title: Organization, replication and modification of the human genome: temporal order of synthesis and methylation of two classes of HeLa nDNA separated in Ag+--Cs2-SO4 gradients.
abstract: During the HeLa S-phase, DNA was methylated, at 1-hr intervals in isolated nuclei and fractionated in Ag+-Cs2SO4 gradients providing a heavy GC-rich peak and a main light AT-rich peak. Both size and specific methylation of these peaks changed during the nDNA duplicative phase. Replication of the heavy GC-rich nDNA fraction, which contains genes for ribosomal RNA, occurred in early S; in contrast, replication of the main AT-rich nDNA fraction was maximal in late S. Concomitantly, specific methylation of the GC-rich nDNA was maximal in the first part of S, while that of the AT-rich nDNA was maximal in the second part of S. This suggested that genes are replicated and methylated with order during the S-phase.
mesh_terms: D002453:Cell Cycle; D002499:Centrifugation, Density Gradient; D003584:Cytological Techniques; D004247:DNA / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D005796:Genes; D006367:HeLa Cells; D007399:Interphase; D008745:Methylation
chemical_list: D004247:DNA