Re: why acidic buffer condition for RNA isolation and basic buffer for DNA?

hi.
its true that the buffers are acidic for RNA work and slightly basic 
(usually around pH 8)for DNA extractions.  If you place DNA in a strong 
bacic solution the bonds between the strands will be cleaved and the 2 
stands will not be able to re join.  RNA can survive and contaminate when 
preforming a DNA extraction, unless specific steps are used to remove the 
RNA ie using RNase.  The acidic conditions for RNA extractions is due to a 
number of problems when dealing with RNA.  DNA is a major contaminant which 
needs to be removed as it can interfer with RT-PCR etc.  Also RNase has 
very little activity in acidic condition and is de-activated by guanidinium 
thiocyanate. Often the RNA extraction buffers are 5M G.thio and pH 4.8ish.

hope that helps
regards
Sean

Admin note:
Another reason for the disparity in nucleic acid extraction buffers is tied to 
the phenol extractions commonly used to purify the nucleic acids away from 
cellular proteins: DNA is soluble in even mildly acidic phenol while RNA is not; 
so a good way to purify RNA's away from genomic DNA is to perform an acidic 
phenol extraction.  Since phenol is itself a buffer, these extraction work best 
if the sample buffer is also acidic.  Conversely, if DNA is being isolated, any 
acidity present during phenol extraction could greatly reduce yield.