New 3/17/03: See also review questions (not assigned) and especially helpful links related to the hand-out of Zimmer's Chapter 5 here
Eukaryote Novelties
Genetic material is inside nucleus
nuclear DNA probably from archaean
debated whether origin of nucleus was part of original cell or endosymbiotic
Cell wall flexible
depends on eukaryotic cytoskeleton system
flexibility depends on steroids in membrane
permitted phagocytosis
permitted endosymbiosis
plants are secondarily rigid
Mitosis
multiple origins of replication
allowed larger genome size
chromosomes not attached to cell wall
chromosomes linear, not circular
chromosome replication supported by spindle fibers
Sexual reproduction
involves shuffling of the DNA in daughter cells (offspring)
depends on reduction division (meiosis)
might be a way to get rid of harmful mutations
one cost is loss of parent's genetic contribution
another cost is built-in break up of any combination of "good genes"
Mitochondria
Endosymbiotic in origin
mitochondria have their own genome (mtDNA)
mtDNA is most similar to DNA of purple nonsulfur bacteria
oxidizing metabolism is also similar
Probably absent in first eukaryotes
Eukaryotes that lack mitochondria tend to be basal in rRNA phylogenies (e.g.,
Giardia)
Recent conflicting evidence suggests that they once might have had mitochondria
(they still make mtDNA proteins)
Too early to resolve this paradox
Plastids
Endosymbiotic in origin
plastids have their own genome (e.g., cpDNA in chloroplasts)
this genome is most similar to cyanobacteria
photosynthetic metabolism is also similar
plastids have been acquired multiple times in eukaryotes
recent evidence demonstrates that plastid-containing eukaryotes were often
the symbionts
in other eukaryotes, not cyanobacteria (secondary endosymbiosis)
Euglena has a typical green chloroplast but it is relatively
unrelated to green algae, suggesting
it is a good example of acquiring plastids by
secondary endosymbiosis
a paper published in May, 2000 presents evidence that only
one primary endosymbiosis
event occurred (from cyanobacteria) (Nature
405: 32-33, and related article on p. 69)
leading to three distinct groups of plastids (green
chloroplasts, red algae, and "glaucophytes"
including Cyanophora),
however this paper has little to do with other sorts of eukaryotic plastids,
for example, those found in kelp, diatoms, and
dinoflagellates.
1. What are the most apparent differences between microbes ("prokaryotes") and eukaryotic organisms?
2. What is the evidence supporting the endosymbiotic theory of mitochondrial and plastid origins?
3. What are the most likely ancestors of mitochondria and plastids and why?
4. Why are the earliest eukaryotic fossils a paradox in terms of what their age would imply?
5. What are some possible advantages and disadvantages of sexual reproduction?
6. What is the Linnean system of classification?
7. What is cladistics?
8. Can you define and use the bold-face terms in the Cladistics section?
9. How do conventional and cladistic classifications differ with respect to how "reptiles" and "birds" are classified?
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