Transcript Slide 1
Dawn of Eukarya
The Steps in the Development
of a Eukaryote Cell
The evolution of eukaryotic cells
included the following components:
The origin of a flexible cell surface
The origin of a cytoskeleton
The origin of a nuclear envelope
The appearance of digestive vesicles
The endosymbiotic acquisition of certain
Revision of the cell – first
The first step toward the eukaryotic
condition may have been the loss of
the cell wall by an ancestral
Steps in the formation of a
A surface that is flexible enough to allow for
infolding lets the cell exchange materials
with its environment rapidly enough to
sustain a larger volume and more rapid
A flexible surface also allows endocytosis.
An infolded plasma membrane attached to a
chromosome within an ancestral prokaryote
may have led to the formation of the nuclear
The development of a
The early steps in the evolution of the
eukaryotic cell likely included three
The formation of ribosome-studded
internal membranes, some of which
surrounded the DNA
The appearance of a cytoskeleton
The evolution of digestive vesicles
Cytoskeleton and size and
A cytoskeleton allowed the now much larger
cell to manage changes in its shape,
distribute daughter chromosomes, and move
materials from one part of the cell to
The origin of the cytoskeleton is a mystery;
the genes that encode it are found in neither
bacteria nor archaea.
A controversial hypothesis suggests that
these genes may have originated in a longextinct fourth domain of life that
transferred them laterally to an ancestor of
the early eukaryotes.
From an intermediate kind of cell, the
next advance was likely to have been a
The first true eukaryotic cell
possessed a cytoskeleton and a nuclear
envelope; it also may have had an
associated endoplasmic reticulum and
Golgi apparatus and perhaps one or
The Effects of Oxygen
During the early stages of eukaryotic evolution, the
O2 levels in the atmosphere were increasing as a
result of the photosynthetic activities of the
Most living things were unable to tolerate this new
aerobic, oxidizing environment, but some prokaryotes
and ancient phagocytes were able to survive.
One hypothesis suggests that the key to the survival
of the early phagocytes was the ingestion of a
prokaryote that became symbiotic and evolved into
the peroxisomes of today.
Blending of the two Domains?
It is clear that the eukaryotic genome
is a mixture of genes with two distinct
Recently, it has been suggested that
the Eukarya may have arisen from the
mutualistic fusion of a Gram-negative
bacterium and an archaean.
Giardia – A link?
Figure 28.1 Three Protists
Most protists are aquatic, occupying a
variety of environments including marine and
fresh waters, the body fluids of other
organisms, and soil water.
Most are unicellular, but some are
multicellular, and a few are very large.
Some protists are heterotrophs, some are
autotrophs, and some switch between these
two modes of nutrition.
The terms protozoan and algae actually lump
together many phylogenetically distant
Apicomplexans - Malaria
Sex – Conjugative exchange
Red Tides- Dinoflagellates