Analysis of DNA looped domains organization during Triturus cristatus spermatogenesis

Chromatin from eukaryotes is organized in DNA loops with sequential attachments to a nucleoskeleton named nuclear matrix. This organization plays major roles in replication, transcription, recombination, DNA repair, chromosome condensation and segregation. During spermatogenesis, chromatin undergoes several dynamic transitions, which are often associated with important changes not only in its physical conformation but even in its compositions and structure. To understand the periodical change in the functional organization of chromatin during spermatogenesis, the higher order organization of chromatin in different testicular cell types (pachytene spermatocytes, round spermatids) and the epididymal sperm of Triturus cristatus have been investigated. The expansion and the contraction of nucleoid DNA were measured with a fluorescence microscope following exposure of nucleoids to increasing concentrations of ethidium bromide (EtBr) (2.5-200 μg/ml) as an intercalating dye to induce DNA-positive supercoils. Nucleoids from all studied cell types exhibited a biphasic change (condensed-relaxed-condensed) in size as a consequence of exposure to increasing concentrations of EtBr, indicating that they contained negatively supercoiled DNA. At higher EtBr concentrations, maximum positive supercoiling occurred in pachytene DNA loops. Our data suggest that pachytene DNA is the most open chromatin conformation in terms of EtBr intercalation.