Why is lower case used in front of a capitalized acronym (e.g. rDNA)?
It's just a stylistic approach because "DNA" or "RNA" is an abbreviation by itself that has become a whole noun. Those terms clearly separate the modifier from the overall DNA/RNA. It makes it easier to read. Terms like DNA and RNA are almost always known just by those acronyms, whereas something like mRNA is known both as "messenger RNA" and "'m' RNA" but almost never as "messenger ribonucleic acid."