The measures leading approximately meiosis are comparable to those that mitosis – the centrioles and chromosomes space replicated. The lot of DNA in the cell has doubled, and also the ploidy that the cell continues to be the very same as before, in ~ 2n. In meiosis I, the phases space analogous come mitosis: prophase I, metaphase I, anaphase I, and also telophase i (below figure). Meiosis i proceeds directly to meiosis II there is no going through interphase.
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Meiosis i is distinct in that hereditary diversity is created through cross over and random positioning of homologous chromosomes (bivalent chromosomes). In addition, in meiosis I, the chromosomal number is diminished from diploid (2n) come haploid (n) throughout this process. (See number below, wherein meiosis I begins with a diploid (2n = 4) cell and ends v two haploid (n = 2) cells.) In humans (2n = 46), who have 23 bag of chromosomes, the variety of chromosomes is lessened by half at the end of meiosis ns (n = 23).
During prophase I, chromosomal condensation enables chromosomes come be perceived under the microscope. In late prophase I, homologous chromosomes (also referred to as bivalent chromosomes, or bivalents) pair laterally, or side-by-side. Right now they are claimed to be in synapsis. During synapsis, crossovers – cross-connections that kind from breakage and rejoining in between sister chromatids – have the right to occur between the paired bivalents, leading to hereditary recombination (exchange of genetic material) in between the strands involved. The allude where a crossover occurs is referred to as a chiasma (plural chiasmata) (see listed below figure). In figure below, complying with crossing over, the blue and red chromosomes, i m sorry originally carried AA and aa alleles, respectively, now carry Aa alleles in both chromosomes at the end of prophase I. Note that this bivalents have two chromosomes and also four chromatids, with one chromosome originating from every parent.
In metaphase I, every pair of bivalents (two chromosomes, 4 chromatids total) align ~ above the metaphase plate. This is various from metaphase in mitosis, whereby all chromosomes align single document on the metaphase plate. The place of each chromosome in the bivalents is arbitrarily - one of two people parental homolog can appear on each side. This means that there is a 50-50 chance for the daughter cell to obtain either the mother"s or father"s homolog because that each chromosome (see number below). As presented in the listed below figure, throughout metaphase I, bivalents from one of two people parent can align top top either next of the cell. In one organism with two sets of chromosomes, over there are 4 ways in which the chromosomes can be arranged, resulting in distinctions in chromosomal distribution in daughter cells after meiosis I. (A diploid organism through 2n chromosomes will have 2n feasible combinations or ways of arranging that is chromosomes throughout metaphase I.)
In a diploid cell v 2 pairs of chromosomes, there are 4 means to kinds the chromosomes during metaphase I.
In anaphase I, homologous chromosomes separate. Homologous chromosomes, every containing two chromatids, move to different poles. Unlike in mitosis, the centromeres execute not split and sister chromatids remain paired in anaphase I.
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Telophase I and Cytokinesis
In telophase I, the homologs of each bivalent arrive at the contrary poles that the cell, and a brand-new nuclear membrane forms roughly each collection of chromosomes. Cytokinesis climate divides the cell right into two daughter cells. Every of the 2 daughter cells is now haploid (n), with half the number of chromosomes per nucleus as in meiosis I. In some species, the nuclear membrane summary forms about the chromosomes, while in others it walk not. The cell now proceeds into meiosis II, v the chromosomes staying condensed.