In: The eukaryote chromosome. Canberra, Australian National Univ Press (1975) pp. 1-33
Numerous models have been proposed to explain why only one or a few cistrons are detected per Drosophila melanogaster chromomere, although these chromomeres contain sufficient DNA to code for a much larger number of polypeptides. Since these models can be distinguished according to the topography that each imposes on the repetitive and non-repetitive sequences, and on the transcribed and translated sequences, we have developed a system for mapping these sequences within and among the chromomeres. It is based on the isolation of segments of chromosomal DNA in homogeneous form. This is accomplished by cloning hybrid molecules consisting of a segment of D. melanogaster DNA inserted into a plasmid DNA which can replicate autonomously in Escherichia coli. Both the poly(dA):poly(dT) and the EcoR1-ligase methods have been used to construct the hybrid plasmids. Nine hybrids have been isolated and the chromosomal segments analyzed for sequence repetition within the segment and within the genome. Their sequences have also been mapped by in situ hybridization to polytene chromosomes. Six segments belong to a single group defined by little or no repetition within the genome and localization to a single chromomeric region. The characteristics of this group are not compatible with tandem repetition models for the chromomere. The remaining three segments contain sequences repeated 30 to 400 times per genome, although one also contains non-repetitive sequences. All contain sequences found in the centromeric heterochromatin. One contains sequences which map at 15 different chromomeric regions in addition to the chromocenter. The third contains the gene for 18-28S rRNA.
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