Protein synthesis occurs on particles called ribosomes which are composed of molecules of proteins and RNA. The RNA associated with ribosomes is termed as ribosomal RNA (MW=20,000). When a cell homogenate is centrifuged at high speed (100,00 x g), about 80% of the RNA bound to the ribosomes is sedimented. Each ribosome is composed of two unequal subunits, the  larger one being composed of one large and one small RNA molecule and a number of protein segments which are smaller. The smaller subunit is composed of one RNA and several protein molecules. Procaryotic ribosomes are 70 S types which are aggregates of two subunits sedimenting with 50 S and 30 S. Eucaryotic ribosomes (mammalian type) are larger having a sedimentation constant of 80 S with subunits of  60 S and 40 S.

Ribosomal RNA has a significant role in the assembly of both ribosomal subunits. RNA extracted from purified ribosomes of Escherichia coli, when subjected to sucrose density centrifugation, sediments at 28S, 18S and 5S. The nucleotide sequence of rRNA has been determined precisely. The 5S RNA contains 120 nucleotides, and the 16S and 23S molecules contain 1542 and 2904 nucleotides sequence but also on their conformation.

The ribosomal RNA exists in a highly folded form, carrying several loops and stems which determine its activity. It contains many regions of nucleotides that are complementary in antiparallel order. A model of rRNA shown the C  ≡  G and A = U ratios in rRNA of different sources vary considerably, and G – C content is found to be always more than 50%.

A point of interest that deserves mention is the striking similarity between the rRNAs of E. coli ribosomes and those of chloroplast mitochondria and eucaryotic ribosomes. Another feature to mark is that 5s component has a nucleotide sequence that is complementary to the on occurring in all transfer RNAs and this feature helps in the attachment of tRNA to the ribosome.