Chromosome cycles turned on in unfertilized sea urchin eggs exposed to NH4OH.

In unfertilized sea urchin eggs treated with NH(4)OH-sea water, the chromosomes condense after a first round of DNA synthesis and go through a chromosome cycle. The chromosomes split visibly, but sister chromosomes are not further separated. They regress into an interphase nucleus. These cycles repeat, producing eggs with large numbers of chromosomes. No mitotic apparatus is seen and the eggs do not divide. There is some microscopic evidence of limited chromosome movement, interpreted as centrifugal movement of the condensed chromosomes before they split and a centripetal movement as the split chromosomes decondense to reconstitute the nucleus. The eggs so treated with NH(4)OH are considered to be unfertilized eggs and can be fertilized later. Such later fertilization permits the introduction of paternal nuclei after the maternal nuclei have progressed some way toward the above-described chromosome condensation. The paternal chromosomes condense prematurely at the time when the maternal chromosomes condense. At the same time, premature with respect to the time of fertilization, mitotic apparatus form.