During Prophase 1 of meiosis, two chromosomes adhere to one another forming a homolog pair. This pair is arranged in a tetrad or bivalent structure because a single chromosome is made up of two genetically identical chromatids. Thus, these two pairs of chromatids forming the chromosome pair are responsible for the tetrad structure. The importance of this is that, the process provides an opportunity for the homolog pair to exchange parts of their separate DNA known as crossing over.
After crossing over, metaphase 1 begins by the arrangement of the chromatid pair facing the same pole. Each of the adjacent chromatids is then connected to spindle fiber from the same pole. This process is important because it forms the basis for separation of the two chromatid pairs forming the chromosome in the next stage.