Final answer:
Crossing-over in eukaryotes occurs during prophase I of meiosis and involves the exchange of genetic material between non-sister chromatids of homologous chromosomes.
Step-by-step explanation:
Crossing-over in eukaryotes occurs during prophase I of meiosis. It is the exchange of genetic material between non-sister chromatids of homologous chromosomes. This process, also known as recombination, happens when homologous chromosomes align and some genetic material from two chromatids switch chromosomes, creating unique combinations of genes.