The stage in nuclear division following *
prophase that commences with the disintegration of the nuclear membrane and the formation of the spindle. In *
mitosis individual chromosomes, each divided into chromatids, gather around the equator of the spindle. At metaphase of the first division of *
meiosis the bivalents formed during prophase gather at the equator. The two centromeres of each bivalent are finally situated one on either side of the equator. The placing of any one homologue on one or the other side of the equator is completely random, no factor operating to discriminate between 'maternal' and 'paternal' chromosomes. At metaphase of the second division of meiosis, spindles form in the haploid daughter cells resulting from the first division and the individual chromosomes become attached to the spindle equator, as in metaphase of mitosis.
The end of metaphase and the beginning of *
anaphase , in mitosis, is marked by the division of the centromeres so that the chromatids are no longer held together. In meiosis, division of the centromeres does not occur until the end of metaphase of the second division.
