Eukaryotes WORLD OF MICROBIOLOGY AND IMMUNOLOGY204•
EEukaryotesUKARYOTES
Eukaryotic organisms encompass a range of organisms, from
humans to single-celled microorganismssuch as protozoa.
Eukaryotes are fundamentally different from prokaryotic
microorganisms, such as bacteria, in their size, structure and
functional organization.
The oldest known eukaryote fossil is about 1.5 billion
years old. Prokaryote fossils that are over 3 billion years old
are known. Thus, prokaryotic cells appeared first on Earth.
The appearance of eukaryotic cells some 1.5 billion years ago
became possible when cellular function was organized into
regions within the cell called organelles.
The eukaryotes are organized into a division of life that
is designated as the Eukaryota. The Eukaryota are one of the
three branches of living organisms. The other two branches are
the Prokaryota and the Archae.
The evolutionary divergence of life into these three
groups has been deduced in the pasts several decades.
Techniques of molecular analysis have been used, in particu-
lar the analysis of the sequence of a component of ribosomal
ribonucleic acid(RNA), which is known as 16S RNA. This
RNA species is highly conserved in life forms. Thus, great dif-
ferences in the sequence of 16 S RNA between a eukaryotic
and a prokaryotic microorganism, for example, indicate that
the two organisms diverged evolutionarily a very long time
ago. A similar 16 S RNA indicates the converse; that evolu-
tionary branching is a relatively recent event.
Eukaryotic cells are about 10 times the size of all but a
few prokaryotes. This translates to an internal volume which
is very much larger, some 1000 times, that the internal volume
of a bacterium. In order to survive, eukaryotes evolved a
highly organized internal structure, in order that all the tasks
necessary for life can be accomplished in the large internal
volume. This internal structure is the fundamental distinguish-
ing aspect of a eukaryote versus a prokaryote.
Functional specialization is the fundamental hallmark
of eukaryotes. In larger organisms, such as humans, this spe-
cialization gives rise to organs such as the heart, lover, and
brain, and to functional organizations such as the immune sys-
tem. But organization is also evident in microscopic, even sin-
gle-celled, eukaryotes.
In a eukaryote, the nuclear material is segregated within
a specialized region called the nucleus. This feature is a key
constituent of eukaryotic cells. Indeed, the word eukaryote
means “true nuclei.” The nucleus exists because of the pres-
ence of the so-called nuclear membrane, which encloses the
nuclear material. The nuclear membrane contains pores,
through which material can enter and leave the nuclear region.
Prokaryotes lack an organized nucleus. Indeed, for many years
the presence of a nucleus was the sole key feature that distin-
guished a eukaryote from a prokaryote.
Most of the eukaryotic DNA (deoxyribonucleic acid) is
present in the nucleus. The remainder is contained within the
energy-generating structures known as the mitochondria. The
organization of the eukaryotic DNA is very different from bac-
terial DNA. In the latter, the genetic material is usually dis-
persed as a large circle throughout the interior of thebacterium, in a gel-like mixture termed the cytoplasm. In con-
trast, eukaryotic DNA is organized into discrete limb-like
structures called chromosomes.
The replication of eukaryotic DNA is also different
from that of prokaryotes. The latter is essentially an unwind-
ing of the double helix of DNA, with ongoing complemen-
tary copies of daughter DNA strands made from each
unwinding parental strand. The result is two double helices.
The replication process in eukaryotes is more complex,
involving several phases of chromosome replication, segre-
gation to areas of the cell, collection together, and enclosure
in a nuclear membrane.
Eukaryotic cells, including microorganisms, contain a
specialized functional region known as the endoplasmic retic-
ulum. This network of tubular structures is involved in the
manufacture of protein from the template of RNA. In many
eukaryotes a region called the Golgi apparatus or Golgi body
is associated with the endoplasmic reticulum. The Golgi body
is involved with the transport of compounds into and out of
the cell.
Another distinctive feature of eukaryotic cells is the
aforementioned mitochondria. These are the energy factories
of the cell. Additionally, some eukaryotes possess structures
called chloroplasts, which use the energy available in light to
change carbon dioxide and water into carbohydrates. The car-
bohydrates provide a ready source of energy for cellular func-A eukaryotic cell.womi_E 5/6/03 2:13 PM Page 204