It is the most stable form of RNA and is found in ribosomes. It has the highest molecular weight. In bacterium Escherichia Coli, there are three kinds of RNA called 23S, 16S, and 5S RNA because of sedimentation behaviour. One molecule of each of these three types of rRNA is present in each ribosome. Ribosomal RNA is abundant of all types of RNAs and makes up about 80% of the total RNA of a cell.

Ribosomal RNA represents about 40%–60% of the total weight of ribosomes. The ribosomes of prokaryotes are found in plastids and mitochondria. Prokaryotes contain three types of RNA: 23S and 5S RNA in the larger (50S) subunit and 16S RNA in the smaller (30S) subunit. Eukaryotes also contain three kinds of RNA: 28S and 5S RNA in the larger (60S) subunit and 18S RNA in the smaller (40S) subunit.

The mammalian ribosomes deserve special rRNA. They contain two major nucleoprotein subunits, a larger one (60S), and a smaller one (40S). The 60S subunit contains a 5S, 5.8S, and a 28S rRNA. There are also probably more than fifty polypeptides. The 40S subunit contains a single 18S rRNA and about thirty polypeptide chains. All types of rRNA, except the 5S rRNA, are processed from a single 45S precursor RNA molecule in the nucleolus.

It is remarkable to note that rRNA from all sources has G−C contents more than 50%. The rRNA molecule appears as a single unbranched polynucleotide strand (primary structure). In low ionic strength, the molecules are like a rod with random coiling, but at high ionic strength, the molecules exhibit a compact helical structure with complementary base pairing and looped outer region. The helical structure results from the folding back of a single-stranded polymer at areas where hydrogen bonding is possible. The function of rRNA is to donate a specific sequence to mRNA where it has to be translated.