While not all bacteria share the same rod-like
shape of the model pictured here, they all have similar
structures. At the center of the cell is usually a single chromosome.
Like the DNA or RNA in animal or human cells, the bacterial chromosome
has full capability to replicate itself.
Unlike higher organisms, the bacteria's chromosome
is not protected from the cytoplasm (fluid of the cell) by a nuclear
membrane (such as in the nucleus of a protozoan).
Also, bacteria may have extra pieces of DNA
(called plasmids), as well as ribosomes (they help the
chromosome replicate) in the cytoplasm. Plasmids confer certain
advantages on their host bacterium. These may include genes for
resistance to antibiotics. Plasmids also have a DNA sequence that
triggers replication of the plasmid independently of the replication
of the main DNA in the chromosome. Genetic engineering techniques
often insert foreign DNA into bacteria in the form of plasmids,
which then multiply in the bacterium.
Lastly, some bacterial species have flagella
to help them move around. Other species may have a rugged cell
wall outside the delicate cell membrane.