16S rDNA is section of prokaryotic DNA that codes for a gene found within all bacteria. Bacteria are prokaryotic, which means they do not contain a cell nucleus or organelles, so DNA floats freely within these single-celled organisms. The 16S rDNA gene codes for a strand of RNA that makes up part of the ribosome. This strand of RNA is termed ribosomal RNA, or rRNA. The r in the 16S rDNA also stands for ribosomal and shows that this is a gene that encodes part of a ribosome within the bacterial cell.
Ribosomes are made up of two subunits, the small subunit and the large subunit. When the two subunits join together, messenger RNA (mRNA) is fed through and translated to form proteins. Within bacterial cells the rRNA transcribed from the 16S rDNA gene forms the small subunit of the ribosome. The 23S rDNA and 5S rDNA genes encode the rRNA that makes up the large subunit of the ribosome.
There are several reasons the 16S rDNA gene is considered useful. The first is that most scientists now use DNA to characterize organisms instead of using physical properties. Secondly, this gene is only found within bacterial cells, which means that is useful for identifying whether a cell is bacterial or from a plant, animal or fungus. Finally, it is quite short in comparison to other genes found within bacteria, which makes it a lot easier, and cheaper, to sequence.
The 16S rDNA gene is one of the most conserved genes of all. This means that it has undergone very little change throughout time, or it varies very little from cell to cell. Even organisms that are distantly related, or that evolved a long time ago, have 16S rDNA sequences that are very similar.
Scientists use this gene, as well as other ribosomal genes, to measure taxonomy, phylogeny and the rate of divergence. Taxonomy is a method of scientifically classifying organisms into similar taxa, or groups. Phylogeny looks at the evolutionary relationships between organisms. The rate of divergence is a method of estimating the rate at which different species of bacteria were formed and diverged from their ancestors.
Within microorganisms, the study of the 16S rDNA gene has been used to look at how single celled organisms are related. In particular, any variations in the gene are noted and compared to other bacterial cells. Examining these differences allows researchers to form evolutionary links between different organisms.