Though meiosis is invisible to the naked eye, human life -- and that of all organisms that sexually reproduce -- depends upon it. Meiosis is the process by which a diploid cell divides into four "daughter" cells. Meiosis creates more egg and sperm cells and actually involves two separate divisions but only one replication of cell DNA.
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In prophase 1, homologous chromosomes beginning crossing over while the DNA tightly coils.
In metaphase 1, the microtubules assemble into a spindle and the nuclear membrane disappears. Pairs of homologous chromosomes (or "homologues") attach themselves to opposite polar microtubules and then align themselves on the metaphase plate.
During anaphase 1, the microtubules are reduced in length, causing the chromosome pairs to be pulled toward the poles, dragging sister chromatids with them. At this point, each pole has a full set of haploid chromosomes.
In telophase 1, the nuclear membrane, which previously disappears, re-forms around the daughter nuclei. At this point, there are two sister chromatids per chromosome in each daughter nucleus.
During meiosis 2, the cell prepares to divide. A new spindle develops as the nuclear membrane degrades. The sister chromatids reorient themselves toward opposite poles. Then nuclear envelopes grow around the daughter chromosome pairs. As nuclear envelopes take shape around the daughter chromosomes, their shape as four distinct cells begins to emerge. The diploid then splits, forming four cells.
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