The chromatin, or genetic material, in bacteria is not housed in a separate nucleus, but in packed, aggregated bundles of fibers of doublestranded DNA floating in the cytoplasm. In E. coli and in many other bacteria, the DNA occurs in one continuous circular loop rather than an open strand.

The enzymes for DNA replication, and transcription of information to messenger RNA, also are free-floating in the cytoplasm. This is the most striking difference between procaryotes and eucaryotes.

In eucaryotes the DNA is organized into chromosomes and is segregated into a nucleus surrounded by a nuclear membrane.

None of this nuclear structure exists in procaryotes.

Bacteria also have special external structures: flagella for motion, and pili (hairs), which are used in sexual conjugation and possibly for other functions.

Bacteria are rather simple living machines, but they contain all of the essentials for survival, and in fact have managed to survive on this planet twice as long as eucaryotes.

They show a biochemical variety and versatility that far surpasses that of eucaryotes.

Part of this variety may reflect the extent to which eucaryotes have "settled down" with the most favorable of the biochemical options, and part may be the result of special chemical adaptations that bacteria made later to survive in competition with eucaryotes.