The first phase of Meiosis is Prophase I. This phase is more complicated and prolonged as compared to the similar stage of mitosis.
Prophase I is divided into five sub-phases: leptotene, zygotene, pachytene, diplotene and diakinesis. Another sub-phase called pre-leptonema is sometimes recognized prior to leptonema. Chromosomes are not distinguishable because of their thinness. Sex chromosomes are often seen in heterochromatic bodies.
At the start of prophase I, the chromosomes have already duplicated. During prophase I, they coil and become shorter and thicker and visible under the light microscope.
The duplicated homologous chromosomes pair and crossing-over (the physical exchange of chromosome parts) occurs. Crossing-over is the process that can give rise to genetic recombination. At this point, each homologous chromosome pair is visible as a bivalent (tetrad), a tight grouping of two chromosomes, each consisting of two sister chromatids. The sites of crossing-over are seen as crisscrossed non-sister chromatids and are called chiasmata (singular: chiasma).
The nucleolus disappears during prophase I.
In the cytoplasm, the meiotic spindle, consisting of microtubules and other proteins, forms between the two pairs of centrioles as they migrate to opposite poles of the cell.
The nuclear envelope disappears at the end of prophase I, allowing the spindle to enter the nucleus.
Prophase I is the longest phase of meiosis, typically consuming 90% of the time for the two divisions.