differs in between meiosis I and meiosis II. In prometaphase I, microtubules affix to the unify kinetochores the homologous chromosomes, and the homologous chromosomes room arranged in ~ the midpoint that the cabinet in metaphase I. In anaphase I, the homologous chromosomes space separated. In prometaphase II, microtubules attach to the kinetochores that sister chromatids, and also the sister chromatids room arranged in ~ the midpoint that the cells in metaphase II. In anaphase II, the sisters chromatids space separated.
Telophase I and Cytokinesis
In telophase, the be separated chromosomes come at the opposite poles. The remainder of the usual telophase events may or may not occur, depending on the species. In some organisms, the chromosomes decondense and nuclear envelopes kind around the chromatids in telophase I. In various other organisms, cytokinesis—the physics separation that the cytoplasmic contents into 2 daughter cells—occurs without reformation of the nuclei. In almost all species of animals and also some fungi, cytokinesis the end the cell contents via a cleavage furrow (constriction that the actin ring that leads come cytoplasmic division). In plants, a cell plate is formed during cell cytokinesis by Golgi engine fusing in ~ the metaphase plate. This cell plate will ultimately lead to the formation of cell walls that separate the 2 daughter cells.
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Two haploid cells space the end result of the very first meiotic division. The cells space haploid because at each pole, there is simply one of every pair the the homologous chromosomes. Therefore, just one full set of the chromosomes is present. This is why the cell are considered haploid—there is only one chromosome set, even though each homolog still consists of 2 sister chromatids. Recall the sister chromatids are merely duplicates of among the two homologous chromosomes (except for changes that arisen during cross over). In meiosis II, these 2 sister chromatids will certainly separate, creating four haploid daughter cells.