DNA hydrogels with a wide range of tunable properties are desirable for applications to make use the characteristics of DNA. This study describes formation conditions of DNA hydrogels using ethylene glycol diglycidyl ether (EGDE) cross-linker and N,N,N',N'-tetramethylethylenediamine (TEMED) catalyst under various reaction conditions. Rheological measurements indicate that the cross-linking of DNA in semidilute solutions proceeds by alternate gel-sol and sol-gel transitions due to two antagonistic effects of EGDE-TEMED pair; the one destroying the physical bonds (denaturation), the other creating chemical bonds (cross-linking). The viscoelastic properties of the hydrogels and the conformation of DNA network chains could be tuned by adjusting the synthesis parameters. Increasing concentration of DNA at the gel preparation stabilizes its structure so that double stranded (ds-) DNA hydrogels form. The average distance between the effective cross-links in single stranded DNA gel is much larger than that in ds-DNA gel making the former gel stable in aqueous solutions. Creep-recovery tests show that heating a semidilute solution of DNA above the DNA melting temperature (87.5 degrees C) and subsequently cooling down to 25 degrees C increases the elastic response of the solution and produces an elastic DNA mesh. DNA hydrogel undergoes a volume phase transition in aqueous solutions of polyethylene glycol's at which the gel changes about 5 times its volume.