[Chromatin structure within sites of DNA replication]

Conformational changes in chromatin structure are nowadays the object of intensive research due to its importance for proper regulation of intranuclear processes. The fine structure of chromatin within the DNA replication sites was studied in in situ fixed cells and cells permebilized by low ionic strength solutions in the presence of divalent cations. The latter method provides visualization of higher level chromatin structures such as globular chromomeres and chromonema fibres. Nascent DNA was detected immunochemically using anti-BrdU antibodies on the surface of ultrathin sections prepared from Epon-embedded material. It was shown that newly replicated DNA preferentially localized within the zones filled with globular and fibrillar elements with characteristic diameter of 30 nm, and not in chromonema fibres, while after replication had been completed DNA became embedded into as thick as 60-80 nm chromonema elements. The results obtained are discussed in the context of conception of hierarchical folding of chromatin fibers.