rRNA (Ribosomal RNA) is the central component of the Ribosome, the protein manufacturing machinery of all living cells. The rRNA is synthesized in the nucleolus. These machines then self-assemble into the two complex folded structures (the large 60S and t ...he small 40S subunits) in the presence of 70 - 80 rProteins (Ribosomal Proteins). The function of the rRNA is to provide a mechanism for decoding mRNA into amino acids (at center of small ribosomal subunit) and to interact with the tRNAs during translation by providing peptidyl transferase activity (large subunit). Accuracy of translation is provided by both subunits. In Bacteria, Archaea, Mitochondria, and Chloroplasts a small ribosomal subunit contains the 16S rRNA, where the S in 16S represents Svedberg units; the large ribosomal subunit contains two rRNA species (the 5S and 23S rRNAs). Bacterial 16S, 23S, and 5S rRNA genes are typically organized as a co-transcribed operon. Eukaryotes generally have many copies of the rRNA genes organized in tandem repeats; in humans approximately 300-400 rDNA repeats are present in five clusters (on chromosomes 13,14,15,21 and 22). The 18S rRNA in most eukaryotes is in the small ribosomal subunit i.e. 40S, and the large subunit i.e. 60S, contains three rRNAs (the 5S, 5.8S and 28S rRNAs). rRNAs are generally transcribed by RNA Pol-I (RNA Polymerase-I) except 5S rRNA, which is transcribed by RNA Pol-III (RNA Polymerase-III)).
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