WHOLE MOUNT ELECTRON MICROSCOPY OF POLYTENE CHROMOSOMES FROM DROSOPHILA MELANOGASTER

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Abstract
A simple method of whole mount EM was developed to study the fine structural organization of D. melanogaster polytene chromosomes. This method preserves the structural features of these chromosomes and may be of use for rapid gene localization at the ultrastructural level. Chromosome bands were electron dense regions composed of closely packed groups of chromomeres; thin bands consisted of a single row of transverse chromomeres, while thicker bands were composed of 2 to several rows of chromomeres. Interband regions contained relatively straight chromatin fibers which traversed the interband zone either singly or in bundles of several fibers. The interband chromatin fibers were generally 130 .ANG. in diameter or thicker, and appeared to be composed of 2 or more thinner (80-90 .ANG.) fibers. Presumptive puff regions were characterized by extended chromatin fibers having a longitudinal orientation, however, some transverse rows of chromomeres were also seen in these regions, suggesting that not all of the chromomeres in a band may be involved in puff formation. The chromatin fibers in the puffs were frequently thinner than those found in the interband regions. In stretched chromosomes, the chromatin fibers were drawn out into a mass of parallel fibers without any distinction between band and interband regions, supporting the hypothesis that individual chromatids are continuous through both band and interband regions and probably extend throughout the whole chromosome. No core fibers were observed in any of the polytene chromosomes studied.