The First four phases of Meiosis
By: Brandon Snyder
Prophase 1:
During Prophase 1 the already duplicated chromosomes become coiled and shorter. The homologous chromosomes pair and cross over. At this point each chromosome pair is visible as a bivalent(a tight grouping of two chromosomes each consisting of two sister chromatids.) The nucleolus disappears during Prophase 1. In the cytoplasm, the meiotic spindle forms between the two pairs of centrioles as they migrate to opposite poles of the cell. The nuclear envelope disappears at the end of prophase I, allowing the spindle to enter the nucleus. Prophase 1 is the longest phase of meiosis, taking 90% of the time for the two divisions.
Metaphase 1:
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The Centrioles appear at opposite sides of the cell. The bivalents become arranged on a plane equally distant from the poles called the metaphase plate. Spindle fibers from one of the poles connect to one chromosome of each pair. Spindle fibers fro the other pole attach to the homologous chromosome.
Anaphase 1:
Anaphase 1 begins when the two cromosomes of each bivalent begin to move towards opposite sides of the cell preparing to split. a key differenc between Mitosis and meiosis accors here in anaphase. In meiosis the chromotids remain attached at their centromeres. in mitosis the chromotids split apart.
Telophase 1:
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The homologous chromosome pairs reach oposite poles of the cell with each cromosome still with two chromatids. A nuclear envelope reforms around each chromosome set. This is followed by cytokinesis. Cytokinesis is the pinching of the cell into two cells. After cytokinesis, each of the two new cells has a nucleus and a set of replicated chromosomes.