Abstract
Experiments were designed to see whether DNA complementary to ribosomal RNA was confined to the nucleolar organizer (NO) region. DNA was prepared from 4 stocks of Drosophila melanogaster carrying 1, 2, 3, and 4 doses of the "NO" segment and hybridized to isotopically labeled ribosomal RNA. The results obtained support the following conclusions. (1) The wild-type genome saturates at 0.27% of the DNA indicating that it contains approximately 200 sites per diplold set for each of the 2 ribosomal components. (2) The nucleolar organizer regions on the X and Y chromosomes contribute equally to the proportion of DNA complementary to ribosomal RNA. (3) The proprotions of the DNA found to be complementary to ribosomal RNA in the different stocks correspond to that predicted from the genetic constitution and the assumption that all the DNA complements of ribosomal RNA are confined to the nucleolar organizer locus. (4) By identifying the "NO" segment as the site of the required DNA templates the data support the assertion that the nucleolus is the site of ribosomal RNA synthesis.

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