The dimethylation of Arg at the 42nd position (R42me2) of H3 histone, a post-translational modification (PTM) in nucleosomes close to the DNA entry/exit region, showed controversial gene regulations. To address this discrepancy, we performed comprehensive all-atom replica-exchange molecular dynamics simulations with and without a single PTM, either symmetric (R42me2s) or asymmetric (R42me2a) dimethylation. Together with a kinetics analysis, our simulations showed that DNA at the entry/exit region in the R42me2a nucleosome adopts a relatively more open conformation than that in the unmodified nucleosome, whereas R42me2s exhibits significantly weaker or even negligible effects on DNA dynamics and structures, which may provide clues of the discrepancy of gene regulation by R42me2. Our approach will be useful to study the mechanism of nucleosome dynamical change induced by a subtle modification.