during fertilization, double-stranded chromosomes are reconstituted. The first division sequence, called meiosis I, begins with prophase I. In
prophase I, as in mitosis, the replication of genetic material has already occurred during the interphase just before prophase; chromosomes become visible as they thicken and shorten but do not become visibly double-stranded until late prophase; nucleoli fade from view; the nuclear membrane disappears; and the spindle is organized. Yet, unlike in mitosis, in meiosis homologous chromosomes pair up on the spindle, laying so close to each other that they often become intertwined. The pairing of homologous chromosomes is called synapsis. Pairs of homologous chromosomes are called tetrads since they have four total chromatids. Each tetrad moves toward the equator of the spindle during late prophase. In metaphase I, unlike in mitosis where homologous chromosomes do not pair up and have their own separate microtubules, in meiosis each homologous pair of chromosomes moves onto a spindle as a single unit. Thus, in meiosis the number of microtubules in the spindle equals the number of tetrads (chromosome pairs) not chromosomes. In anaphase I, the two double-stranded chromosomes of each of the synapsed pairs move away from each other; centromeres do not uncouple. Therefore, half of the total number of chromosomes move toward each pole of the cell unlike in mitosis where the total number of chromosomes moves toward each pole. But, because of the synaptic pairing of homologous chromosomes each new nuclei gets one of each type of chromosome. Cytokinesis also begins during
