Chromatin circles in amphibian provitellogenic oocytes.

Previtellogenic oocytes of Odontophrynus americanus display hundreds of chromatin circles. Electron microscopy of spread preparations of isolated nuclei shows that the circles originate from the chromatin. The circles change their morphology and form new copies. The length of the DNA packed in the nucleosomal circles is about 2.5--3.5 micrometer or multiples of this value. Assuming that histones need not be removed from chromatin before DNA replication we suggest that the circles might belong to the process of rDNA amplification.