Topic I: RIBOSOMAL RNA
To infer relationships that span the diversity of known life, it is necessary to look at genes conserved through the billions of years of evolutionary divergence. Examples of genes in this category are those that define the ribosomal RNAs (rRNAs). In Bacteria, Archaea, Mitochondria, and Chloroplasts, the 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).
Most prokaryotes have three rRNAs, called the 5S, 16S and 23S rRNA. Bacterial 16S, 23S, and 5S rRNA genes are typically organized as a co-transcribed operon. There may be one or more copies of the operon dispersed in the genome (for example, E coli has seven). The Archaea contains either a single rDNA operon or multiple copies of the operon.