According to the textbook description of Prophase I stages:

• Diakinesis is marked by the completion of terminalisation of chiasmata .
• Pachytene is characterised by the appearance of recombination nodules, the sites at which crossing over occurs .
• Zygotene involves chromosome pairing (synapsis) accompanied by the formation of synaptonemal complex .
• Leptotene is the stage where chromosomes become gradually visible under the light microscope (appearing as thin threads) .