DNA concentration is measured by OD260,protein usually is OD280.Some proteins have peak absorbency around 310 nm.Therefore it can be used for judging protein contamination.
EDTA is able to chelate Mg2+ (Magnesium) which is required for most DNase.After Mg2+ is chelated,DNase will not work,this help to preserve DNA..
Sodium acetate and sodium chloride can help to precipitate DNA.