Fig. 5. Ultrastructural identification of initiation sites of rDNA transcription within the nucleolus. (A,B) Immunogold detection of BrUTP-labelled rRNAs on ultrathin sections of isolated nucleoli from ELT cells, pulse-labelled for 15 minutes with BrUTP after a 15 minute incubation with transcription medium (A) or pulse-labelled for 15 minutes with BrUTP after a 15 minute incubation in the presence of an elongation inhibitor, cordycepin, instead of ATP (B). Bar, 0.25 µm. (C) Density (gold particles/µm²) on the FC, DFC, GC and resin (R) in nucleoli isolated from ELT cells. The results represent means±s.e.m. 24, 24 and 14 random micrographs were analyzed and 464, 381 and 141 gold particles were counted, respectively. Student's t-test for nucleolar components versus resin (+ P<0.05, ++ P<0.01) for each nucleolar component in untreated versus treated nucleoli (^* P<0.05, ^** P<0.01) and for FC versus DFC in untreated or treated nucleoli (^ P<0.05, ^^ P<0.01) was used.