Animation 17.9 Prophase I of Meiosis
Textbook Reference: Meiosis and Fertilization, p. 681
Many important events occur during prophase I of meiosis. In the first stage, leptotene, the chromatin of the chromosomes is condensing. Recombination occurs at a high frequency during meiosis, and is initiated by double strand breaks that are induced in this stage.
The close association of homologous chromosomes, called synapsis, begins during the next stage, zygotene. During zygotene, a zipperlike protein structure, called the synaptonemal complex, forms along the length of the paired chromosomes.
The synaptonemal complex keeps the homologous chromosomes closely associated and aligned with one another through the pachytene stage, during which recombination between homologous chromosomes is completed, leaving the chromosomes linked at the sites of crossing over, called chiasmata.
The complex disappears at the diplotene stage and the homologous chromosomes separate along their length. At this stage, each chromosome pair (called a bivalent) consists of four chromatids with clearly evident chiasmata. The chiasmata are critical for the correct alignment of chromosomes at metaphase.
Diakinesis, the final stage of prophase I, represents the transition to metaphase during which the chromosomes become fully condensed.