ABSTRACT
The genomes of eukaryotes contain, in addition to the four nucleobases, a single minor modified base 5-methylcytosine that occurs in differing amounts in the DNAs of organisms and their tissues. In mammalian cells, enzymatic methylation and replication of DNA are closely coupled. DNA methyltransferase (DMase) can be isolated from chromatin that is prepared from nuclear lysates by pelleting through dense sucrose gradients at low ionic strength, but is generally isolated from nuclei. In regenerating liver, the amount of DMase trebles during DNA replication. Flat bed isoelectric focusing on granulated gel was used to study the heterogeneity of fraction-III enzyme. In the nucleus of eukaryotes, DNA is packaged into nucleosomes which in turn are organized into higher-order structures of varying degrees of complexity and compaction. In addition, various individual nonhistone chromatin proteins are postulated to be distributed either widely or selectively in chromatin domains on the basis of their function.
