The rRNAs form extensive secondary structures and play an active role in recognizing conserved portions of mRNAs and tRNAs. In eukaryotes (organisms that possess a clearly defined nucleus), anywhere from 50 to 5,000 sets of rRNA genes and as many as 10 million ribosomes may be present in a single cell. In contrast, prokaryotes (organisms that lack a nucleus) generally have fewer sets of rRNA genes and ribosomes per cell.