MICROBIOLOGY 685
nucleic acid and form a protective coat or capsid.
The smallest viruses appear spherical but magnifi-
cation in the order of 150,000–700,000 reveals
that they are icosahedrons (20 triangular faces and
12 corners) for example, wart virus. Other viruses,
for example, tobacco mosaic virus (TMV), the first
virus crystallized in 1935 by Wendell Stanley, is
grossly rodlike. Tobacco mosaic virus is composed
of a central, spirally-attached RNA to which capso-
meres are attached to form the outside of a cylinder.
The center of the RNA spiral of TMV is hollow.
Structurally, the most complicated viruses are some
which attack bacteria and blue-green algae.These complicated viruses are composed of an icosahedral
head, containing DNA, a protenaceous tail and sometimes
accessory tail structures which are important for the attach-
ment of the virus to its host cell.2) Mycoplasma are prokaryotes which overlap viruses
in size. They range from 100–300 nm in size. They
are highly pleomorphic: they do not have one typi-
cal shape but rather can appear coccoid, filamentous,
or highly branched. Unlike most other prokaryotes,
they do not have cell walls external to their cell
membranes. Their cell membranes usually contain
sterols, which are thought to lend strength to these
cell-limiting membranes (see also Table 2).
3) The true bacteria or Eubacteriales are prokaryotes
which are built on three general geometric forms:
spheres or cocci, rods, and spirals (including spi-
ral helices). All true bacteria have rigid cell walls.
They are either permanently immotile or move
by means of one to many flagella. They may be
aerobes or anaerobes. Some of the anaerobes are
photosynthetic. Their sizes and shapes are usuallyconstant except among the rods, in which rapidly-
multiplying cells may be somewhat smaller than
usual. When the cells divide, they often remain
attached to each other and form characteristic,
multicellular clusters. The shape of the cluster is
determined by the number of division planes.When cocci divide in only one plane, they form chains which
may be as much as 20 cells long. Diplococcus pneumoniae
forms chains only two cells long while Streptococcus is
an example of the long-chain forming type. On the other
hand, cocci which divide along two planes, at right angles
to each other, form sheets of cells, and cocci which divide in
three planes form cube-shaped packets. If there is no regu-
lar pattern of the orientation of successive division planes,
a randomly-shaped cluster is formed. Staphylococcus is an
example of a coccus which forms random clusters. A typical
coccus is in the size range of 0.15–1.5 m in diameter.
Rods always divide in only one plane. They may appear
as single cells or groups of only two when they separate rap-
idly. The common intestinal bacterium Escherichia coli (size
0.5 2.0 m) is an example of this type. Frequently rods
form long chains or streptobacilli. Bacillus megaterium (size
1.35 3.0 m), the organism responsible for the “bloody
bread” of ancient times, is an example of a chain forming
rod. Some basically rodshaped bacteria are either curved or
helical rods. Their sizes range from almost as small as the
smallest straight rod shaped form to close to twice the length
of the largest straight rod.
True bacteria always divide by binary fi ssion after their
single circular chromosome replicates in a semiconservative
fashion.
Some true bacteria have complicated life cycles which
includes spore-formation. Spore-formers are all rods but
belong to diverse genera. They are ecologically related
in that they are found primarily in soil. Since that naturalTABLE 2
Some characteristics of prokaryotic and eukaryotic cellsStructure Prokaryote EukaryoteWeight Chromosome 0.001–1.0 pg
one, single circular DNA double helix not
complexed with histones10–10,000 pg
several linear DNA double helices (several
chromosomes usually complex with histones)
Nucleus No true nucleus. Chromosomes not separated from
cytoplasm by a membraneTrue nucleus. Chromosomes enclosed in a nuclear
membrane
Reproduction Usually asexual, conjugation takes place rarely, no
mitosis or meiosisAsexually by mitosis or sexually after meiosisMembranes Only cell limiting membrane present. Usually lacks
sterols (except for mycoplasma)Cell limiting membrane plus membrane limited
organelles present. Composition includes sterols
Organelles None Many including mitochondria, chloroplasts (plants
only), Golgi apparatus, lysosomes, etc.
Apparatus for protein synthesis Ribosomes, 70 S type usually not associated with
membranesRibosomes, 80 S type in cytoplasm associated with
endoplasmic reticulum. 70 S type in mitochondria
and chloroplasts not associated with membranesC013_004_r03.indd 685C013_004_r03.indd 685 11/18/2005 10:41:57 AM11/18/2005 10:41:57 AM