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During Prophase I tetrad formation takes place. Tetrad contains four sister chromatids.

Chromosomes numbers are duplicated during the interphase of meiotic division. Sister chromatids that were formed during synthesis are joined together at the centromere with the help of cohesion proteins. Nuclear envelopes begin to fragment when the cell enters prophase I and protein holding homologous chromosomes locate each other. These four sister chromatids arrange themselves lengthwise and synaptonemal complex which is a protein lattice that binds them together. This complex helps in crossing over between non-sister chromatids which can be seen as chiasmata. Then this synaptonemal complex breaks down and sister chromatids are liberated except chiasmata attachment. In this stage, these four sister chromatids become visible in each homologous pair which is known as a tetrad.