FIG. 1.

Map of the human rRNA gene repeat. The human rRNA gene forms a 44-kb repeat unit, with the four segments defined by EcoRI (E) sites. Boxes show the positions of sequences coding for the 18S, 5.8S, and 28S rRNA genes. Spacer, nontranscribed region. The horizontal arrow indicates the transcribed region. Numbers for genomic positions are according to GenBank (accession no. U13369). 1 to 3656, 5′ external spacer; 3657 to 5527, 18S segment; 5528 to 6622, internal spacer I; 6623 to 6779, 5.8S segment; 6780 to 7943, internal spacer II; 7944 to 12969, 28S segment; 12970 to 13314, 3′ external spacer. Replication starts bidirectionally in the nontranscribed intergenic spacer (18, 28, 74). Unidirectional replication fork barriers are located at the 3′ end of the transcribed region (23, 28).