THE STRUCTURE OF THE SALIVARY GLAND CHROMOSOMES OF DROSOPHILA MELANOGASTER

Abstract

The salivary glands of Drosophila melanogaster were treated with 50%, 30% and 10% solutions of urea-alkali mixture for varying lengths of time at room temperature. The various appearances of the chromosomes after treatment are classifiable into three major series of transformations: α. β and γ which are described and analyzed in detail. These three types of transformation are shown to be due to different modes of migration of some of the nucleic acid from its peripheral position in the untreated chromosomes to the chromonemata which are located in the center of the chromosomes. Several observations show decisively that the number of chromonemata is four. The structure of the β₄-chromosomes is considered for a number of reasons to represent very closely the structure of the normal chromosome. From this and other facts learned in the study, the structure of the normal chromosome is described (see Conclusion for details). The different appearance of the coil and its nucleic acid bulbs at the free ends of the β₄-chromosomes substantiates the “telomere” concept. Experiments indicate that a “skeleton” exists in the discs of normal chromosomes which is resistant to nucleases but is destroyed by urea-alkali treatment. The structure of the lampbrush chromosomes produced by urea-alkali treatment is compared with the structure of the natural lampbrush chromosomes of Triton and chick described by Kolt-zoff.