The process you are referring to is meiosis, a reduction division that results in haploid gametes--sperm and egg cells. Before meiosis can occur, the chromosomes are replicated. In meiosis during prophase I, homologous chromosomes exchange DNA in a process called crossing over. This leads to genetic variation. The chromosomes that are paired and replicated form tetrads. In the tetrad are four chromatids, that are from two chromosomes--one from each parent. This process is called synapsis and non-sister chromatids can cross over, exchanging DNA resulting in new combinations of alleles. Since humans have 46 chromosomes, when they replicate, there will be 46 pairs of chromosomes. These will form 23 tetrads during meiosis.