Beaded organization of chromatin in the salivary gland chromosome bands of Drosophila melanogaster

Abstract

Beads on a string type of organization were found in chromomeric fibrils by electron microscopic studies on thin‐sectioned polytene chromosomes of Drosophila melanogaster. The beaded fibrils occur parallelly and are probably supercoiled pairs. In certain small bands the orientation of paired fibrils is clearly transversal to the longitudinal axis of the chromosome. The transverse position and some hairpin‐like ends of paired fibrils point to the cruciform palindromes or closed loop type or chromomere organization in these bands. In larger bands the coiling organization of chromomeric fibrils is more complicated. Some hypothetical models for the uni‐ and multireplicon type of chromomeres are presented. The obvious branching of chromomeric fibrils at the border region of certain hands indicates a local polyteny in these bands.