Exam 3 Scripts.docx - EXAM 3 Lecture 27 Slide 1 Welcome to...

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EXAM 3 Lecture 27 Slide 1: Welcome to this video on viruses. In this lesson we will briefly discuss what a virus is, its structure, and a little about the viral genome. Slide 2: First, let’s answer the question: “What is a virus?” Viruses challenge our notion of what is and is not considered living-kind of like zombies. On the one hand they can infect cells where they are replicated and produce new virus particles. Nevertheless viruses are considered acellular entities. They lack many of the components found in cells including cytoplasm, components of the protein synthesis machinery such as ribosomes and other organelles like mitochondria. Without these constituents, viruses have no means to perform metabolic activities on their own and they cannot replicate without the machinery and energy provided by a host cell. Slide 3: If a virus isn’t a cell-what is it? All viruses have the same basic structure-a structure that is relatively simple compared to the prokaryotic and eukaryotic cells they infect. Viruses are comprised of a nucleic acid genome made of either DNA or RNA that is packaged within a protein coat called a capsid. The capsid is made up of repeating units of either one or several different proteins. Viruses composed only of a nucleic acid and a capsid, together called a nucleocapsid, are often referred to as naked viruses. Slide 4: Other viruses, called enveloped viruses, have a phospholipid membrane surrounding the nucelocapsid. This membrane is derived from the host but virally-encoded proteins are embedded within it. Slide 5: Let’s talk briefly about the nucleic acid contained within the virus structure. While cells store their genetic information as DNA, viruses are more diverse. Viral genomes can be either DNA or RNA. This DNA or RNA can be either single or double-stranded and while many viral genomes are a single molecule some viruses, like influenza virus, have a genome that is segmented into several separate nucleic acid pieces. Slide 6: Viral genomes are also small compared to the genome of their host cells. Some viruses encode just a few genes while others have several hundred genes but viral genomes are incomplete and they don’t encode most of the machinery needed to self-replicate. If the virus doesn’t encode this machinery... Slide 7: What genes does the viral genome encode? A viral genome often encodes proteins needed for viral replication that the host cell does not make. Viruses can also encode proteins involved in the regulation of viral gene expression so the correct proteins are produced when they are needed during viral replication. The virus encodes structural proteins, such as capsid proteins, needed to assemble new virus particles. And finally, many viruses encode proteins needed for their release from the host cell.
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