Prophase I

Prophase-I lasts far longer than the prophase of mitosis, and it is where the features that make meiosis genetically special all occur. It is conventionally divided into five sub-stages:

• Leptotene: the nuclear volume increases, and the chromosomes become distinct — long, thread-like, and coiled. They adopt a characteristic arrangement inside the nucleus called the bouquet stage, in which the ends of all the chromosomes converge toward the side of the nucleus where the centrosome lies. The centrosome itself duplicates during this sub-stage and begins migrating toward the two opposite poles.
• Zygotene: homologous chromosomes — one maternal, one paternal in each pair — come together in an intimate, point-by-point pairing called synapsis, brought about by the formation of a structure called the synaptonemal complex.
• Pachytene: each paired homologue is now seen to consist of two sister chromatids, so that a paired set shows four visible chromatids together, called a tetrad (or tetravalent). The homologous chromosomes of each pair begin to separate from one another at this stage, but they do not come apart completely — they stay joined together at one or more points, which is where crossing-over (also called recombination) takes place: the chromatids break at these contact points and the broken segments are exchanged between non-sister chromatids of the two homologues.
• Diplotene: although the crossover points (chiasmata) are actually formed back in pachytene, they only become clearly visible now, in diplotene, as the paired homologous chromosomes begin to repel each other and pull apart — a process called desynapsis, in which the synaptonemal complex breaks down. The cross-shaped points where the homologues remain joined are the chiasmata. …