Complex sentence: Synapsis takes place during prophase I. When homologous chromosomes synapse, their nends are first attached to the nuclear envelope, and then ends migrate, assisted by the extranuclear cytoskeleton, until matching ends have been paired.
Split into simpler sentences: Synapsis takes place during prophase I. When homologous chromosomes synapse, their ends are first attached to the nuclear envelope. These end-membrane complexes then migrate, assisted by the extranuclear cytoskeleton, until matching ends have been paired.