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Origin of Eukaryotes
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Lecture notes about eukaryotes from first year.
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Lecture 3: Origin of Eukaryotic organellesEndosymbiosis probably led to mitochondria, plastids and perhaps other eukaryotic features. Under
one evolutionary scenario, the endomembrane system of eukaryotes may have evolved from inner
foldings of the plasma membrane.
Most protists move with flagella or cilia during some time in their life cycles. They probably lost them
as they evolved into plants, fungi and animals.
The eukaryotic flagella are not homologous to those of prokaryotes and is an example of parallel
evolution. They probably arose from the eukaryotic cytoskeleton. The eukaryotic flagella are
extensions of the cytoplasm with a support of the 9 + 2 microtubule system.
What is the arrangement of microtubules in prokaryotic flagella?
Mitochondria and plastids evolved from endosymbiotic bacteria in a process called endosymbiosis.
Plastids evolved from endo bacteria more than once.
Lots of evidence support a close similarity between bacteria, chloroplasts and mitochondria of euk.
The organelles are similar in size
Enzymes and transport systems in the inner membrane of the organelles resemble those in
the plasma membrane of prokaryotes.
Replication by mitochondria and chloroplasts resembles binary fission
All 3 domains seem to have genomes that are chimeric mixes of DNA that was transferred across the
boundaries of the domains.
Protists
Protists are eukaryotes and therefore much more complex than prokaryotes. The first eukaryotes
were unicellular.
Eukaryotes have 4 groups:
Excavata - protists only
SAR - protists only
Archaeplastida - (protists and land plants)
Unikonta - (protists, fungi and animals)
Because the root of the eukaryote is unknown, all four supergroups are shown as diverging
simultaneously from a common ancestor. It's not correct but we aren't unsure which evolved first.
Protists show molecular relationships with fungi, animals and plants so therefore can't have a
separate kingdom.
They're defined partly by:
Structural level
Exclusion from definitions of plants, fungi and animals
DNA analysis
Protists are the most diverse of all eukaryotes. While unicellular protists would be considered to be
the simplest, they're actually very complex. Ost protists are aerobic with mitochondria for cellular
respiration. Some are photoautotrophs with chloroplasts. Others are heterotrophs that absorb
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