Zygos means pair and tene means thread. At this stage the homologous chromosomes are arranged in pairs. Attraction begins between homologous chromosomes. Chromosomes come close to each other and pair up (one from the father and the other from the mother). The process of pairing of chromosomes is called synapsis and a pair of chromosomes is called bivalent. Bivalent chromosomes are linked by two synaptonemal complexes. The pairing process may start from one end to the other end, or may begin at the centromere and gradually spread to both sides, or may begin at a site. Bivalents form half the number of chromosomes in each cell. At this stage the chromosomes are more compressed, thicker and shorter. Nuclear membrane and nucleolus are clear. Dr. Siddique Publications
[Chromosomes having the same structure are called homologous chromosomes. One of the homologous chromosomes comes from the father and the other from the mother]