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Proportionate representation of ribosomal dna and balbiani ring dna in polytene chromosomes of chironomus tentans


, : Proportionate representation of ribosomal dna and balbiani ring dna in polytene chromosomes of chironomus tentans. Chromosoma (Berlin) 57(2): 185-197

Attention was focused on the question whether the most active genes of the polytene genome of C. tentans, i.e., rDNA [ribosomal DNA] and Balbiani rings, have undergone the same number of duplications as the bulk of the genome. RNA was extracted from isolated salivary glands, enriched for messenger RNA by poly U sepharose chromatography, and labeled with 125I. In situ hybridization showed that, apart from rRNA, Balbiani ring [BR] 1 and 2 transcripts were the major hybridizing RNA spp. present in this enriched preparation. BR1 hybridized only at the b-region. Transcripts of BR6 and puff I 17B were salivary gland specific and hybridized in situ to an extent of less than 5% of the RNA hybridized to BR1 plus BR2. Saturation hybridization levels of rRNA and Balbiani ring (BR1 plus BR2) RNA with polytene DNA from salivary glands were determined and compared with the saturation levels measured for DNA from adult flies and larvae. rRNA hybridized to the same level (0.09%) with polytene DNA as with DNA from primarily diploid cells (adult fly DNA) or from larvae. An equal hybridization level (0.04%) with each of these 3 types of DNA was also obtained for BR RNA. During polytenization in C. tentans, the genes for rRNA and BR1 and 2 were duplicated to the same extent as the bulk of the genome.

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