plicated but individual chromatids are not visible. Instead of lining up on a metaphase, as in mitosis, chromosomes come together in pairs (2). Each chromosome in a pair is similar in structure (homologous), but would have come originally from different parents. Later in prophase the homologous pairs twist round each other and chromatids may cross over (3). Breaks occur at these crossovers or chiasmata, and pieces of chromatid are exchanged (4). Chromosome pairs line up across the equator of the spindle at metaphase I (5). In anaphase I the chromosomes separate and travel to opposite ends of the spindle. The chromosomes migrate to the equators of two new spindles for metaphase 2 (7). Next the chromatids are pulled apart in anaphase 2 to form four clusters of chromosomes in telophase 2. The nuclear envelopes reform around four haploid nuclei that will give rise gameteThe leptotene. This phase differs only slightly from the early stages of mitosis. Usually are the cells and nuclei of meiotic tissues bigger than that of their neighbouring tissues and often do they seem to be longer and are longitudinally structured. At regular intervals can thickenings be found, like beads on a string: the chromomeres. Their number, size and positioning is constant in each species. The zygotene. During this phase begins the pairing of homologous chromosomes. It is also called synapsis and the resulting structure synaptic complex. Directly after initiation of the process does the pairing spread like a zipper across the whole length of the chromosome. The pachytene. During the pachytene does the pairing stabilize. The number of synaptic complexes corresponds to the number of chromosomes in a haploid set of the respective species. The pairs are also called bivalents. The diplotene. The bivalents separate again. During this does it become visible that each chromosome is built of two chromatids, so that the whole complex stands still, four strands during the se...
