| | Re: Degradation of mRNA after cell lysis?
First, there are nucleases in the extracellular matrix, too. For instance, skin has nucleases (skin is formed by death keratinized cells, well its outer part is). Those nucleases will stay even after the cells die.
Second, the first step of RNA extraction involves destruction or denaturing of nucleases by strong denaturing agents like guanidine isotiocyanate, which is added prior to cell lysis. Reagents like TRIzol do both things at the same time (get rid of nucleases and lyse the cells). Even if your cells are death, nucleases will be active, so the same first step must be given to extract viral RNA.