Distribution of repetitive DNA sequences in the polytene chromosomes of Rhynchosciara americana

Cytochemical properties of euchromatin and heterochromatin

Two classic cytochemical tests, the Feulgen-Schiff reaction and Toluidine Blue basophilia, have been employed for investigating the differential characteristics of heterochromatin and euchromatin. Differences have been detected in the Feulgen hydrolysis kinetics, the Feulgen absorption spectrum, the image analysis of Feulgen-stained material, and the binding of Toluidine Blue under ordinary and Mg2+ competitive staining conditions. The differences are assumed to be a function of the composition and stereo-arrangement of the DNA and DNA-protein complexes present in these chromatin types and are possibly associated with physiological activities whose whole meaning is far from being clear. Differences in optical retardations in Toluidine Blue-stained material were also found. These are interpreted as being due to chromatin packing state and selective removal of histones promoted by the acetic acid-ethanol fixative.

Acid lability of deoxyribonucleic acids of some polytene chromosome regions of Rhynchosciara americana

An analysis of profiles of Feulgen hydrolysis curves has been carried out in two heterochromatic (B-15 and C-11) and one euchromatic (C-1) regions of polytene chromosomes of salivary glands of 66 day old Rhynchosciara americana larvae. Differences among these three regions have been found mostly concerning the period of predominant apurinic acid breakdown including the "lag" phase of teh apurinic acid depolymerization. The hydrolysis time for which maximal Feulgen-DNA values are attained also differs when comparing the hetero- and euchromatic regions of the C chromosome and the heterochromatic regions C11 and B15 to each other. Different labilities of DNA to acid hydrolysis have therefore been demonstrated for these chromosome regions. This has been tentatively correlated with differences in non-histone proteins some of which are assumed to be involved in chromatin compactness. Others (probably not concerned with chromatin compactness) are suggested to be associated with certain specific DNA types which are known to occur in these chromosome regions.