[DNA degradation in vivo in dying thymocytes, model for the study of chromatin structure].

Properties of chromatin fragments which are formed as a result of in vivo DNA degradation in thymocytes of gamma-irradiated rats were studied. It is shown that under these conditions chromatin breakdown is accounted for by internucleosomal DNA scission without proteolysis and disturbance of nucleosome structure as judged from how they are split by DNAase I. Amount of mononucleosomes and their dimers, trimers and tetramers is in average about 10% of the fragments formed, and that of oligomers composed of about 8--10 nucleosomes equals about 80%. Core-particles and subnucleosomal fragments are never observed. Chromatin fragments of different size have been purified and their composition was studied. Electrophoresis in 3% PAAG--0.5% agarose divides mononucleosomes into two fractions. The less mobile fraction contains all histones, non-histone proteins and DNA fragments of 178 base pairs long. More mobile mononucleosomes do not contain at least fragments of 178 base pairs long. More mobile mononucleosomes do not contain at least one of the histone H1 subfraction and the size of their DNA is 168 base pairs. Chromatin fragments of different size contain both similar and different non-histone proteins. In mononucleosomes and their dimers, trimers and tetramers proteins are found which are absent in chromatin fragments of high molecular weight. It is assumed that mononucleosomes and their dimers, trimers and tetramers are products of active chromatin degradation and that high molecular weight oligomers are supernucleosomal structures of inactive chromatin.