During meiosis I, the chromosomes start pairing at:

During zygotene phase of Meiosis I, the chromosomes becomes shorter and thicker. The homologous chromosomes come to lie side-byside in pairs, called synapsis and form bivalent. During pachytene, crossing over occurs. During diplotene, the paired chromosomes begin to separate into two pairs of chromatids. During Leptotene, each chromosome becomes visible as two fine threads (chromatids). Related Theory Zygotene is that phase wherein the homologous chromosomes pair or come together in synapse. The pairing or coming together of homologous chromosomes is called synapsis. It may be facilitated by the synaptonemal complex. The zygotene stage is also described as bouquet stage since the telomeres cluster at one end of the nucleus.