is one of the three domains of life contianing organisms whose
cells contain complex structures
complex membranes call endomembranes.
These organisms are called Eukaryotes and are either
singular- or multiple-celled. The
structure that differentiates eukaryotic cells
from prokaryotic cells is the cell nucleus,
within which the genetic material and many other cell bodies
are contained. The presence of a nucleus gives eukaryotes their
name that derives from the Greek (eu, "good")
and (karyon, "kernel").
Most eukaryotic cells also contain other membrane-bound
as mitochondria, chloroplasts and the Golgi apparatus.
All species of large complex
organisms are eukaryotes, including animals, plants
and fungi, although most species of eukaryote are
Domain Eukaryota is taxonomically devided into seven kingdoms.
Cell division in eukaryotes
is different from prokaryotes that lack a nucleus. It involves
separating the duplicated chromosomes, through movements
directed by microtubules. There are two types of division processes.
In mitosis, one cell divides to produce two genetically identical
cells. In meiosis, which is required in sexual reproduction,
one diploid cell (having two instances of each chromosome, one
from each parent) undergoes recombination of each pair of parental
chromosomes, and then two stages of cell division, resulting
in four haploid cells (gametes). Each gamete has just one complement
of chromosomes, each a unique mix of the corresponding pair of
parental chromosomes. Eukaryotes appear to be monophyletic, comprising
one of the three
domains of life; Bacteria and
Archaea are prokaryotes that have none of the characteristics.
Eukaryotes represent a tiny minority of all living things. It
could well be said that the eukaryotes are guests in a prokaryotic
world. For example, a human body contains 10 fold more prokaryotes
than eukaryotic human cells. Eukaryote and prokaryote biomass
on earth is approximately equal.
The earliest fossil evidence of
animals dates from the Vendian Period (650 to 544 million years ago), with
coelenterate-type animals that left traces of their soft bodies in shallow-water
sediments. The first mass extinction ended that period, but during the Cambrian
Period which followed, an
explosion of new forms began the evolutionary radiation
that produced most of the major groups, or phyla, known today.
Kingdoms of Domain Eukaryota
are a major group of multicellular, eukaryotic organisms
of the Kingdom Animalia or Metazoa.
Their body plan eventually becomes fixed as they develop,
although some undergo a process of metamorphosis later on
in their life. Most animals are motile, meaning they can
move spontaneously and independently. All animals are also
heterotrophs, i.e., they eat other organisms
or their byproducts for sustenance. Most known animal phyla
in the fossil record as
marine species during the Cambrian
542 million years ago.
||Plants are eukaryotic organisms
belonging to the Kingdom Plantae. Plants comprise familiar
organisms such as trees,
flowers, herbs, bushes, grasses, vines, ferns, mosses, and
green algae. They obtain most of their energy from sunlight
via photosynthesis using chlorophyll contained in chloroplasts,
which gives them their green color. Some plants are parasitic
and may not produce normal amounts chlorophyll
or photosynthesize. Evidence suggests that an algal scum formed
on the land 1,200 million years ago, but it was not until
Period, around 450 million years ago, that
land plants appeared.
fungus is a member of a large group of eukaryotic organisms
includes microorganisms such as yeasts and molds as well
as the more familiar mushrooms. These organisms are classified
as a kingdom, Fungi, which
is separate from plants, animals, and bacteria. One major
difference is that fungal cells have cell walls that contain
chitin, unlike the cell walls of plants that contain cellulose.
These and other differences show that the fungi form a single
group of related organisms, named the Eumycota (true fungi
or Eumycetes), that share a common ancestor (a monophyletic
group). This fungal group is distinct from the structurally
similar myxomycetes (slime molds) and oomycetes (water molds).
In contrast to plants and animals, the early fossil
record of the fungi is sparse. Factors
that likely contribute to the under-representation of fungal
species among fossils
include the nature of fungal fruiting bodies that are
soft, fleshy, and easily degradable tissues and the morphological
nature of fungi that are difficult to distinguish
from those of other microbes.
||Chromalveolata is a eukaryote supergroup first proposed
by Thomas Cavalier-Smith in 1981 as a refinement to Kingdom
Chromista. Historically, many chromalveolates were considered
plants, because of their cell walls, photosynthetic ability,
and in some cases their morphological resemblance to the
land plants (the Embryophyta). Chromalveolata was proposed
to represent the result of a single secondary endosymbiosis between
a line descending from a bikont and a red alga that became
the progenitor of chlorophyll C containing plastids.
In a major classification produced in 2005, Chromalveolata
was regarded as one of the six major groups within the eukaryotes,
a phylogeny that has been challenged.
||The Rhizaria are a species-rich supergroup of unicellular
eukaryotes. This supergroup was proposed by Cavalier-Smith
in 2002. They vary considerably in form, but for the most
part they are amoeboids with filose, reticulose, or microtubule-supported
pseudopods. Many produce shells or skeletons, which may be
quite complex in structure, and these make up the vast majority
of protozoan fossils. Nearly all have mitochondria with tubular
cristae. Historically, many rhizarians were considered animals,
with their motility and heterotrophy as justification. However,
when the five-kingdom system took prevalence over the animal-plant
dichotomy, the rhizarians were put into the kingdom Protista.
Then, after Woese
published his three-domain system, because
of the paraphyly of the Kingdom Monera, taxonomists turned
their attention to the eukaryote domain, and the inherent
paraphyly of Protista. After much debate, which continues
to this day, Rhizaria emerged as a monophyletic group.
||The excavates are a major kingdom of unicellular eukaryotes,
The phylogenetic category Excavata that was erected by
Cavalier-Smith in 2002 contains a variety of free-living
forms, and also includes some important
parasites of humans. Many excavates lack typical mitochondria,
although most retain a mitochondrial-like organelle.. Most
excavates have two, four, or more flagella and many have
ventral feeding groove with a characteristic ultrastructure,
supported by microtubules. Still others that
lack these traits are considered excavates based on genetic
evidence from molecular sequencing. The closest that the
excavates come to multicellularity are the Acrasidae slime
other cellular slime
molds, they live most of their life as single cells, but
will sometimes assemble into a larger cluster. Excavate relationships
are still uncertain, and it is possible that they are not a
monophyletic group. Certain excavates are often considered
among the most primitive eukaryotes.
||The Amoebozoa are a major group of amoeboid protozoa, including
the majority that move by means of internal cytoplasmic flow.
Their pseudopodia are characteristically blunt and finger-like,
called lobopodia. Most are unicellular, and are common in
soils and aquatic habitats, with some found as symbiotes
of other organisms, including several pathogens. Vase-shaped
microfossils (VSMs) discovered around the world show that
amoebozoans have existed since the Neoproterozoic Era. The
fossil species Melanocyrillium hexodiadema, Palaeoarcella
athanata, and Hemisphaeriella ornata come from rocks 750
million years old. All three VSMs share a hemispherical shape,
invaginated aperture, and regular indentations, that strongly
resemble modern arcellinids, which are shell-bearing amoeboids.
P. athanata in particular looks the same as the extant genus
Eukaryotic cells are typically much larger than those of prokaryotes.
Eukaryotic cells also contain many internal membrane-bound structures
called organelles. These organelles such as the mitochondrion
or chloroplast serve to perform metabolic functions and energy
conversion. Other organelles like intracellular filaments provide
structural support and cellular motility. Eukaryotic DNA is divided
into several linear bundles called chromosomes that are separated
by a microtubular spindle during nuclear division. Eukaryote cells
include a variety of membrane-bound structures. Many cells ingest
food and other materials through a process of endocytosis, where
the outer membrane invaginates and then
pinches off to form a vesicle. It is probable that most other
membrane-bound organelles are ultimately derived from such vesicles.
The nucleus is surrounded by a double membrane (commonly referred
to as a nuclear envelope), with pores that allow material to
move in and out. Various tube- and sheet-like extensions of the
nuclear membrane form what is called the endoplasmic reticulum
or ER, which is involved in protein transport and maturation.
It includes the rough ER where ribosomes are attached to synthesize
proteins, which enter the interior space or lumen. Subsequently,
they generally enter vesicles, which bud off from the smooth
ER. In most eukaryotes, these protein-carrying vesicles are released
and further modified in stacks of flattened vesicles, called
Golgi bodies or dictyosomes.
Vesicles may be specialized for various purposes. For instance,
lysosomes contain enzymes that break down the contents of food
vacuoles, and peroxisomes are used to break down peroxide, which
is toxic otherwise. Many protozoa have contractile vacuoles,
which collect and expel excess water, and extrusomes, which expel
material used to deflect predators or capture prey. In multicellular
organisms, hormones are often produced in vesicles. In higher
plants, most of a cell's volume is taken up by a central vacuole,
which primarily maintains its osmotic pressure.
are organelles found in nearly all eukaryotes. They are surrounded
by two membranes (each a phospholipid bi-layer),
the inner of which is folded into invaginations called cristae,
where aerobic respiration takes place. Mitochondria contain their
own DNA. They are now generally held to have developed
from endosymbiotic prokaryotes, probably proteobacteria.
The few protozoa that lack mitochondria have been found to contain
organelles, such as hydrogenosomes and mitosomes; and thus probably
secondarily lost their mitochondria.
and various groups of algae also have plastids with their own
DNA and, like animals, are accepted to have developed from
this case cyanobacteria. They usually take
the form of chloroplasts,
which like cyanobacteria contain chlorophyll and produce organic
compounds (such as glucose) through photosynthesis. Others are
involved in storing food. Although plastids likely had a single
origin, not all plastid-containing groups are closely related.
Instead, some eukaryotes have obtained them from others through
secondary endosymbiosis or ingestion.
Coleman, and Wiebe, Prokaryotes: The unseen majority, Proc.
Sci. USA, Vol. 95, pp.
FD, Doerks T, von Mering C, Creevey CJ, Snel B, Bork P (2006). "Toward automatic reconstruction of a highly resolved
tree of life". Science 311 (5765): 1283–7.
T. (1998). "A revised six-kingdom system
of life". Biological Reviews of the Cambridge Philosophical
Society 73 (3): 203–266.
T. & Chao, E.E. (1996). "Molecular phylogeny
of the free-living archezoan Trepomonas agilis and the nature of
the first eukaryote". Journal of Molecular Evolution 43 (6):
Thomas (2002). "The phagotrophic origin of
eukaryotes and phylogenetic classification of Protozoa". International
Journal of Systematic and Evolutionary Microbiology 52 (2): 297–354.
- Fabien Burki, Kamran Shalchian-Tabrizi, Marianne
Skjæveland, Sergey I. Nikolaev, Kjetill S. Jakobsen, Jan
Pawlowski (2007). "Phylogenomics Reshuffles the Eukaryotic
Supergroups". PLoS ONE 2 (8): e790.
Susannah H., Meisterfeld, Ralf, and Knoll, Andrew H. (2003). "Vase-shaped
microfossils from the Neoproterozoic Chuar Group, Grand Canyon:
a classification guided by modern testate amoebae". Journal
of Paleontology 77 (3): 409–429.
- Thomas Cavalier-Smith (2006). "Protist phylogeny and the
high-level classification of Protozoa". European Journal of
Protistology 39 (4): 338–348.