Microbiology - Viruses, Viroids, and Prions (Ch. 13)

A virus is a tiny infectious acellular agent composed nucleic acid surrounded by proteinaceous capsomeres that form a covering called a capsid.

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A capsid is a protein coat surrounding the nucleic acid core of a virus.

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An envelope is a phospholipid membrane that surrounds the nucleocapsid.

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A nucleocapsid is collectively the nucleic acid and its capsid of a virion.

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• 1) ssDNA
• 2) dsDNA
• 3) ssRNA
• 4) dsRNA

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If a virus is a generalist, it means it can infect many different kinds of cells in many different hosts.

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A bacteriophage is a virus that infects bacteria.

It is also known simply as a phage.

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Capsomeres are the proteinaceous subunits which comprise the capsid of a virus.

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• 1) Helical
• 2) Polyhedral
• 3) Complex

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They acquire their envelope from the host cell. The envelope is a portion of the membrane system of the host cell.

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The lytic replication cycle is a process of viral replication consisting of five stages, ending with lysis of the host cell as well as the release of new virions.

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• 1) Attachment of the virion to the host cell.
• 2) Entry of the virion and its genome into the host cell.
• 3) Synthesis of new nucleic acids and viral proteins by the host cell's enzymes and ribosomes.
• 4) Assembly of new virions within the host cell.
• 5) Release of new virions from the host cell.

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• Burst time is the time required for a virus to complete its entire process, from attachment to the host cell to release.
• Bust size is the number of virions released from each lysed host cell.

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• The lysogenic replication cycle is a process of viral replication in which the bacteriophage enters a bacterial cell, inserts its DNA into the host, then remains inactive. Thus, every time the bacteria replicates its chromosome, so too does the phage replicate. Later, the phage may leave the chromosome. 
• It is also known as lysogeny.

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• Temperate phages are involved in the lysogenic replication cycle.
• Thus, they are also known as lysogenic phages.

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A prophage is an inactive bacteriophage.

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Lysogenic conversion is a process whereby lysogenic phages can change the phenotype of a bacterium, for example from a harmless form into a pathogen.

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Induction is the exision of a prophage from the host chromosome, at which point the prophage reenters the lytic phase.

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• 1) Direct penetration
• 2) Membrane fusion
• 3) Endocytosis

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Uncoating is the removal of a viral capsid once it is within a host cell.

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Budding is the extrusion of virions through a host cell's membrane.

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• Persistant infections are infections with enveloped viruses in which the host cells shed the viruses slowly and relatively steadily.
• *This is in direct contrast with lytic replication cylces, where there is a sudden burst of infection.

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Latency is a process where an animal virus, sometimes not incorporated into the chromosomes of the cell, remains inactive in the cell.

*This can occur even for years before the virus becomes active again.

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Latent viruses are viruses involved in latency. They are also known as proviruses.

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No. When a provirus is incorporated into eukaryotic DNA, the condition is permanent.

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Reverse transcriptase is a complex enzyme that allows retroviruses to make dsDNA from RNA templates.

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Neoplasia is uncontrolled cell division in a multicellular animal.

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A tumor is a mass of neoplastic cells.

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• 1) Benign
• 2) Malignant (Cancers)

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Metastasis is the spreading of malignant cancer cells to the nonadjacent organs and tissues, where they produce new tumors.

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• Protooncogenes are genes that play a role in cell division.
• When they are active, they are known as oncogenes.

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20-25%

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• 1) Diploid cell cultures
• These are created from embryonic animal, plant, or human cells. They typically last no more than 100 generations (cell divisions) before they die.
• 2) Continuous cell cultures
• These are derived from tumor cells and thus provide a never-ending supply of cells (since tumor cells don't know when to stop dividing). 
• *Henrietta Lacks and HeLa cells (1951)

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Viroids are extremely small, circular pieces of RNA that are infectious and pathogenic in plants.

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They lack capsids.

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They are infectious, pathogenic RNA particles that lack capsids but infect fungi, not plants. Thus, they cannot be considered viroids.

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A prion is a proteinaceous infectious particle that lacks nucleic acids and replicates by converting similar normal proteins into new prions.

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Prions affect a cytoplasmic membrane protein called PrP. The prion changes the functional structure of several prominent alpha-helices into beta-pleated sheets.

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Spongiform encephalopathies.

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• 1) Are not cells.
• 2) Are all obligate intracellular parasites.
• 3) Have no metabolism outside of the host cell.
• 4) Do not divide - only propagate by invading the host cell.
• 5) Have no mechanisms to make proteins (outside of hijacking the host’s
• mechanisms).
• 6) Are biologically inert.
• 7) Do not necessarily have a genome of dsDNA (though some do).

(Class Notes)