Prophase I is the first stage of meiosis, a type of cell division that reduces the chromosome number by half. During this phase, chromosomes condense and become visible under a microscope. Each chromosome pairs with its homologous partner, forming structures called tetrads. This pairing is crucial for genetic diversity. Additionally, during Prophase I, crossing over occurs, where segments of DNA are exchanged between homologous chromosomes. This process increases genetic variation in the resulting gametes. The nuclear envelope also begins to break down, and spindle fibers start to form, preparing the cell for the next stages of meiosis.