Localization of ribosomal RNA genes by high resolution autoradiography

In the myxomycete Physarum polycephalum, the genes for ribosomal RNA replicate to a large extent in the G2 phase of the mitotic cycle. In mitotically synchronous plasmodia this fact allows the rRNA genes to be localized by electron microscopic autoradiography. After labelling with thymidine in the G2 phase, silver grains were concentrated over the fibrillar and not over the granular regions of the nucleoli. This shows the presence of DNA in these fibrillar regions or their immediate vicinity. Following labelling and chase during the G2 period, nucleoli are dispersed during mitosis and silver grains concentrate over some regions of condensed chromatin.