Crossing over creates genetic variation by exchanging DNA between two nonsister chromatids to produce genetically unique chromosomes. The process of crossing over occurs during meiosis.
Meiosis is a type of cell division that produces four haploid gametes from a parent cell. These gametes contain 23 chromosomes, which is half of the genetic information of the parent. During fertilization, a zygote is formed when these gametes combine with the gametes of the opposite sex. The zygote will have the complete or diploid number of chromosomes, which is 46. The two ways wherein meiosis increases genetic diversity in a species are crossing over and independent assortment of homologous chromosomes.
Crossing over takes place during meiosis I when two nonsister chromatids exchange DNA material. Approximately two or three crossovers occur on a single chromosome. A homologous pair refers to two chromosomes that paired up in a process called synapsis. During synapsis, a nucleoprotein lattice is created to ensure that the DNA of non-sister chromatids are aligned. As the lattice breaks down, the chromatids remain attached at regions called chiasmata. These points of cross allow the exchange of genes between the chromatids. When the chromatids separate as chromosomes, the resulting haploid gametes will be genetically unique and varied.