Final answer:
The region where crossing over occurs between homologous non-sister chromatids is called a chiasma, which is microscopically visible during prophase I of meiosis.
Step-by-step explanation:
The microscopically visible region where crossing over has occurred between homologous non-sister chromatids is called a chiasma. Crossing over happens during prophase I of meiosis, when homologous chromosomes pair up tightly in a process called synapsis, facilitated by the synaptonemal complex. This complex promotes the exchange of genetic material between non-sister chromatids, leading to genetic variation. Recombination nodules located along the synaptonemal complex facilitate the process, where the DNA is cleaved, modified, and then reconnected, forming the visible chiasmata.
As prophase I progresses and the synaptonemal complex dissolves, these homologous chromosomes remain attached at points known as chiasmata. These points are critical for maintaining the proper alignment and eventual separation of the chromosomes during meiosis I. Importantly, at least one chiasma is formed on every homologous pair of chromosomes to ensure correct segregation, although multiple chiasmata can be present, depending on the species and the length of the chromosome.