INTRODUCTION
The three domains of life are bacteria, archaea, and eukarya (Fig. 1.1) (Rising and
Reysenbach, 2002; Woese, 1987). Bacteria, along with actinomycetes and cyanobacteria
(blue-green algae) belong to the prokaryotes while eukaryotes or eukarya include fungi,
protozoa, algae, plant, and animal cells.
Viruses are obligate intracellular parasites that belong to neither of these two groups.
The main characteristics that distinguish prokaryotes from eukaryotes are the following
(Fig. 1.2):
1. Eukaryotic cells are generally more complex than prokaryotic cells.
2. DNA is enclosed in a nuclear membrane and is associated with histones and other
proteins only in eukaryotes.
3. Organelles are membrane-bound in eukaryotes.
4. Prokaryotes divide by binary fission whereas eukaryotes divide by mitosis.
5. Some structures are absent in prokaryotes: for example, Golgi complex, endoplasmic
reticulum, mitochondria, and chloroplasts