Prophase 1 is the first stage of meiosis 1, which is the process of cell division that produces gametes (sperm and egg cells) with half the number of chromosomes as the parent cell. During prophase 1, the chromatin, which is the complex of DNA and protein, condenses to form chromosomes. The pairs of replicated chromosomes, known as sister chromatids, remain joined at a central point called the centromere. Homologous chromosomes, which are pairs of chromosomes that contain the same genes but may have different versions of those genes, come together and form synapses, a step unique to meiosis. The paired homologous chromosomes then line up side by side, forming tetrads. Within the tetrad, any pair of chromatid arms can overlap and fuse in a process called crossing-over or recombination. Recombination is a process that breaks, recombines, and rejoins sections of DNA to produce new combinations of genes, which promotes genetic diversity. As prophase 1 continues, the centrosomes move to opposite poles of the cell, and spindle apparatus form.