Prophase I of meiosis is split into five stages. Zygotene is the second stage in this process. It comes right after the Leptotene stage (stage 1) in which Chromatin fibres begin to condense. During the Zygotene stage, condensation continues to make distinguishable Chromosomes. Homologous chromosomes also begin to pair up with one another through the process of Synapsis to form bivalents (or the synaptonemal complex).
Crossing over is possible once the Homologous_chromosomes are in close proximity with one another. This is important as the exchange of genetic information between corresponding regions of the Homologous_chromosomes is crucial to producing variation in a population.