during the zygotene stage of prophase I, chromosomes start pairing together (synapsis). This process is accompanied by the formation of a complex structure called the synaptonemal complex. The text states, 'The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad' . Although they are more clearly visible as tetrads in the subsequent pachytene stage, the formation of the bivalent/tetrad structure via the synaptonemal complex is the defining characteristic of the zygotene stage.