Prokaryotes are unicellular organisms that lack nuclear membranes and membrane-bounded cell organelles. For many years, biologists considered all prokaryotes to be related, but genome sequence information now provides convincing evidence that prokaryotes are divided into at least two distinct groups: the archaea and the eubacteria. The archaea are a group of diverse prokaryotes that are frequently found in extreme environments, such as hot springs and the bottoms of oceans. The eubacteria are the remaining prokaryotes, including most of the familiar bacteria. Although superficially similar in their cell structure, eubacteria and archaea are distinct in their genetic makeup, and the differences between them are as great as those between eubacteria and eukaryotes. In fact, the archaea are more similar to eukaryotes than to eubacteria in a number of molecular features and genetic processes. In this book, the term bacteria will generally be used to refer to eubacteria.

Bacteria are extremely diverse and come in a variety of shapes and sizes. Some are rod-shaped and others are spherical or helical. Most are much smaller than eukaryotic cells, but at least one species isolated from the guts of fish is almost 1 mm long and can be seen with the naked eye. Some bacteria are photosynthetic. Others produce stalks and spores, superficially resembling fungi.

Bacteria have long been considered simple organisms that lack much of the cellular complexity of eukaryotes. However, recent evidence points to a number of similarities and parallels in the structure of bacteria and eukaryotes. For example, a bacterial protein termed FtsZ, which plays an integral part in cell division, is structurally similar to tubulin in eukaryotic cells, a protein that is found in microtubules and helps to segregate chromosomes in mitosis and meiosis. Like eukaryotes, bacteria have proteins that help condense DNA. Other bacterial proteins function much as cytoskeletal proteins do in eukaryotes, helping to give bacterial cells shape and structure. And, though bacteria don’t go through mitosis and meiosis, replication of the bacterial chromosome precedes binary fission, and there are bacterial processes that ensure that one copy of the chromosome gets allocated to each daughter cell.