A student studying meiosis in a locust testis recorded the following descriptions of four cells at different stages. (Note: The descriptions are not in chronological order.)
Cell P: Condensed chromosomes are visible as bivalents. The nuclear envelope is breaking down. Chiasmata are present between some homologous chromosomes. Spindle microtubules are starting to form.
Cell Q: Chromosomes, each consisting of two sister chromatids, are aligned singly on the equatorial plate. Spindle fibers attach to the centromeres from opposite poles.
Cell R: Homologous chromosomes are being pulled to opposite poles by the spindle. Sister chromatids remain attached at the centromere. The cell is elongated.
Cell S: Two groups of chromosomes have reached the poles. Nuclear envelopes are reforming around each set. The chromosomes are decondensing. The cell membrane is beginning to invaginate.
Place these cells in the correct chronological order and describe the key events occurring at each stage.