(a) The salt concentration dependence of the charge Qb of the tightly bound counterions (per nucleotide) for a B-DNA oligomer of length L=24 in [Mg2+] (upper line) and in [Na+] (bottom line) solutions. Lines, from the present model; symbols, from the Monte Carlo simulation. (b) The salt concentration dependence of the total charge Qtot(Rb) of the bound ions per nucleotide for a B-DNA oligomer of length L=24 in [Mg2+] (upper lines) and [Na+] (bottom lines) solutions. Solid lines, from the present model; dotted lines, from the counterion condensation theory for an infinite-length DNA in [Mg2+] (upper line) and in [Na+] (bottom line) (Ref. 22); dashed line, from the counterion condensation theory for an oligomer in monovalent ionic solution (Ref. 88); symbols, from the Monte Carlo simulation.