In general, prokaryotes differs from the eukaryotic organism in every sense except its organelles i.e. mitochondria and chloroplast. It has to be believed that these organelles have been derived from bacteria as they share many common features (endosymbiotic theory).

16S ribosomal RNA acts as a signature sequence and is present in all the cells. These are approximately 1500bp in length. The organism of one taxonomic group has different conserved sequences than the other group.

Carl Woese proposed the reorganization of five kingdoms into three kingdoms, based on the ribosomal RNA signature sequences. He placed protista, fungi, plants, and animals into a single group called eukaryotes.

Thiomargarita namibiensis and epulopiscium fishelsoni are the largest bacteria of 600 micrometers in length and 75 micrometers in diameter and are easily visible to eyes.

There are four basic shapes of bacteria: rod shape is bacillus bacteria, comma shaped is vibrio bacteria, spiral shape is spirilla, and sphere shape of coccus.