Studying the phylogenetic relationships among organisms within natural communities using rRNA sequences

RRNAs - specify structural RNA products of ribosomes. Bacterial ribosomes comprise 3 rRNAs : 23S (~ 2.4 kb) 16S (1.5 kb) & 5S (0.12 kb). Most conserved & widely used is the 16S rRNA : databases currently contain 2,500 sequences (cultured bacteria number 2,600).