Fig. 1.

The global DNA and POLD1 promoter methylation levels in the replicative senescence of 2BS and WI-38 cells. a, b Global genome DNA methylation (%) in different PDs of 2BS and WI-38 cells. Global DNA methylation was measured using the Methylamp™ Global DNA methylation Quantification Kit. c, d DNA methylation status of CpG islands located in the region of the POLD1 promoter in different PDs of 2BS and WI-38 cells, measured by bisulfite DNA sequencing analysis. e, f The percentage of cytosine methylation of each CpG island of the POLD1 promoter in different PDs of 2BS and WI-38 cells. The data were analyzed by one-way ANOVA, three independent experiments in each group, *p < 0.05, **p < 0.01, vs. the young cells (25PD). g, h The DNA methylation pattern of part of CpG island 3 in the POLD1 promoter at different PDs of 2BS and WI-38 cells. The methylation levels of the sequence between the 33 CpG site and 38 CpG site and the methylation of the 36 CpG site increased significantly in replicative senescence. Empty circle, unmethylated cytosine; filled circle, and methylated cytosine