What is the expected composition of RNA in the results of RNA-Seq? The assumption is that no rRNA depletion was performed.
I've come across information stating that the composition of a typical eukaryote cell is:
~80% rRNA, ~15% tRNA, and the rest mRNA. (http://www.ncbi.nlm.nih.gov/books/NBK21729/)
Why is this so? As I understand, only the RNA molecules which which will constitute the ribosome in their RNA form are called ribosomal RNA, not those that code for ribosomal proteins. Where, then, does this rRNA abundance come from? Why aren't there more mRNA that code for ribosomal proteins?