Ch 9t - Chapter 9 Viruses Mobile Elements Molecular...

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Chapter 9: Molecular Genetics Lecture Outlines Viral Genomes: Fig. 9.1: Bacteriophage particles ( virions ) are usually just a piece of DNA or RNA surrounded by protective proteins. The protein coat (capsid) protects against nucleases and the environment. Icosahedra have 20 sides and 12 vertices or variations on that theme. Filamentous virions (e.g., M13) have the coat proteins coating the viral nucleic acid helix. Other bacteriophage virions have a head of coat proteins protecting a dsDNA core, attached to a proteinaceous tail with (T4) or without (lambda) tail fibers. Table 9.1: All creatures have dsDNA genomes, but viruses can have dsDNA, ssDNA, dsRNA or ssRNA formats for their genomes. Also, the genomes can be linear or circular, and tiny or large, depending on the viral species. Sometimes the viral genome is segmented , with one segment per one or two genes. If nonsegmented, the whole viral genome serves as an operon. Fig. 9.2: Many of the smaller viruses have overlapping genes. One example would be a gene within a gene in another reading frame. A more common example would be the first few codons of one gene being shared, in a different reading frame, with the last few codons of the preceding genes. Fig. 9.4: You can synchronously infect a batch of bateria with a batch of virions and each stage of the infection cycle will occur at the same time with all the bacteria in the culture. There is latent period during which the virus is being replicated, then coat proteins are being made, then virion assembly occurs. Finally the little buggers lyse the host cell and are released to the surrounding medium as infectious particles. If you assay for infected cells (Fig. 9.4A), the latent period has a steady number of infected cells, followed by
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Ch 9t - Chapter 9 Viruses Mobile Elements Molecular...

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