Chapter 9
Eukaryotic Cells and
Multicellular Organisms
Figure CO: Oblong shaped Giardia
Courtesy of Dr. Stan Erlandsen/CDC
Overview
• The origin of cells with eukaryotic organization, some 2.5
Bya, facilitated the evolution of multicellularity
• Endosymbiosis was important in the origin of eukaryotes
• Five (or more) supergroups of eukaryotes are recognized
• DNA in eukaryotic cells is dispersed among several linear
chromosomes
• There are separate mitochondrial and chloroplast genomes
• Meiosis and some form of sexual reproduction are almost
universal in eukaryotes
• Some eukaryotes are multicellular
Evolution of Eukaryotes
• As early as 1.5 Bya eukaryotic
cells appear as fossils
Figure 01A: Microfossils of
probable eukaryotic cells
Figure 01B: Microfossils of
probable eukaryotic cells
Reproduced from Schopf, J.W., Scientific American 239 (1978): 111-138. Courtesy
of J. William Schopf, Professor of Paleobiology & Director of IGPP CSEOL
Figure 01C: Microfossils of
probable eukaryotic cells
Evolution of Eukaryotes
• Grypania spiralis has
been found in ancient
rocks in Michigan
• This fossil species is
preserved because it
formed simple shells
Still Another Tree of Life
• A Tree of Life was established
using nucleotide sequences
from 5S rRNA of over 30
species of prokaryotes and
eukaryotes
• This tree is from 1979
• There are still three grades
recognized here: animals,
plants and fungi
• Unfortunately, protistans are
omitted from this analysis
Adapted from Hori, H. and S. Osawa, Proc. Natl Acad. Sci. USA 76 (1979): 381-385.
Figure 02: Phylogenetic tree
Single-Celled Eukaryotes: Protistans
• Early eukaryotes were single-celled organisms or
simple filaments
• Today, most eukaryotes are multicellular
• All unicellular eukaryotes can be classified in the
kingdom Protista
• Endosymbiotic events provided mitochondria and
chloroplasts
• Microtubules drive the nuclear chromosomal
divisions (mitosis and meiosis)
• But the Kindgom Protista does not appear to be
monophyletic
Five Eukaryotic Supergroups
alveolates
*
*
chromalveolates
Figure B01: Eukaryotic tree of life
Adapted from Keeling, P.J., et al., Trends Ecol. Evol. 20 (2005): 670-676.
Others would
establish six
supergroups
Five Eukaryotic Supergroups
• Plantae = Archaeplastida: Charophyta (stem group), red
algae, green algae, and land plants
• Excavata: Various Protistans, many with parasitic lifestyles
(e.g., Giardia, Trichomonas, Trypanosoma)
• Chromalveolata: Many of the algae, heterotrophic ciliates,
and other Protistan parasites such as Plasmodium
falciparum
• Rhizaria: A group advocated for by Cavalier-Smith
containing heterotrophic Protistans such as foraminiferans
and radiolarians
• Unikonta: Still other parasitic Protistans, choanoflagellates,
fungi, animals, and amebozoans including slime molds
Five Eukaryotic Supergroups:
Plantae = Archaeplastida
Charophyta
(stem group)
red and green algae
Red algae
Viridiplantae
Chlorophytes
Plantae
Embryophytes
Streptophyta
Charophytes
land plants
Five Eukaryotic Supergroups: Excavata
Trichomonas vaginalis
Giardia lamblia
Trypanosoma sp.
Five Eukaryotic Supergroups:
Chromalveolata
dinoflagellates
brown algae
water molds
diatoms
Plasmodium falciparum
Five Eukaryotic Supergroups: Rhizaria
foraminiferans
Figure B03: Diversity of forms of
foraminiferans
Reproduced from E. Haeckel. Art Forms in Nature. New York: Dover Publications, Inc., 1974.
radiolarians
Five Eukaryotic Supergroups: Unikonta
choanoflagellates
animals
amoeba
cellular slime mold
fungi
plasmodial slime mold
Six Eukaryotic Supergroups
As more data is
collected,
especially DNA
sequence data,
from more
example
organisms, and
more data about
Horizontal Gene
Transfer, these
groups will be
revised -- probably
many times
Unikontans
Figure B02: Eukaryotic tree of life
Adapted from Adl, S.M., Simpson, A.G.B., et al., J. Eukaryot. Microbiol. 52 (2005): 399-451.
Unikontans & Bikontans
one flagellum and basal body versus two
Lots of competing hypotheses!
Unikontans & Bikontans
Origin of the Eukaryotes?
We may never know the
correct pathway or how many
steps were involved
Endosymbiosis is very likely an
important part of this process
Which came first: the nucleus,
mitochondria or chloroplasts
as organelles?
Origin of the Eukaryotes
• Free-living bacteria
developed mutually
beneficial relationships
within a host prokaryotic cell
• Some aerobic bacteria
developed into mitochondria
and cyanobacteria into
chloroplasts, eventually
producing the eukaryotic
cells of animals and plants
Origin of the Eukaryotes
Origin of the Eukaryotes
Origin and Evolution of
Mitochondria and Chloroplasts
• Ancient anaerobic eukaryotic cells evolved the
ability to engulf (endocytose or phagocytize)
prokaryotes
Figure 03: Symbiotic relationships
between a eukaryote and its
photosynthetic organelles
The ciliate Paramecium bursaria houses
hundreds of symbiotic green algae which
can be liberated from the Protistan cell and
the algal cells will live independently
Courtesy of Anthony L. Swinehart, Hillsdale College
Organelle DNA Differs
from Nuclear DNA
1) In location: organelle vs. nucleus
2) In organization: single circular vs. multiple linear
strands
3) In function: which proteins are coded for and
how are they regulated
4) In mode of replication and inheritance: organelle
DNA transmitted maternally during cell division
during cytokinesis while nuclear DNA is sorted
during nuclear division (mitosis and meiosis)
Mitochondrial DNA (mtDNA)
•
•
•
•
•
•
•
Mt DNA is a single double-stranded circular DNA
molecule
There are several copies in each mitochondrion
and there are many mitochondria in each
eukaryotic cell
Mt DNA is similar to prokaryotic DNA: there are
no histones or any other protein associated with
mt DNA and Mt DNA genes contain no introns
Because Mt DNA is in a highly oxidizing
environment, Mt DNA has a much higher
mutation rate than nuclear DNA
Mt DNA genes code for mitochondrial
ribosomes and transfer RNAs
Some Mt DNA genes code for polypeptide
subunits of the electron transport chain
common to all mitochondria
Mt DNA relies on nuclear gene products for
replication and transcription
Chloroplast DNA (cpDNA)
•
•
•
•
•
•
CP DNA is a single double-stranded circular DNA
molecule (the smallest of the three plant
genomes)
20-200 copies in every chloroplast; several
thousand copies in each green leaf cell; CP DNA
constitutes one-fourth of all DNA in a plant cell
Consists of large (LSC) and small (SSC) single-copy
regions separated by two inverted repeat regions
Inherited uniparentally from the maternal (seed)
parent
CP DNA contains some 113 genes, 20 of which
contain introns; most of these genes are involved
with photosynthesis and plastid gene expression
Structural rearrangments of the genome are rare
(but when they occur, they are useful in
establishing relationships phylogenetically; e.g.,
losses of genes and introns, inversions, IR
expansions or contractions)
Origin of Various
Photosynthetic Eukaryotes
Figure 04: Primary, secondary and tertiary endosymbiosis
Adapted Cracraft, J. and M. J. Donoghue (Eds). Assembling the Tree of Life. Oxford University Press, 2004.
The Origin of early
Eukaryotic Ancestors
leading to the lineages of
animals and fungi was
probably an independent
event from that of the
origin of plants
Transfer of Genes Between
Organelles and Nucleus
• Many genes were transferred to the eukaryotic nucleus;
conversely, some nuclear genes were transferred to
organelle genomes
• Two examples are genes for anaerobic glycolysis and
genes for amino acid synthesis
• Chloroplasts synthesize only a small portion of the
proteins they use
• Transfer of nuclear genes coding for symbiotic organelle
proteins
• Such gene transfers improve efficiency and reduce the
likelihood of mutations
Transfer of Genes Between
Organelles and Nucleus
• Genes transferred to and from the eukaryotic nucleus
and internal organelles are a form of horizontal gene
transfer
• The transfer of genes between the nucleus and the
organelles complicates their use in establishing
phylogenies
• Despite many potential problems, DNA sequences have
become important characters in the study of evolutionary
relationships
The Molecular Clock
• Molecular clocks use mutations to
estimate evolutionary time
• Mutations add up at a “constant
rate” in related species
– This rate is the ticking of the
molecular clock
– As more time passes, there will be
more mutations
• Scientists estimate mutation rates
by linking molecular data and real
(geological) time
Organelle DNA as a Molecular Clock
When a stretch of DNA serves as molecular clock, it becomes a
powerful tool for estimating the dates of lineage-splitting events
• Imagine that a length of DNA found in
two species differs by four bases and we
know that this entire length of DNA
changes at a rate of approximately one
base per 25 million years
• That means that the two DNA versions
differ by 100 million years of evolution
and that their common ancestor lived
50 million years ago
• Since each lineage experienced its own
evolution, the two species must have
descended from a common ancestor
that lived at least 50 million years ago
Mitochondrial DNA and Ribosomal RNA
.
Provide Two Types of Molecular Clocks
• Different molecules have different mutation rates
– higher rate, better for studying closely related species
– lower rate, better for studying distantly related species
• Ribosomal RNA is used to study distantly related species
– many conservative regions because the shape is so important
– lower mutation rate than most DNA loci
Mutations add up at a fairly
constant rate in the DNA of
species that evolved from a
common ancestor.
DNA sequence from a
hypothetical ancestor
Ten million years later—
one mutation in each lineage
Another ten million years later—
one more mutation in each lineage
The DNA sequences from two
descendant species show mutations
that have accumulated (black).
The mutation rate of this
sequence equals one mutation
per ten million years.
Organelle DNA as a Molecular Clock
• Mitochondrial DNA is used to study closely related species
– Mt DNA’s mutation rate is ten times faster than that of nuclear DNA due to the
reactive oxygen species in the mitochondrial matrix
– Mt DNA is passed down from mother to offspring without recombination
grandparents
mitochondrial
DNA
nuclear DNA
parents
Mitochondrial DNA is
passed down only from
the mother of each generation, so it is
not subject to recombination.
child
Nuclear DNA is inherited from both
parents, making it more difficult to
trace back through generations.
Using DNA as a Molecular Clock
• It is relatively easy to use DNA from living species
to draw conclusions about phylogeny and times
of divergence
• It is more difficult to use DNA from preserved
museum and fossil material
• First, museum and fossil material may be
contaminated by other DNA, especially microbial
DNA
• Second, fossil material is likely to have only tiny
quantities of DNA from which to work
DNA Reveals the Aboriginal Australians
Are the First Humans to Leave Africa
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An international team of researchers has for the first time
sequenced the genome of a man who was an Aboriginal
Australian (Science: 22 September 2011)
They have shown that modern day Aboriginal Australians are
the direct descendents of the first people who arrived on the
continent some 50,000 years ago and that those ancestors
left Africa earlier than their European and Asian counterparts
Although there is good archaeological evidence that shows
humans in Australia around 50,000 years ago, this genome
study re-writes the story of their journey there
The study provides good evidence that Aboriginal Australians
are descendents of the earliest modern explorers, leaving
Africa around 24,000 years before their Asian and European
counterparts
This is contrary to the previous and most widely accepted
theory that all modern humans derive from a single out-ofAfrica migration wave into Europe, Asia, and Australia
The study derived from a lock of hair collected by a British
anthropologist one hundred years ago from an Aboriginal man from
the Goldfields region of Western Australia in the early 20th century
The Polymerase Chain Reaction
Figure B04A: The polymerase chain reaction
Eukaryote Origins Remain Unclear
Which came first – nucleus or organelle?
Other details of the transition?
Eukaryote Characteristics
• DNA organized as linear chromosomes; various
states of ploidy
• many cytoplasmic membrane-bound organelles
• eukaryotic cytoskeleton (microtubules, actin, and
intermediate filaments)
• eukaryotic ribosomes
• presence of external cell wall - variable
• sexual reproduction predominates and various
means of gene recombination available
• unicellular or multicellular organisms
Eukaryotes
There Is No Generalized Eukaryotic Protistan Cell
Generalized
Eukaryotic Cell
(Animal)
• Plasma Membrane
– microvilli
• Cytoplasm
• Cytoplasmic Organelles
–
–
–
–
cytoskeleton
ribosomes
mitochondria
rough endoplasmic
reticulum
– smooth endoplasmic
reticulum
– Golgi apparatus
– lysosomes, etc.
• Nuclear Envelope with
pores
• Nucleoplasm and nucleoli
• DNA in chromosomes
Generalized Eukaryotic Cell (Plant)
• The same basic
components and
organelles as the animal
cell with the addition of
a cellulose cell wall, a
central water vacuole,
which sequesters
various chemicals, and
chloroplasts that carry
out photosynthesis
Generalized Eukaryotic Cell (Fungus)
The same basic
components and
organelles as the plant cell
but the substitution of a
chitin cell wall and no
central water vacuole
Eukaryotes Package
DNA Differently
Transcription and
Translation in
Prokaryotes and
Eukaryotes
• Prokaryote genes lack introns and, therefore, no pre-mRNA
processing is required
• Prokaryotes have no nucleus, no separation between DNA
and the cytoplasm
• Prokaryotic ribosomes are different in structure
• Methods of gene regulation differ (prokaryotic operons)
Review: Gene Expression
• DNA contains a sequence of nitrogenous bases
which codes for the sequence of amino acids in a
protein
– A triplet code, in which each codon is composed of 3
nitrogenous bases, forms the “genetic code”
• During transcription
– one strand of DNA serves as a template for formation of
messenger RNA
– mRNA has bases complementary to the base sequence
in the DNA
• Messenger RNA is processed, with intron removal,
before leaving the nucleus
Review: Gene Expression (cont.)
• mRNA carries the codon sequence to the ribosomes
(rRNA and protein) in the cytoplasm
• Each tRNA carries a particular kind of amino acid
– each tRNA also carries a 3-base anticodon which pairs
complementarily to a codon of the mRNA
• During translation
– the linear sequence of codons in the mRNA determines
the order of tRNAs and their attached amino acids
– sequential peptide bond formation produces the
primary structure of the protein at the ribosome
Oxidative
Nutrient
Metabolism
• Breakdown products of carbohydrates, fats, and proteins
enter various metabolic pathways where energy is
harvested
• Oxygen (O2) is used up; carbon dioxide (CO2) is given off
Nutrient Catabolism Pathways Are All
Interconnected
Nucleic acids can also be broken down and the products sent to these or related pathways
Photosynthesis
Photosynthesis
• Plant cells contain numerous
chloroplasts
• In chloroplasts, light energy is
used eventually to produce
energy transfer molecules,
ATP and NADP+
• These energy transfer
molecules power the Calvin
cycle, which in turn produces
glucose
• Glucose is used in cellular
respiration and starch
synthesis
Landmarks in Time
• As early as ~3.5 Bya, some prokaryotes
develop early photosynthetic metabolism
• ~ 2.0 Bya: eukaryotes develop from
prokaryotes by complex means including
endosymbiosis
• ~ 2.0 Bya : eukaryotes develop sexual
reproduction and colonial lifeforms
• ~1.8 Bya : O2 levels rise sufficiently that the
atmosphere becomes oxidizing
• ~1.3 – 0.6 Bya : multicellular (metazoan) life
evolves, perhaps several times
almost 2 billion
years of strictly
unicellular life!
What’s Left? The Macroscopic
Multicellular Minorities
Chapter 9
End