[Age- and tissue-specific differences in intragenome distribution of 5'-methylcytosine in cow's DNA]

In aged animals the content of 5'-methylcytosine (m5C) in total thymus DNA (1,29 mol.%) and heart muscle (1,10 mol.%) is decreased by 30 and 40%, respectively as compared to that in the 8-months-old embryos. No differences in the GC-content and reassociation kinetics of these DNAs have been revealed. Consequently, the age differences in the m5C content are caused by the decrease in DNA methylation upon ageing. The degree of DNA methylation in various organs (thymus, heart) is different. Upon ageing the degree of genome methylation is decreased and the pattern of methylation is changed: the m5C content is decreased in the repeated sequences and remains practically unchanged in the unique ones. The level of methylation in thymus DNA of moderately (C0t = 4--400) and highly (C0t less than 4) repeated sequences is thereby decreased almost in the same degree. In heart DNA of aged cows the hypomethylation of highly repeated sequences is more pronounced as compared to the moderately repeated ones. The age- and tissue-specific decrease of DNA methylation is regarded as a possible mechanism responsible for the decrease of transcription and functional activity of the cells upon ageing.