Crossing-Over

Homologous chromosomes move together during synapsis in prophase of meiosis I. Proteins hold these chromosome pairs very close together.

Sometimes homologous chromosomes exchange segments of DNA. This is called crossing-over. When crossing-over occurs, each chromatid ends up with a unique sequence of DNA.

At the completion of meiosis, the new combinations of crossed-over genes are found in the sex cells.

Crossing-over adds variation to the offspring and ultimately contributes to the evolution of a species.