Calling "prettyPrint()" adds a class to your PRE tags named "prettyPrinted" after it has been prettified. The line below will remove all instances of the "prettyPrinted" class on your page so that the prettyPrint() function can re-prettify you're PRE tags. This can be done without dynamically adding PRE tags to DIVs.
$('.prettyprinted').removeClass('prettyprinted');