Final answer:
Crossing over during prophase I of meiosis is shown microscopically by the structures called chiasmata, where non-sister homologous chromatids exchange genetic material.
Step-by-step explanation:
During prophase I of meiosis, crossing over is indicated by the microscopically visible structure known as chiasmata. This process involves the synaptonemal complex mediating the exchange of chromosomal segments between non-sister homologous chromatids, leading to genetic variation. During prophase I of meiosis, crossing over is indicated by the chiasmata, which can be observed visually under a microscope. The chiasmata are large protein assemblies located along the synaptonemal complex, which support the exchange of chromosomal segments between non-sister homologous chromatids.
As prophase I progresses, the synaptonemal complex begins to break down, and the resulting crossover points, where chromosomal segments have been exchanged, are called chiasmata, which are visible under a microscope. These chiasmata are essential for the proper separation of homologous chromosomes during meiosis I.