VIRAL GENOMES AND THEIR IMPLICATIONS IN DISEASE There doesn’t seem much rhyme or reason why some viruses have one type of genome and others have yet another type altogether. In some cases, the genome type determines what the virus can do inside an infected cell and how it causes disease. One commonly studied virus that does not have major implications in human diseases is the T phage that infects E. coli bacteria. It consists of one molecule of double-stranded DNA. These viruses have an icosahedral head and a helical protein tail. The DNA is injected into the cell, where it can make about a hundred new virions every twenty minutes. The cell then lyses to release these phages. Within 20 minutes of infection, the metabolic processes in the cell will be turned over to make phage proteins only. Temperate phages like the lambda phage in E. coli are also well-studied. These are double-stranded DNA viruses that might become circular to create ordinary progeny that are released through lysing. They might also enter the genome of the bacterium to assume a lysogenic cycle that does not involve immediate lysing. We will talk more about this in a minute. The environment around the cell will determine what type of life cycle happens in the cell. Small DNA phages generally only code for less than twelve proteins and have also been extensively studied with regard to their life cycle. These are simple viruses that require a great deal of help from the host in order to make the proteins necessary to make the phage progeny. Research on these small DNA phages has helped researchers determine which cellular proteins are necessary to participate in DNA replication. RNA phages also infect E. coli but have an RNA genome. Most of these have their genomes directly made into proteins as is the case with eukaryotic messenger RNA. The phage RNA can make many but not all of the proteins necessary for the making of phage proteins. Some will just make RNA polymerase to transcribe viral RNA, an enzyme that dissolves cell walls of bacteria, and two capsid proteins used to make the phage capsids. As you learn about the animal viruses and, in particular, the human viruses, you’ll see that these are largely randomly named or named after the diseases they cause in the human host. It can be confusing because, in the case of respiratory viruses, the
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