Control of nucleic acid charge on the X -chromosome of the hamster

Abstract

The differential segment of the X -chromosome of Cricetus auratus when its nucleic acid charge is reduced appears as a thin, unspiralized and understained thread. This happens under all conditions which lower the frequency of spermatogenesis and in proportion to their effect in doing so, viz. winter conditions (anoestrus), undernourishment, disease and old age. The frequency of the effect is variable, ranging from nil to 90% of cells in a given testis. In up to 3% of cells, the starvation of the differential segment is recognizable but incomplete. In 10 % the sex bivalent could not be recognized and its differential segment was assumed to be fully charged. When conditions lead to extreme nucleic acid depletion there is some failure of chiasma formation and metaphase pairing of both autosomes and sex chromosomes. This indirect deprivation of nucleic acid, like that directly induced by cold, suppresses spiralization and hinders division or separation of the chromosome threads. Extreme food starvation, combined with senility, stops spermatogenesis at the pachytene stage, presumably owing to protein as well as nucleic acid starvation. The variability of nucleic acid charge in a warm-blooded animal as opposed to its regularity when reduced by cold in plants and Amphibia seems to be due to its dependence on the supply of ribose nucleic acid and not on the simpler chemical process of conversion of this supply into the desoxyribose form. The sensitivity of heterochromatin to variation in nucleic acid supply makes it possible to use its behaviour as an indicator of nucleic acid metabolism.