Chapter 9 Eukaryotic Cells and Multicellular Organisms

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Chapter 9 Eukaryotic Cells andMulticellular
Organisms
Figure CO Oblong shaped Giardia
Courtesy of Dr. Stan Erlandsen/CDC
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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 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

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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
Figure 01C 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
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Evolution of Eukaryotes

• Grypania spiralis has been found in ancient rocks
  in Michigan
• This fossil is preserved because it formed simple
  shells

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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

Figure 02 Phylogenetic tree
Adapted from Hori, H. and S. Osawa, Proc. Natl
Acad. Sci. USA 76 (1979) 381-385.
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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,
  chloroplasts
• Microtubules drive the nuclear chromosomal
  division (mitosis)
• But the Kindgom Protista does not appear to be
  monophyletic

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Five Eukaryotic Supergroups
alveolates chromalveolates
Others would establish six supergroups
Figure B01 Eukaryotic tree of life
Adapted from Keeling, P.J., et al., Trends Ecol.
Evol. 20 (2005) 670-676.
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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 parastitic Protistans,
  choanoflagellates, fungi, animals, and Amebozoans
  including slime molds

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Five Eukaryotic SupergroupsPlantae
Archaeplastida
Charophyta (stem group)
red and green algae
Red algae
Chlorophytes
Viridiplantae
Charophytes
Streptophyta
land plants
Plantae
Embryophytes
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Five Eukaryotic Supergroups Excavata
Trichomonas
Giardia
Trypanosoma
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Five Eukaryotic Supergroups Chromalveolata
dinoflagellates
brown algae
diatoms
Plasmodium falciparum
water molds
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Five Eukaryotic Supergroups Rhizaria
foraminiferans
Figure B03 Diversity of forms of foraminiferans
radiolarians
Reproduced from E. Haeckel. Art Forms in Nature.
New York Dover Publications, Inc., 1974.
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Five Eukaryotic Supergroups Unikonta
choanoflagellates
animals
amoeba
cellular slime mold
plasmodial slime mold
fungi
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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.
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Bikontans Unikontans
Lots of competing hypotheses!
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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?

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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.

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Origin of the Eukaryotes
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Origin of the Eukaryotes
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Origin and Evolution of Mitochondria and
Chloroplasts

• Ancient anaerobic eukaryotic cells evolved the
  ability to engulf (endocytose or phagocytize)
  prokaryotes

The ciliate Paramecium bursaria houses hundreds
of symbiotic green algae which can be liberated
from the Protistan cell and will live
independently
Figure 03 Symbiotic relationships between a
eukaryote and its photosynthetic organelles.
Courtesy of Anthony L. Swinehart, Hillsdale
College
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Organelle DNA Differsfrom Nuclear DNA

• In location organelle vs. nucleus
• In organization single circular vs. multiple
  linear strands
• In function which proteins are coded for and
  how are they regulated
• 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)

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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

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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)

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Origin of VariousPhotosynthetic Eukaryotes
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
Figure 04 Primary, secondary and tertiary
endosymbiosis
Adapted Cracraft, J. and M. J. Donoghue (Eds).
Assembling the Tree of Life. Oxford University
Press, 2004.
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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

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Transfer of Genes Between Organelles and Nucleus

• Genes transferred to and from the eukaryotic
  nucleus 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

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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 time

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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

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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
• lower mutation rate than most DNA

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.
DNA sequence from a hypothetical ancestor
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Organelle DNA as a Molecular Clock

• Mitochondrial DNA is used to study closely
  related species

• Mt DNAs mutation rate is ten times faster than
  that of nuclear DNA
• Mt DNA is passed down unshuffled from mother to
  offspring

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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 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

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DNA Reveals the Aboriginal Australians Are the
First Humans to Leave Africa

• 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-of-Africa 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.
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The Polymerase Chain Reaction
Figure B04A The polymerase chain reaction
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Eukaryote Origins Remain Unclear
Which came first nucleus or organelle?
Other details of the transition?
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Eukaryote Characteristics

• DNA organized as linear chromosomes various
  states of ploidy
• many cytoplasmic membrane-bound organelles
• eukaryotic cytoskeleton and ribosomes
• presence of external cell wall - variable
• sexual reproduction predominates and various
  means of gene recombination available
• unicellular or multicellular

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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

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Generalized Eukaryotic Cell (Plant)

• The same basic components and organelles as the
  animal cell plus the addition of a cellulose cell
  wall, a central vacuole, which sequesters various
  chemicals, and chloroplasts that carry out
  photosynthesis

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Eukaryotes Package DNA Differently
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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

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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

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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

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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

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Nutrient Catabolism Pathways Are All
Interconnected
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Photosynthesis
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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

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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

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almost 2 billion years of strictly unicellular
life!
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Whats Left? The Macroscopic Multicellular
Minorities
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Chapter 9End