Cellular life is detected in almost every environment on Earth.
As life has colonized and adapted to the vast number of niches, cells
have evolved an incredible amount of diversity in regard to size [6],
form [7], lifestyle [8, 9], and complexity [10]. Understanding the
basis of such diversity remains one of the central aims of biology.
Readers interested in the latest understanding of Earth’s biodiver-
sity, the unique characteristics of its organisms, and how both extant
and extinct forms are related to each other are encouraged to explore
the following resources: the University of California Museum
of Paleontology “History of life through time” exhibit [11],
the Tree of Life Web Project [12], the Encyclopedia of Life [13].
2.1 Viruses
Viruses are infectious agents of living cells that are unable to repro-
duce in the absence of a host. Viruses are not considered cellular
entities since they lack two of the essential attributes that define a
cell; they possess neither a cell membrane nor cytoplasm. The
discovery of virophages, viruses that parasitize other viruses, resur-
rected the debate on their classification as living organisms
[14]. Some consider viruses to be living entities since they can be
hosts to other viruses, with a virophage infection leading to the
eventual death of the host virus, implying an initial “living” state
[15]. The opposing view asserts that a virus’ inability to reproduce
outside of a cellular host makes them nonliving entities
[16, 17]. Irrespective of their delineation as living or nonliving,
viruses are relevant to this review as they possess genomes and are
the most abundant biological replicators in the biosphere [18].
Outside of their host, viruses exist as viral particles (virions)
consisting of a protein capsule that protects and encloses their
genome. Once a virion has entered a host cell, it “hijacks” the
host’s cellular structures and processes to carry out the metaboli-
cally active phase of the viral life cycle. At this stage, the virus
exhibits physiological properties reminiscent of living cells; they
metabolize, grow, and reproduce. There is a wide range of viral
lifestyles, with corresponding diversity in viral forms, sizes, hosts,
and genomes [16]. The largest known virus, the mimivirus, was
originally identified as an infectious agent of an amoeba [19] and
can itself become a host for virophages [14]. To put this in context,
the virion of a mimivirus can be larger than some prokaryotic cells
[16]. At the other end of the scale are viruses such as the circo-
viruses, some of which have small genomes made up of less than
2000 nucleotides [20]. A more detailed account of viral diversity
can be found at the ViralZone website [21].
2.2 Bacteria
The bacterial cell is prokaryotic, and it is relatively simple as com-
pared to eukaryotic cells. It has no membrane-bound organelles,
and the chromosome (usually one) is not separated from the other
components of the cell. While predominantly unicellular, they often
live in biofilms, a community of cells bound together by a secreted
Introduction to Genome Biology and Diversity 5
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