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Eukaryotic Origins
• Eukaryotic cells differ from prokaryotes– Presence of a cytoskeleton– Compartmentalization (nucleus and organelles)
• Appearance of eukaryotes in microfossils occurred about 1.5 BYA
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50 µm© Andrew H. Knoll/Harvard University
• The nucleus and endoplasmic reticulum arose from infoldings of prokaryotic cell membrane
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DNAPlasma membrane
plasma membrane
Endoplasmic reticulum (ER) Nuclear
Nucleus
Plasma membrane
Eukaryotic cell
envelope
Infolding of the
of eukaryotic cellsProkaryotic ancestor
Prokaryotic cell
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• Many organelles evolved via endosymbiosis between an ancestral eukaryote and a bacterial cell– Mitochondria
• Aerobic bacteria engulfed by larger bacteria
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Chloroplast
Eukaryotic cell with chloroplastand mitochondrion
Endosymbiosis
Photosynthetic bacterium
Mitochondrion
Aerobicbacterium
Eukaryotic cell with mitochondrion
Endosymbiosis
Internal membrane system
eukaryotic cellAncestral
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• Chloroplasts– Larger bacteria
engulfed smaller photosynthetic bacteria
– Chloroplasts come from single line of cyanobacteria
– Hosts are not monophyletic
• Brown algae engulfed red algae that already had chloroplasts– Secondary
endosymbiosis
Organelle withfour membranes
Red algal nucleus lostBrown alga
Nucleus
Nucleus
SecondaryEndosymbiosis
Chloroplastwith twomembranes Eukaryotic cell
PrimaryEndosymbiosis
Eukaryotic cell
Nucleus
Cyanobacteria Red alga
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• Endosymbiosis supported by– DNA inside mitochondria and chloroplasts
• DNA similar to bacteria DNA in size and character
– Ribosomes inside mitochondria similar to bacterial ribosomes
– Chloroplasts and mitochondria replicate by binary fission – not mitosis
• Mitosis evolved in eukaryotes
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Defining Protists
• Most diverse of the four eukaryotic kingdoms
• United on the basis that they are not fungi, plants, or animals
• Vary considerably in every other aspect– Unicellular, colonial, and multicellular groups– Most are microscopic but some are huge– All symmetries– All types of nutrition
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8
Working model for protists classification
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Cil
iate
s
Ap
ico
mp
lex
ans
ArchaeaEubacteria Chromalveolata Rhizaria Archaeplastida Excavata Amoeboza Opisthokonta
Par
aba
sal
id
Din
ofl
ag
ella
tes
Alveolata Stramenopila Rhodophyta Chlorophytes Diplomonads Euglenozoa Choanoflagellates
An
ima
ls
Fu
ng
i
Am
oeb
ozo
a
La
nd
pla
nts
Ch
aro
ph
yte
s
Ce
rco
zoa
Fo
ram
inif
era
Ra
dio
lara
Bro
wn
alg
ae
Dia
tom
s
Oo
myc
ete
s
• Locomotion– Flagella
• One or more
– Cilia• Shorter and more numerous than flagella
– Pseudopodia (“false feet”)• Lobopods – large, blunt• Filopods – thin, branching• Axopods – thin, long
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• Asexual reproduction– Typical mode of reproduction– Some species have an unusual mitosis
• Mitosis – equal size daughter cells• Budding – one daughter cell smaller• Schizogony – cell division preceded by several nuclear
divisions; produces several individuals
• Sexual reproduction – May be obligate, or only under stress– Meiosis is a major eukaryote innovation– Union of haploid gametes which are produced by
meiosis– Advantage in allowing frequent genetic recombination
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• Multicellularity– From single cells to colonies to true
multicellularity– Arisen multiple times– Fosters specialization– Few innovations have had as great an
influence on the history of life
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• Euglena– Two anterior (and unequal) flagella
• Attached at reservoir
– Contractile vacuoles – collect excess water
– Stigma – movement towards light– Numerous small chloroplasts
• From ingestion of green algae
– Concept of a single Euglena genus is now being debated
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a. 6.5 µm© Andrew Syred/Photo Researchers, Inc.
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Second flagellumStigma
Contractile vacuole
Paramylon granule
Nucleus
ChloroplastFlagellum
Pellicle
Mitochondrion
Basal bodies
Reservoir
b.
Apicomplexans
• Spore-forming animal parasites• Apical complex is a unique arrangement of
organelles at one end of the cell– Enables the cell to invade its host
• Plasmodium causes malaria– Complex life cycle – sexual, asexual, different
hosts– Eradication focused on eliminating mosquito
vector, drug development, vaccines• DDT-resistant mosquitoes
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Inside Mosquito Inside Mammal
Sporozoite
Merozoite
Gametocyte
Fertilization
Host ’sliver cell
Host ’s redblood cell
1. While feeding, mosquito injects Plasmodium sporozoites into human.
2. Sporozoites enter the liver, reproduce asexually and release merozoites into the bloodstream.
6. The game-tocytesdevelop intogametes andreproducesexually,formingsporozoiteswithin themosquito.
5. Gametocytes are ingested by another, previously uninfected mosquito.
4. Certain merozoites develop into gametocytes.
3. MerozoitesmultiplyInside redblood cellsand arereleased.The cyclerepeats.
Oocyst
Sporozoite
Gametes
Brown algae
• Conspicuous seaweeds of northern regions
• Life cycle involves alternation of generations– Sporophyte – multicellular
and diploid– Gametophyte –
multicellular and haploid
• Not plants
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ME
IOS
ISn 2n
Sporophyte (2n)
Zygote (2n)Sperm
Egg
Zoospores (n)
Gametophytes (n)
Developingsporophyte
Germinatingzoospores
MIT
OS
IS
FE
RT
ILIZ
AT
ION
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Green Algae
• Land plants arose from an ancestral green alga only once during evolution
• Green alga consist of 2 monophyletic groups– Chlorophyta– Charophytes
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5 µm© Dr. Richard Kessel & Dr. Gene Shih/Visuals Unlimited
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Chlamydomonas life cycle.
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–
+
–
Gamete
Asexualreproduction
MITOSIS
MITOSISStrain
Strain FERTILIZATION
Zygospore(diploid)
+GametePairing of positiveand negativemating strains
n
2n
MEIO
SOS
• Cell specialization in colonial chlorophytes– Multicellularity arose many times in the
eukaryotes – Colonial chlorophytes are examples of cellular
specialization• Volvox - hollow sphere made up of a single layer of
500 to 60,000 individual cells each with 2 flagella
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Vegetative cells
Reproductive cells
© Wim van Egmond/Visuals Unlimited
20 nm
Haplodiplontic Life Cycle
• Multicellular diploid stage – sporophyte– Produces haploid spores by meiosis
– Diploid spore mother cells (sporocytes) undergo meiosis in sporangia
• Produce 4 haploid spores• First cells of gametophyte generation
• Multicellular haploid stage – gametophyte– Spores divide by mitosis
– Produces gametes by mitosis
– Gametes fuse to form diploid zygote• First cell of next sporophyte generation
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