Examples: rDNA, mRNA, tRNA.
Why do we use small case for "r" in "rDNA", etc? Any particular reason?
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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."
These letters are used to indicate the different types of RNA and DNA. Especially RNA has lots of (functionally) different variants, so this helps discering between them.
Your exmaples in particular: - mRNA is short for messenger RNA - tRNA is short for transfer RNA - rDNA could be ribosomal or recombinant DNA (writing rDNA is very uncommon, mostly ssDNA or dsDNA are used)