Unformatted text preview: Figure 25.3 (Campbell 9th ed) 34 2. Origin of life How did life begin?
**Heredity is essential to life**
- pass on beneficial characteristics for survival/reproduction DNA allows us to pass on our traits?
- But DNA is a complex molecule
- Likely preceded by a less complex molecule 35 Why RNA is believed to be
the original unit of heredity
RNA monomers spontaneously
assemble into short RNA
polymers (i.e. a simple gene). Certain RNA sequences act as catalysts (ribozymes)
e.g. with some catalyzing copying of other RNA Tomas Cech
1982 40 Key Events:
3. Atmospheric O2 • 1st aquatic photosynthetic bacteria ~2.1 - 2.7 bya
How do we know?
• O2 is a byproduct of photosynthesis
• O2 reacted w/ dissolved iron in H2O
forming iron oxide sediments
Figure 25.8 (Campbell 8th ed) • depleted dissolved iron
and then saturated the H2O
• then released into atmosphere
rusting terrestrial rocks (2.7 bya) 41 Key Events:
3. Atmospheric O2 ~2 bya atmospheric [O2] began to increase rapidly…WHY?
> atmospheric [O2] made aerobic respiration possible
→ larger cells & organisms w/ > energy demands (i.e. eukaryotes)
→ > atmospheric [O2] Anerobic
respiration 42 Key Events:
3. Atmospheric O2 Higher [O2]:
• Killed many prokaryotes
• (O2 still toxic to many bacteria) • Formed ozone layer
→ terrestrial life possible! 43 Key Events:
4. Eukaryotes • Oldest fossil eukaryotic cell ~1.6- 2.1 bya
• but steroids, a constituent of euk cell membranes,
found in sediments from 2.7 bya
• Evolved from prokaryotes in several hypothesized steps: 44 Evolution of eukaryote morphology
A. Nuclear Envelope
Origin: infoldings of prok plasma membranes
- Extant bacteria w/ infoldings
- Nuclear membrane &a...
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- Fall '10
- Evolution, Extinction event