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Bi1_2009_Lecture13_full

Bi1_2009_Lecture13_full - Viruses are parasites Viral...

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Viruses are parasites • Viral genomes small -- contain few genes compared with living cells – HIV genome has 9749 nucleotides - Human cells have ~3.2 x 10 9 base pairs (~30,000 genes) • Viruses must hijack host cells. They can only reproduce if they force the host cell to make viral copies. • New viruses leave host cell and infect similar cells. • Some viruses cause diseases in their hosts.
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Clicker question Viruses can infect 1) Humans 2) Animals 3) Plants 4) Bacteria 5) Bi 1 students 6) All of the above
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Raison d’etre of a virus Deliver nucleic acid (single or double stranded; RNA or DNA) to host cell. Take over host’s biosynthetic machinery for replication, transcription, and translation of viral genes. Assembly progeny viruses in infected cell. Release progeny to infect other cells. Avoid host immune system.
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Clicker question Are you currently symptomatic of a rhinovirus infection? 1) Yes 2) No 3) Decline to reveal 4) Huh?
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Baker et al., 1999 Adding the third dimension to virus life cycles: 3D reconstructions of icosahedral viruses from cryoelectron microscopy. Microbiol. Mol. Biol. Rev . 63: 862-922 Many viruses are icosahedral We will look at two icosahedral viruses: rhinovirus (causes common cold) and poliovirus . Both are much smaller than HIV (diameters ~300 Å or 30 nm for rhino- and polioviruses compared with ~1000 Å for HIV) and both are non- enveloped (no lipid bilayer) viruses.
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Icosahedral viruses Foxtrot by Bill Amend Human rhinovirus (HRV) and poliovirus are both picorna viruses (pico = small; rna refers to RNA). Lots of people are interested in picorna viruses.
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Poliovirus Inserts RNA Directly into Target Cell Poliovirus RNA is then translated in the cytoplasm of the host cell Principals of Virology. Flint et al. Fig. 5.13
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Clicker question Poliovirus and rhinovirus are retroviruses. 1) True 2) False
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Clicker question Poliovirus and rhinovirus are retroviruses. 1) True 2) False -- Picornaviruses are plus strand RNA viruses, so their genomic RNA can serve directly as messenger RNA to translate viral proteins. One such viral protein is an RNA-dependent RNA polymerase, which makes a negative strand copy of the viral RNA that then serves as a template to make more viral RNA for packaging into new viruses.
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Viral nucleic acid must be protected Viruses have protein shells to enclose their nucleic acid. Can’t make a single protein that is big enough to enclose and protect nucleic acid. Protein shells of viruses built up from many copies of one or more polypeptide chains, often resulting in rod-like or spherical shapes.
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Spherical viruses Genetic economy -- build shell from many copies of a few kinds of subunit. Specificity -- subunits recognize each other to form interface of non-covalent interactions. Leads to SYMMETRY because of specific repeated bonding patterns of identical building blocks.
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