Re: What are the effects of a diphenylamine soln. for a DNA test of an onion?

 It is generally important to know the degree of purity of sample subjected 
 to DNA test with diphenylamine. If it is crude extract this test is
 useless, because there are too many ‘non-DNA’ factors that can give
 positive results. In such cases only negative results may assure
 you that there is no DNA in the sample or its level is below the range
 of sensitivity of the method. 
Diphenylamine reagent is very acidic (it contains high concentration
 of sulfuric acid and acetic acid). This is another cause to give color
 effects on crude preparations of plants. I wouldn’t recommend this method
 as the only one for monitoring DNA isolation from plant material from
 very early stage of preparation. It would be useful to support it with
 another test.  For example very simple and quite useful is checking
 the ratio of absorbance at 260 nm and 280. The value A260/A280 >1.8
 suggests that the sample is pretty pure (with respect to nucleic acids
 versus protein and phenol impurities) and then it is sense to use
 the diphenylamine method. The lower value shows that amount of proteins
 and/or phenolic compounds is still very high.
 Diphenylamine reagent gives deep blue complexes with furfural which
 is a product of deoxyribose conversion at very acidic conditions.
 However you must remember that this reagent gives deep-blue color
 in presence of many oxidizing substances, organic as well as inorganic.
 It is obvious that onion (as well as other plants) contains lots of such
 substances and you must keep it in mind.
 And remember about very important rule in biochemistry: just one test
 means nothing unless it is not supported by another! Good luck!