Tissue sonication for RNA extraction - Help :) (Feb/13/2009 )

Help!!!!! I have a question. After using a sonicator in preparation for RNA extraction I achieved results that showed the RNA had been sheared and there was contamination with genomic DNA. Does anyone know why the RNA is damaged yet the DNA is not?????
Thanks

presence of rnases? maybe not sufficiently inhibited (may explain why sheared rather than gone).

are you talking about shearing of the 28s and 18s rrnas or a smear of mrnas?

a smear of mrnas is normal.