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BIO153 Lecture 1.pdf

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University of Toronto Mississauga
Christoph Richter

BIO153: Lecture 1 Origins of Life January 5 th, 2009 Life on earth is diverse… where did it all come from??? In the beginning..... ▯ origin of universe: ~ 13.7 billion years ago ▯ earth formed ~ 4.5 billion years ago ▯ life arose from non-life ~ 4.4 billion years ago (perhaps) What is life? “I am not going to answer this question.” — J. B. S. Haldane Life is cellular: ▯ cells are enclosed by an osmotic barrier (cell membrane) ▯ cells have internal reactions catalyzed by proteins ▯ within cells, there occur the formation/hydrolysis of P –bonds in energy-transforming reactions ▯ cells have a conservative biochemistry (all cells use a limited repertoire of biochemical reactions and components) Cells show all the characteristics of life: 1. Cells are highly structured - diverse, but similar biochemistry 2. Cells metabolize - transform energy, possess metabolic pathways, process nutrients 3. Cells self-regulate - adjust to environment, osmoregulate, communicate 1 4. Cells reproduce - grow, divide and differentiate 5. Cells can evolve - replication is not perfect; variations subject to selection What about viruses? ▯ not cellular ▯ no metabolism: no ATP, respiration, fermentation, photosynthesis ▯ no growth or division ▯ tiny (thousands can fit in an average bacterium!) ▯ obligate intracellular parasites ▯ replicate by “hijacking” host DNA/RNA ▯ recently implicated in cancer ▯ not primitive - multiple origins; host genomes … and prions? ▯ infectious agent made entirely of protein ▯ PrPC: c = common (innocuous form) ▯ PrP : Sc =scrapie (infectious form) ▯ causes damage to cells (especially brain tissue) ▯ recently found in fungi – not harmful? ▯ all known prion diseases are fatal: ▯ e.g. BSE, Creutzfeldt-Jakob, kuru ▯ immune system cannot detect C infectious agent because PrP is a normal cell membrane protein Life from non-life: the chemical evolution hypothesis This hypothesis proposes that complexity (eventually, cells) arose from an assembly of large molecules Principles guiding the hypothesis: Any model of abiogenesis (life arising from non-living matter) must conform to the following principles: 2 1. Continuity: The transition from non-life ▯ life was gradual (sudden appearance of complex cells is not supportable) 2. Signature: compatible with contemporary biochemistry: early life must have used the same or similar biochemicals that we see in cells living today 3. “No free lunch”: life requires energy; thus any model of early life must account for energy-transforming reactions Postulated steps in abiogenesis: Stage 1: The origin of biological monomers Stage 2: The origin of biological polymers Stage 3: The evolution from molecules to cell Conditions on the early earth: Not a nice place to visit, and couldn’t live there! ▯ active volcanoes, hot lava flows ▯ early earth bombarded by extraterrestrial debris ▯ meteorites may have brought H 2O and C-compounds ▯ In formation of the earth, there was a gradual cooling; gases held by gravitational field to create and atmosphere ▯ Compounds present in the early atmosphere: methane (CH ), CO , 4 2 ammonia (NH ) 3 , 22, H O 2initially as vapour ▯ eventually became liquid) ▯ little free2O reducing (i.e. electron adding) atmosphere (Life may have begun in…) “a warm little
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