80 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”to showing that, even in the microbial world, self is well in place and
clearly expressed.
Microorganisms are classified into bacteria, archaea, and protists.
The first two are prokaryotes (having no nucleus), whereas the third are
eukaryotes (with a nucleus). Despite the name, archaea are not the oldest
in evolutionary history; bacteria are. Prokaryotes do not possess well-
formed structures within the cell. They have a circular DNA and only
one set of genes (haploid). They reproduce asexually by binary fission.
Nonetheless, genetic mixing is frequent through transfer of DNA across
individuals, resulting in a flexible gene pool. Archaea differ from bacte-
ria in two major respects: the genome of archaea contains introns as well
as exons; their DNA is bound to a special protein called histone. (Introns
are portions of the DNA chain that do not encode protein.) These fea-
tures put them closer to eukaryotes than bacteria. Unlike bacteria, which
are widely distributed, archaea are restricted to thrive in more extreme
conditions, such as high temperature, high pressure, and high acidity.
Hence they are frequently associated with the term “extremophiles.”
Unlike the prokaryotes, eukaryotes have two sets of genes (diploid)
that can undergo meiosis (splitting into haploids) during reproduction.
Protists are unicellular eukaryotes that are neither animals nor plants.
They are larger than bacteria and archaea. Protists are classified into
protozoans, slime molds and algae. Examples of protozoans are amoe-
bas, paramecia, and stentors. Slime molds are unique in that they are
both unicellular and multicellular depending on food supply. Algae con-
tain chloroplasts and are capable of photosynthesis.^1
4.1 Bacterial Expression of Self
Despite being the simplest free-living organisms, bacteria do express a
rudimentary self, as can be seen in the following examples:
4.1.1 Defense against adverse environment
When the environmental condition is unfavorable, some bacteria can
protect themselves by forming a spore. They produce a thick coat to