Viruses -General Principles; Focus On Bacterial Viruses

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VIRUSES (structure, size, specificity)

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-Always contain nucleic acid (DNA or RNA) and a protein coat (the "capside"), and SOMETIMES have an envelope made of lipids, carbohydrates, and proteins (basically similar in structure to an enhanced plasma membrane, but with a DIFFERENT FUNCTION!)
-Viruses are very specific regarding the types of cells they can infect; they generally infect certain types of cells in a particular species. This is their HOST RANGE. (ex. HIV->t cells)
-Viruses can be seen by electron microscopy ONLY from 20 to 1,000 nM in length.

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VIRION

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-Mature, infective particle consisting of nucleic acid and protein coat (and envelope depending upon the species).

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What is the arrangement of nucleic acids in viruses?

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-Many arrangements!
-Either RNA or DNA (never both); nucleic acid may be single or double stranded, linear or circular.

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CAPSID

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-Protein coat, which consists of subunits called capsomeres.

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If the virus has an envelope...

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Spikes are usually present. (carbohydrate-lipid complexes whose function we will discuss later.)

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HOST RANGE

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-Viruses are very specific regarding what types of cells they can infect; they generally infect certain types of cells in a particular species.
*HIV = t-cells
*Rabiers = neurons

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Helical Viruses

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-Capsomeres are assembled in a helical configuration
-Usually resemble long rods, and their capsids are hollow cylinders surrounding the nucleic acid.

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Polyhedral Viruses

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-Viruses are many-sided; usually the capsid is an icosahedron (20 triangular faces)

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Enveloped Viruses

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-May be helical or polyhedral, and acquire their envelope by taking part of the plasma membrane of the host cell as they "bud" from the cell (leave by exocytosis) If a virus lacks an envelope, it is said to be "naked." Enveloped viruses are animal viruses.

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Complex viruses

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Have a polyhedral head with a complex tail attached; bacteriophages are complex viruses.

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What characteristics is the taxonomy of viruses based upon?

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-Type of nucleic acid
-Life-cycle strategy
-Morphology

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How are viruses named?

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-Viruses not assigned a place in the "normal" overall classification system; however, they are generally grouped into families, genera, and species.
-Not given binomial names!! Given normal names (like influenza, HIV)

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What is a viral species?

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-Group of viruses sharing the same genetic information and ecological niche (HOST RANGE)

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Species: HIV
Family: ____
(RNA OR DNA VIRUS?)

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Retroviridae - RNA

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Species: Poliovirus
Family: ____
(RNA OR DNA VIRUS?)

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Picornaviridae - RNA

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Species: Rabies Virus
Family: ____
(RNA OR DNA VIRUS?)

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Rhabdoviridae - RNA

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Species: HPV(Human Papilloma)
Family: ____
(RNA OR DNA VIRUS?)

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Papoviridae - DNA

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Species: HSV (Herpes Simplex)
Family: ____
(RNA OR DNA VIRUS?)

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Herpesviridae - DNA

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How do we make Viral Cultures? Phage & Animal Viruses

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-To grow viruses in the lab, its host cells must be present.
-It is easiest to grow bacteriophages; phages are incubated on a lawn of bacteria and plaques develop where they are multiplying.
-Animal viruses may require a living host in order to grow; otherwise, cell cultures of host cells can be used.

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VIRAL REPLICATION : Life Cycle

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-Viruses do not have enzymes for key metabolic reactions (e.g. protein synthesis or energy production)
-Therefore, they must take over a host cell and use itse cellular machinery to produce viral enzymes and other components. Viral infection turns a cell into a factory for producing more viruses.

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Bacteriophages, aka Phages.

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"Virus eater" - viruses that infect bacteria.
-Discovered in 1915
-All carry their genomes as double-stranded DNA
-Always have a "complex" morphology. Most widely studied are the viruses of E.coli, including the "T-even" phages and phage lambda.

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The Lytic Cycle: What happens?

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-A phage causes the lysis and death of its host bacterium as it replicates.
-T-even phages (T2 and T4 are widely studied) use the lytic cycle

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The Lysogenic Cycle

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-Phage incorporates its nucleic acid into the host cell chromosome and remains dormant for a period of time.
-Phage lambda is a popular example of this type.
-These are called TEMPERATE PHAGES.

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Stages of the Lytic Cycle (5)

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1) Attachment
2) Penetration
3) Biosynthesis
4) Maturation
5) Release

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Attachment
(Lytic Cycle)

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-Sites on the bacteriophage tail fibers attach to receptor sites on the bacterium.

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Penetration
(Lytic Cycle)

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-The tail sheath contracts to force its core through the cell wall, and phage DNA enters the cell (the capsid stays outside!!)

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Bacteriophages, aka Phages.

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"Virus eater" - viruses that infect bacteria.
-Discovered in 1915
-All carry their genomes as double-stranded DNA
-Always have a "complex" morphology. Most widely studied are the viruses of E.coli, including the "T-even" phages and phage lambda.

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The Lytic Cycle: What happens?

Back 28

-A phage causes the lysis and death of its host bacterium as it replicates.
-T-even phages (T2 and T4 are widely studied) use the lytic cycle

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The Lysogenic Cycle

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-Phage incorporates its nucleic acid into the host cell chromosome and remains dormant for a period of time.
-Phage lambda is a popular example of this type.
-These are called TEMPERATE PHAGES.

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Stages of the Lytic Cycle (5)

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1) Attachment
2) Penetration
3) Biosynthesis
4) Maturation
5) Release

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Attachment
(Lytic Cycle)

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-Sites on the bacteriophage tail fibers attach to receptor sites on the bacterium.

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Penetration
(Lytic Cycle)

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-The tail sheath contracts to force its core through the cell wall, and phage DNA enters the cell (the capsid stays outside!!)

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Biosynthesis
(Lytic Cycle)

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-Transcription, translation, and replication of viral DNA occurs.

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Maturation
(Lytic Cycle)

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-New Phage DNA and capsids are assembled into virions.

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Release
(Lytic Cycle)

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-Phage lysozyme destroys the cell wall, the cell bursts, and the new virions are released.

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What are the stages of the Lysogenic Cycle? (7) *2 new stages*

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1) Attachment
2) Penetration
3) Prophage formation*
4) Prophage replication*
5) Biosynthesis
6) Maturation
7) Release

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Prophage Formation
(Lysogenic Cycle)

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-Viral DNA recombines with the bacterial chromosome to form a PROPHAGE.

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Prophage Replication
(Lysogenic Cycle)

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-The prophage replicates with the bacterial DNA, and will continue to do so until something (an adverse environment for example) triggers it to excise itself; it then continues with the steps of the lytic cycle.