497A Lecture 4

497A Lecture 4 - 1/16/11 Lecture 4 The Origin of Viruses...

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1/16/11 1 Lecture 4 The Origin of Viruses Are Viruses Alive? Definitions of Life • Many definitions of life exist. • Classic definition: physically connected unit that has metabolism, can reproduce and evolve by natural selection. • Personal favourite - “CITROENS” (“Complex Information-Transforming Reproductive Objects That Evolve by Natural Selection”). - Leslie Orgel (1973) • As viruses fit the “CITROENS” definition they can be considered alive. • Like most of the universe, life is composed of CHONSP - carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus (therefore all life in the universe is likely to be carbon-based). Time-Line for the Origin of Life ~15 bya - Big Bang! ~4.7 bya - Solar system forms ~4.5 bya - Formation of Earth ~3.5 bya - First fossils on Earth What happened here?
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1/16/11 2 Does Life have a Hot or a Cold Origin? Fungi Plants Animals Archaea Bacteria Eucarya Root? • “Hot” Origin: - Long intellectual history - Archaea (archaebacteria) which are often extreme thermophiles, are throught to be closest to the root of cellular life - Deepest branches are G+C rich which suggests a hot origin (G+C are more thermally stable) “Tree of Life” Based on rRNA Does Life have a Hot or a Cold Origin? • “Cold” Origin: - More recent (i.e. better!) phylogenetic trees reveal that the archaebacteria fall into two groups (one closer to eukaryotes) and the thermophiles are not closest to the root - Deepest branches are A+T rich which suggests a cold origin (i.e. mesophilic bacteria) - Galtier et al . (1999). Science 283 , 220-221. “Tree of Life” Based on rRNA Fungi Plants Animals Archaea Bacteria Eucarya The Origin of Viruses • Studies of viral origins are hampered by lack of a fossil record, so analysis can only be conducted on contemporary samples or those stored (in freezers) from past epidemics. In most cases, like influenza A virus, this storage goes back ~90 years at a maximum. • An even bigger problem for RNA viruses is that rapid evolution makes signals of their origins even harder to recover. For example, although it is possible to identify families of RNA viruses, extreme sequence divergence makes it difficult to construct trees linking them together; there is often no more similarity among them than expected by chance alone (i.e. extensive multiple substitution). • Recombination between viral families will also complicate attempts to construct evolutionary history (no single tree!).
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1/16/11 3 The Origin of Viruses • Three main theories for the origin of viruses; (i) “Pre-Cellular Life”: Viruses represent descendants of pre- cellular life forms, with ancestries dating back billions of years, and so have always existed independently of host genomes.
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497A Lecture 4 - 1/16/11 Lecture 4 The Origin of Viruses...

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