Nucleolar DNA in Oocytes of Xenopus Laevis

The ovaries of newly metamorphosed Xenopus females contain oocytes in all stages of early meiotic prophase. In pachytene nuclei extrachromosomal nucleolar DNA appears in the form of a thin cap covering one side of the nucleus. During pachytene this cap of DNA enlarges to occupy half the nucleus. After pachytene the nucleus grows rapidly and the cap of DNA disperses into numerous tiny granules which become scattered throughout the nucleoplasm. Autoradiographs of cells which have been incubated with [3H]thymidine, and microspectrophotometric measurements of the Feulgen dye contents of nuclei in various stages of meiosis, show that the extrachromosomal nucleolar DNA is synthesized during the pachytene and immediate post-pachytene stages. Microspectrophotometric comparison of the Feulgen dye contents of post-pachytene nuclei in which the DNA cap has dispersed, and similarly prepared mouse liver nuclei, show that the post-pachytene nuclei have about 30 µµg of extrachromosomal pachytene nucleolar DNA. In [3H]thymidine autoradiographs early pachytene nuclei are less heavily labelled than late pachytene and early diplotene nuclei. Consequently it is proposed that primary replicas of the chromosomal nucleolar organizer undergo a series of post-detachment replications.