At the start of prophase I, chromosomes have duplicated already and during the prophase I, they coil and become thicker and shorter which can be seen under light microscope. After which, crossing-over follows and eventually the nucleolus disappears. In the cytoplasm, the meiotic spindle forms and the nuclear envelope disappears. This phase is considered as the longest phase of meiosis consuming almost 90 percent of the time for the two divisions.
The unique event that happens in prophase I of first meiotic division is the synapsis of homologous pairs.