(a) The polyelectrolyte length L dependence of the charge Qb of the tightly bound counterions per nucleotide in [Mg2+] (upper line) and [Na+] (bottom line) solutions at bulk salt concentration 0.01M. Solid lines, from the present model; symbols, from the Monte Carlo simulation. (b) The polyelectrolyte length L dependence of the total charge Qtot(Rb) of the bound ions per nucleotide for a B-DNA oligomer in [Mg2+] (upper two lines) and [Na+] (bottom two lines) solutions at bulk salt concentration 0.01M. Solid lines, from the present model; dotted lines, from the counterion condensation theory for the DNA of infinite length in [Na+] (bottom line) and [Mg2+] (upper line) solutions (Ref. 22); dashed line, from the counterion condensation theory for ionic oligomer in monovalent ionic solution (Ref. 88); symbols, from the Monte Carlo simulation.