Kinetochore-to-pole connections during prometaphase of the meiotic divisions in Urechis eggs

Abstract

Electron microscope observations of kinetochores and chromosomal spindle filaments during prometaphase of the meiotic divisions in eggs of Urechis caupo are presented. In the first meiotic division, sister kinetochores lie close together on the same side of the chromosome. By prometaphase of the second meiotic division, sister kinetochores have separated and moved to opposite sides of the chromosome. In early prometaphase of the first meiotic division, the kinetochore region of one member of a chromosome-pair may have connections to both poles, or only to one pole. In early prometaphase of the second meiotic division, sister kinetochores may be connected to the same pole or to opposite poles. A single kinetochore may have connections to both poles simultaneously. These observations strongly support Östergren's “orientation-by-pulling” hypothesis, by showing that (i) initial kinetochore-to-pole connections are made at random; (ii) the precise orientation of chromosomes by metaphase is brought about through the loss of some kinetochore-to-pole connections, and the stabilization of others; and (iii) the difference between meiosis and mitosis with respect to chromatid distribution is correlated with a difference in the structure of the kinetochore region.