A unique group of eubacteria that bears mentioning is the mycoplasmas.
Classified as Gram-positive based on their relatedness to other Gram-positives,
because mycoplasmas lack a cell wall they are functionally gram-negative.
Mycoplasmas are both the smallest eubacteria and the smallest organisms capable
of independent reproduction. They are barely larger than some
viruses.
Mycoplasmas have an extremely simple cell
structure, a small genome, and are therefore of special evolutionary interest.
As we just saw, eubacteria are extremely diverse and specialized to their
environments. Surprisingly, the structure of most eubacterial cells is
relatively simple.
Figure %: Structure of Eubacteria
Rather than the complex chromosomes consisting of protein and DNA found in
plants and animals, eubacteria have prokaryotic chromosomes, which are
smaller and have fewer associated proteins. Eubacteria also have circular DNA
molecules called plasmids. Prokaryotic chromosomes and plasmids are not
housed in a centralized nucleus because eubacteria, as prokaryotes, lack a
nuclear membrane. Instead, plasmids are usually found in relatively clear areas
in the cytoplasm called nucleoids. The rest of the cytoplasm is filled with
ribosomes, the cell's protein synthesis machinery. While eubacteria lack
the organized organelles found in
eukaryotic
cells, many eubacteria have specialized internal membranes. For example,
cyanobacteria have membranes that contain chlorophyll and other chemicals
required to carry out photosynthesis.
Many eubacteria have cell walls that lie outside of their plasma membranes.
These are similar to the cell walls found in plants and fungi, but are composed
of peptidoglycan rather than cellulose or chitin. In some eubacteria,
this cell wall is covered by another layer called the outer membrane. Many
eubacteria have yet another coating layer called a capsule. It is composed
mostly of complex sugars and serves to protect the cell against environmental
dangers, such as attack by host immune defenses or dehydration.
Motility
Many eubacteria are motile. In most cases, rotating structures called
flagella enable them to move. The term flagella is also used to refer to
similar motility structures in protists and other eukaryotic cells, but the two
are not the same and should not be confused. Prokaryotic flagella are composed
of protein subunits called flagellin, while eukaryotic flagella are made of
arrays of microtubules made of tubulin. Prokaryotic flagella are anchored
in the plasma membrane and move in a spiral motion. Eukaryotic flagella are
enclosed by the plasma membrane and can only move by beating back and
forth. Exceptions to this structure of prokaryotic flagella are found in some
species of spirochetes, whose flagella resemble those of eukaryotes. It is
believed that eukaryotes may have developed flagella through symbiotic
relationships with these spirochetes.
Figure%: Comparison of Eukaryotic and Prokaryotic Flagella
Shape
Eubacteria are often classified by their shape. They fall into three main shape
categories. Spherical eubacteria are called cocci; rod-shaped eubacteria
are known as bacilli; spiral or helically-shaped eubacteria are
spirilla.
Figure%: Common shapes of eubacteria
Unlike eukaryotic cells, which divide by
mitosis or
meiosis, eubacteria reproduce by
binary
fission. In this process, the genetic material is replicated, and the two
copies move to separate nucleoid regions. Next, the plasma membrane
pinches inward, producing two equal daughter cells. While these daughter
cells are completely independent of each other, in some species they remain
together, forming colonies and filaments. Binary fission can take place very
rapidly, on the order of about one split every 20 minutes, accounting for the
amazing replicative ability of eubacteria.