The procedures leading up to meiosis are comparable to those of mitosis – the centrioles and also chromosomes room replicated. The amount of DNA in the cell has actually doubled, and also the ploidy the the cell stays the exact same as before, at 2n. In meiosis I, the phases are analogous to mitosis: prophase I, metaphase I, anaphase I, and also telophase ns (below figure). Meiosis i proceeds straight to meiosis II without going v interphase.
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Meiosis ns is distinct in that genetic diversity is produced through cross over and random placing of homologous chromosomes (bivalent chromosomes). In addition, in meiosis I, the chromosomal number is reduced from diploid (2n) to haploid (n) throughout this process. (See figure below, where meiosis I begins with a diploid (2n = 4) cell and ends v two haploid (n = 2) cells.) In people (2n = 46), who have actually 23 pairs of chromosomes, the variety of chromosomes is reduced by half at the finish of meiosis ns (n = 23).
During prophase I, chromosomal condensation enables chromosomes come be perceived under the microscope. In so late prophase I, homologous chromosomes (also dubbed bivalent chromosomes, or bivalents) pair laterally, or side-by-side. Right now they are said to it is in in synapsis. During synapsis, crossovers – cross-connections that form from breakage and rejoining in between sister chromatids – can occur between the paired bivalents, top to hereditary recombination (exchange of genetic material) between the strands involved. The allude where a crossover occurs is dubbed a chiasma (plural chiasmata) (see listed below figure). In number below, adhering to crossing over, the blue and red chromosomes, i beg your pardon originally brought AA and aa alleles, respectively, now bring Aa alleles in both chromosomes at the end of prophase I. Note that this bivalents have two chromosomes and four chromatids, with one chromosome originating from each parent.
In metaphase I, each pair that bivalents (two chromosomes, 4 chromatids total) align top top the metaphase plate. This is different from metaphase in mitosis, wherein all chromosomes align single paper on the metaphase plate. The place of each chromosome in the bivalents is random - one of two people parental homolog can appear on every side. This means that over there is a 50-50 possibility for the daughter cell to get either the mother"s or father"s homolog because that each chromosome (see number below). As displayed in the below figure, throughout metaphase I, bivalents from either parent deserve to align top top either side of the cell. In one organism through two set of chromosomes, over there are 4 ways in i beg your pardon the chromosomes can be arranged, result in distinctions in chromosomal distribution in daughter cells after meiosis I. (A diploid organism v 2n chromosomes will have actually 2n possible combinations or means of arranging that chromosomes throughout metaphase I.)
In a diploid cell through 2 bag of chromosomes, there space 4 ways to arrange the chromosomes during metaphase I.
In anaphase I, homologous chromosomes separate. Homologous chromosomes, each containing two chromatids, move to separate poles. Unlike in mitosis, the centromeres do not split and also sister chromatids remain paired in anaphase I.
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Telophase I and also Cytokinesis
In telophase I, the homologs of each bivalent come at opposite poles the the cell, and a new nuclear membrane forms around each set of chromosomes. Cytokinesis climate divides the cell right into two daughter cells. Every of the two daughter cells is currently haploid (n), with fifty percent the variety of chromosomes per nucleus together in meiosis I. In part species, the nuclear membrane briefly forms about the chromosomes, while in others it does not. The cell currently proceeds into meiosis II, through the chromosomes remaining condensed.