Cytological Studies on the Suspensor Polytene Chromosomes ofPhaseolus: DNA Content and Synthesis, and the Ribosomal Cistrons

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Abstract
Feulgen spectrophotometry determinations of Phaseolus polytene chromosomes demonstrate that in all suspensor cells examined there is an exact doubling of the genome in each DNA synthesis cycle and the maximum amount of DNA per suspensor cell is 8,192C. 3H-thymidine incorporation studies demonstrate that there is differential replication of these chromosomes with synthesis of the euchromatin, followed by the β-heterochromatin adjacent to the centromere and finally by the α-heterochromatin of the centromeres. During the replication process the chromosomes assume a diffuse appearance. After 90 minutes the first labeled chromosomes which are contracted appear. These chromosomes have silver grains exclusively over the centromeres. After 4–6 hours incubation in 3H-thymidine contracted chromosomes which are totally labeled are observed. No evidence for underreplication or amplification is indicated. The ribosomal cistrons were cytolocalized by in situ hybridization with P. vulgaris rRNA. These were found only in the nucleolar organizing system: the attachment sites to the nucleolus of the four nucleolar organizer chromosomes, in the nucleolus proper and in the satellites of these four chromosomes.