This study investigates how ionic strength and multivalent cations affect the elasticity of single DNA molecules. Using a force-measuring optical trap, researchers determined the elastic properties of λ bacteriophage DNA under varying ionic conditions. The electrostatic contribution to the persistence length (P) varied inversely with ionic strength in monovalent salt, as predicted by the worm-like polyelectrolyte model. However, ionic strength is not always the dominant factor in DNA elasticity. Multivalent ions, such as Mg²⁺ and Co(NH₃)₆³⁺, significantly reduce P, leading to values as low as 250–300 Å, much lower than the 450–500 Å observed in monovalent salt. In contrast, polyamines like putrescine²⁺ and spermidine³⁺, with linear charge distribution, result in higher P values. The elastic stretch modulus (S) and P show opposite trends with ionic strength, contradicting macroscopic elasticity theory. DNA remains well described by the worm-like chain (WLC) model at trivalent cation concentrations that induce condensation, provided the molecule is stretched. A retractile force appears in the presence of multivalent cations at molecular extensions allowing intramolecular contacts, suggesting a "thermal ratchet" mechanism for DNA condensation. Ions significantly influence DNA's biological functions, such as wrapping around nucleosomes and binding to transcriptional proteins. While ion exchange equilibria are often cited, ions may also affect DNA structure and mechanical properties, including bending and torsional rigidity. Recent advances in nanomanipulation allow the mechanical behavior of single DNA molecules to be studied. The WLC model describes DNA as intermediate between a rigid rod and a flexible coil, with P representing the distance over which two segments remain directionally correlated. The study shows that DNA elasticity is influenced by ion concentration, valence, and structure, with multivalent cations having a more pronounced effect than monovalent ions. The findings highlight the importance of understanding ionic effects on DNA elasticity to comprehend key biological processes.This study investigates how ionic strength and multivalent cations affect the elasticity of single DNA molecules. Using a force-measuring optical trap, researchers determined the elastic properties of λ bacteriophage DNA under varying ionic conditions. The electrostatic contribution to the persistence length (P) varied inversely with ionic strength in monovalent salt, as predicted by the worm-like polyelectrolyte model. However, ionic strength is not always the dominant factor in DNA elasticity. Multivalent ions, such as Mg²⁺ and Co(NH₃)₆³⁺, significantly reduce P, leading to values as low as 250–300 Å, much lower than the 450–500 Å observed in monovalent salt. In contrast, polyamines like putrescine²⁺ and spermidine³⁺, with linear charge distribution, result in higher P values. The elastic stretch modulus (S) and P show opposite trends with ionic strength, contradicting macroscopic elasticity theory. DNA remains well described by the worm-like chain (WLC) model at trivalent cation concentrations that induce condensation, provided the molecule is stretched. A retractile force appears in the presence of multivalent cations at molecular extensions allowing intramolecular contacts, suggesting a "thermal ratchet" mechanism for DNA condensation. Ions significantly influence DNA's biological functions, such as wrapping around nucleosomes and binding to transcriptional proteins. While ion exchange equilibria are often cited, ions may also affect DNA structure and mechanical properties, including bending and torsional rigidity. Recent advances in nanomanipulation allow the mechanical behavior of single DNA molecules to be studied. The WLC model describes DNA as intermediate between a rigid rod and a flexible coil, with P representing the distance over which two segments remain directionally correlated. The study shows that DNA elasticity is influenced by ion concentration, valence, and structure, with multivalent cations having a more pronounced effect than monovalent ions. The findings highlight the importance of understanding ionic effects on DNA elasticity to comprehend key biological processes.