title: Long spacers among ribosomal genes of Drosophila melanogaster.
abstract: The fruit fly, Drosophila melanogaster, has 200 tandemly arranged copies of the ribosomal RNA genes (rDNA) per haploid genome. One such cluster of rRNA genes occurs on the X and one on the Y chromosome. The basic repeating unit of the rRNA gene consists of a segment coding for 18S rRNA and 28S rRNA followed by a non-transcribed spacer (Fig.1). In the X chromosome, there are two major size classes (12 and 17 kilobases) and numerous minor size classes of rDNA repeats. Most of this length heterogeneity is generated by insertions at a specific site in the 28S gene. The frequency and size patterns of these insertions in the 28S gene differ in the X and Y chromosomes. Electron microscopic analysis of rDNA-rRNA hybrids has shown that there is also length heterogeneity in the rDNA non-transcribed spacer. This heterogeneity is due in part to internal sequence repetition. We have now examined further the length heterogeneity of the rDNA spacer and have observed a class of spacers that we shall refer to as 'long spacers'. The size and frequency of these long spacers are different in the X and Y rDNA.
mesh_terms: D000818:Animals; D002874:Chromosome Mapping; D004262:DNA Restriction Enzymes; D004331:Drosophila melanogaster / Q000235:genetics*; D005260:Female; D005796:Genes; D008040:Genetic Linkage; D008297:Male; D008970:Molecular Weight; D012335:RNA, Ribosomal / Q000235:genetics*; D014960:X Chromosome; D014998:Y Chromosome
chemical_list: D012335:RNA, Ribosomal; D004262:DNA Restriction Enzymes