DNA associates strongly with cationic cosolutes: surfactant, lipids, proteins and polymers.The binding patterns for several systems are presented, illustrating the consequences of this binding in solution in terms of phase behaviour, compaction/decompaction and microstructure.These systems are characterized by a strong associative phase separation.In an attempt to mimic the DNA-histone interactions in chromatin, the phase behaviour and aggregate structures in different aqueous mixtures of DNA and a cationic protein were investigated.We also describe the preparation of covalent DNA gels and present their swelling-deswelling behaviour.It is found that covalent gels offer novel opportunities for monitoring DNA-cosolute interactions.A local phase separation in covalent gels can lead the formation of a surface phase, as a"skin".Based on the associative phase separation, the preparation of novel DNA particles by mixing DNA with surfaetant and protein solutions can be achieved.Surface morphology, degree of DNA entrapment, swelling/deswelling behaviour and kinetics of DNA/protein release are also described.Cationic lipids and surfactants are efficient in compacting DNA and can also be efficient transfection agents.The effect of aminoacid-based surfactants on different formulations in transfection studies is presented.The different types of studies are performed for both double-and single-stranded DNA.Throughout, a stronger interaction is observed with denatured DNA.On the basis of these results and other observations it is found useful to view DNA as an amphiphilic polymer self-assembling and interacting, by hydrophobic interactions.