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Genetics of Viruses
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Viruses -small infectious particles made of nucleic acid (DNA or RNA) enclosed in a protein coat obligate intracellular
parasites: can only
reproduce within a host
cell much smaller than bacteria edu/content/cells/scale/
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Types of Viruses (Protein) RNA Capsomere of capsid Glycoproteins
Figure 17.2 RNA Capsomere of capsid Glycoproteins Capsomere Membranous envelope Capsid Head DNA Tail sheath Tail fiber Glycoprotein 80 225 nm 80–200 nm (diameter) 70–90 nm (diameter) 18 250 nm (a) Tobacco mosaic virus 20 nm (b) Adenoviruses 50 nm (c) Influenza viruses (d) Bacteriophage T4 (Protein)
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Figure 17.2a RNA Capsomere of capsid Capsomere DNA Glycoprotein 70–90 nm (diameter) 18 250 nm (a) Tobacco mosaic virus 20 nm (b) Adenoviruses 50 nm Capsids are built from protein subunits called capsomeres -host range, a limited number of host cells that it can infect
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Some viruses have membranous envelopes that help them infect hosts
Figure 17.2b Membranous envelope RNA Capsid Head DNA Tail sheath Tail fiber 80 225 nm 80–200 nm (diameter) (c) Influenza viruses 50 nm (d) Bacteriophage T4 Glycoproteins Some viruses have membranous envelopes that help them infect hosts host range, a limited number of host cells that it can infect
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Bacteriophages (phages)
viruses that infect bacteria DNA injected into host Icosahedral head 20 sides
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Viral Replication Entry and uncoating
Figure 17.3 VIRUS Replication Entry and uncoating DNA Capsid Transcription and manufacture of capsid proteins HOST CELL Viral DNA mRNA Capsid proteins 1 2 3 4 Self-assembly of new virus particles and their exit from the cell Viral Replication
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Lytic Cycle Attachment Entry of phage DNA and degradation of host DNA
Figure 1 Attachment Lytic Cycle 2 Entry of phage DNA and degradation of host DNA New phages lyse cell wall, killing cell Virulent phages (cause cell death) 5 Release Phage assembly 4 Assembly 3 Synthesis of viral genomes and proteins Head Tail Tail fibers
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lysogenic cycle Figure 17.5b Daughter cell with prophage Many cell divisions create many infected bacteria. Prophage is copied with bacterial chromosome. Phage DNA integrates into bacterial chromosome. Lysogenic cycle Prophage exits chromosome. Prophage replicates the phage genome without destroying the host (temperate phages) viral DNA molecule is incorporated into the host cell’s chromosome (prophage) Every time the host divides, it copies the phage DNA and passes the copies to daughter cells
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Lytic vs lysogenic An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode The phage injects its DNA. Daughter cell with prophage Many cell divisions create many infected bacteria. Prophage is copied with bacterial chromosome. Phage DNA integrates into bacterial chromosome. Phage DNA and proteins are synthesized and assembled. The cell lyses, releasing phages. Lytic cycle Lysogenic cycle Prophage exits chromosome. Phage DNA circularizes. Phage DNA Phage Bacterial chromosome Prophage
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Viral envelopes Used to enter host cell Capsid RNA HOST CELL
Figure 17.6 Capsid RNA Envelope (with glycoproteins) HOST CELL (RNA) New virus Copy of genome Viral genome (RNA) Template mRNA ER Glycoproteins Capsid proteins Used to enter host cell Influenza-RNA
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Figure 17.7 Reverse transcriptase HIV Glycoprotein Viral envelope Capsid RNA (two identical strands) HOST CELL Viral RNA RNA-DNA hybrid DNA NUCLEUS Chromosomal DNA RNA genome for the next viral generation mRNA Membrane of white blood cell HIV entering a cell 0.25 m Provirus New virus New HIV leaving a cell RNA viruses Retroviruses use reverse transcriptase to copy their RNA genome into DNA HIV Viral DNA that is integrated into the host genome permanently – provirus
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Chemotherapy
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Evolution of Viruses Are viruses alive?
Since viruses can replicate only within cells, they probably evolved after the first cells appeared
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