Feulgen-DNA absorption curves of polytene chromosome regions of Rhynchosciara americana.

Feulgen absorption spectra of polytene chromosome regions, differing in types of DNA composition and of chromatin compactness, were determined microspectrophotometrically in squashed salivary glands of Rhynchosciara americana. The absorption curves exhibited a secondary maximum at their ascending branch after the chromatins were maximally depurinated. The degree of shoulder prominence could not be specifically correlated with the packing state or even with richness in satellite repetitive DNA of the chromatins. It is assumed that other factors may exist which play a part in the phenomenon that gives rise to the Feulgen absorption shoulder characteristics. Differences in the extraction rate of the apurinic acid fragments due to their binding to specific non-histone proteins are suggested as contributing to the above-mentioned absorption patterns when comparing the various chromatin types.