Q.Synapsis is pairing of .............

Prophase-I is the longest and most genetically significant phase of meiosis, and it is where every event unique to meiosis takes place. It is divided into five sub-stages. In Leptotene, the chromosomes become visible as long, thin, coiled threads and adopt the 'bouquet stage' arrangement, with their ends converged toward the side of the nucleus where the centrosome lies. In Zygotene, homologous chromosomes undergo synapsis — an intimate, point-by-point pairing brought about by the synaptonemal complex, with each pair consisting of one maternal and one paternal chromosome. In Pachytene, each paired homologue is seen to consist of two chromatids, so the pair together forms a four-stranded tetrad; the homologues begin to separate but remain joined at one or more points, where crossing-over (recombination) occurs — chromatids break at these points and exchange segments with the non-sister chromatid of the homologous partner. In Diplotene, the chiasmata formed during pachytene become clearly visible as the paired chromosomes start to repel each other and pull apart (desynapsis), with the synaptonemal complex breaking down. In Diakinesis, the chiasmata slide along the chromosome arms toward the chromosome ends (terminalization) and persist there until metaphase, while the nucleolus and nuclear membrane disappear and spindle fibres begin to appear. Prophase-I is therefore where homologous chromosomes are recognised, paired, and allowed to exchange genetic material — the essential first step behind both the reduction in chromosome number and the genetic variation that meiosis produces.