[Intragenomic specificity of DNA methylation in animals. Qualitative differences in tissues and changes in methylation of repeating sequences during aging, carcinogenesis and hormonal induction].

5-methylcytosine (m5C) is nonrandomly distributed in mammalian genome. The unique sequences in DNA of all species studied (mouse, rat, cow) methylated to a similar extent and are characterized by a minimal m5C content (0.8--0.9 mole%). The very renaturating sequences (including satellite DNA) possess a maximal m5C amount. The rarely repeated (10 to 1000 folds) sequences which are known to contain genes for rRNA and tRNA as well as for histones are characterized by an elevated level of methylation. It is established that tissue differences in m5C content in DNA as well as decrease in the DNA methylation level with age and on spontaneous lympholeukosis in cattle and, on the contrary, increase in DNA methylation degree in rat liver as a result of induction with hydrocortisone, are due to tissue specific changes in the methylation level of repeated but not unique sequences. The methylation level of sequences differing in the reiteration degree seems to be mainly due to CG-dublet frequencies. In the unique sequences this particular dinucleotide is almost completely methylated. Nevertheless, the multiplicity of DNA methylases in the animal nucleus is not ruled out. Methylation of the CG sequence is supposed to protect animal DNA's, especially structural genes, against restriction endonucleases. The decrease in DNA methylation in animals with age and on lympholeukosis detected by us is considered to be one of the possible reasons for chromosome lesions and for distortions in DNA replication and transcription.