Synapsis (also calle syndesis) is the pairing of two homologous chromosomes that occurs during meiosis. It is a form of chromosomal crossover. Synapsis takes place during prophase I. When homologous chromosomes synapse, they come closer together until they are connected by a protein complex called the synaptonemal complex, which contains central and lateral elements. While autosomes undergo synapsis during meiosis sex chromosomes usually remain unpaired.