The human ribosomal RNA genes: structure and organization of the complete repeating unit

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Abstract
The complete repeating unit of the human ribosomal RNA gene has been reconstructed by the cloning of approximately 27 kilobases (kb) of non-transcribed spacer. The structure of this tandemly repeated gene can now be studied in its entirety. We report the analysis of spacer DNA by molecular cloning and its organization in the genome by Southern transfer analysis. These studies reveal both length and sequence variation of the spacer. Sequence variations are distributed throughout the spacer while the length variations exist near the 5′ end of the transcript and just beyond the 3′ end. The human spacer shares extensive homology with primates but little with other mammals. Within the primates the degree of homology reflects the rapid evolutionary changes characteristic of the primate group.