Microbiology chapter 8 review .docx

However they infect and lyse cells so you can see

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by themselves. However, they infect and lyse cells so you can see plaques in a layer of cultured cells Plaque forming unit Quantification: How do you know how many viruses you have? In the virus lyses the cell it infects, you can dilute the virus sample and plate it on cells. Clear areas (plaques) will be formed in the lawn of cell caused by the lysis of the cells. Each plaque is a single virus infectious unit This clear area is called a plaque, or and the number of viruses per mi is called the titer Titers usually refer to plaque forming units (pfu) per mi and not viruses per mi, because not all of the viruses in a sample will successfully infect a cell and cause a plaque Viral life cycle and growth curve Latent period: Eclipse-virus genome is separated from capsid. Multiplication occurs here. Maturation: assembly of virus particles T4 bacteriophage Large genome Icosahedral head with a collar, tail, endplate, tail fibers Genome dsDNA and very large 650X length of head Bacteriophage penetration Attachment: Tail fiber proteins on virus interact with specific receptor molecules on the cell surface core polysaccharides o Digestion: Tail fibers retract and the tail pin proteins contact cell wall to form a small hole (degrade peptidoglycan-similar to lysozyme) Penetration: Tail sheath contracts and the genome is injected through cell envelope
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Genome assembly and processing As the genome is being replicated the partially completed fragments recombine at homologous regions (regions where the sequences are the same). This is called homologous recombination This forms large concatamers of viral genomic DNA in which several copies of the viral genome are connected in a single large DNA molecule Next, the concatamer is fed into the phage head and when the head is filled with DNA, an endonuclease cuts the DNA. This is called head-full packaging dsDNA T even phages: Lifecycle After infection, transcription begins and the EARLY proteins are made. These are involved in replicating the virus genome and transcription of viral genes. Immediate early: first to be transcribed Delayed early: 1-2 minutes after infection Late proteins: head, tail, and lysis proteins. The head, tail, and tail fibers are assembled separately and then when each is complete they are stuck together. Lysis protein (T4 lysozyme) is also a late protein. What does a lysozyme do? Viruses that always cause lysis are called virulent phage Viral genome and mRNA production Synthesis of proteins and replication are quite a bit different from what we have just studied in bacteria since the viral genome in many cases is not dsDNA as it is in bacteria
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