Final answer:
Crossing-over occurs during prophase I of meiosis and is less frequent in mitosis. Mitosis involves the production of identical cells, while meiosis creates genetic diversity in gametes.
Step-by-step explanation:
Crossing-over is a process where genetic material is exchanged between non-sister chromatids of homologous chromosomes. This typically occurs during prophase I of meiosis, where the chromosomes line up in pairs, forming a tetrad, and genetic recombination can occur multiple times. In contrast, mitosis does not involve crossing-over as frequently as meiosis does.
Therefore, in answering the question, crossing-over occurs much less frequently during mitosis than during meiosis. Mitosis is mainly for growth and maintenance of the organism and produces genetically identical cells, whereas meiosis involves reduction division producing gametes with varied genetic combinations contributing to genetic diversity.