Bacteria are prokaryotic cells, which are characterized by a lack of a membrane-bound nucleus. They are typically simpler than eukaryotic cells, which have a membrane-bound nucleus and organelles responsible for cell function.
Bacteria contain a small strain of nucleoid DNA in their cytoplasm and may also contain plasmid DNA enclosed in a circular loop. The DNA and ribosomes enclosed in the bacteria's cell walls are responsible for reproduction and expression of specific traits in new cells. A bacterium's locomotion and adhesion to its environment are performed by a small tail-like appendage called a flagellum. Prokaryotic cells may have a round shape or resemble rods or spirals.