Mim: Properties And Replication Of Viruses

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Bacteria vs. Virus

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Nature of Viruses

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Genome- either RNA or DNA.

Surrounded by a protective protein shell.
Shell is enclosed within an envelope that contains both protein and lipid making it an enveloped virus.

Viruses multiply only inside cells.
-They are completely dependent on the host cell's synthetic and energy-yielding apparatus.

The replication of viruses involves an initial step, the separation of their genomes from their protective shells.

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naked virus

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only contains DNA and protein, no protective protein shell

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Replication of a virus

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Involves first the attachment of the virus to the cell. This can be done via glycoproteins and receptors on the surface of the envelope that bind to receptors on the surface of the host.

Naked viruses such as the adenovirus sometimes use tail fibers to aide in attachment.

After attachment, the virus must be taken up into the cell, and then it must uncoat itself.

Nucleic acids replicate and RNA transcripts are made resulting in the formation of proteins.

The proteins must come together and assemble and virus leaves the cell either via cell lysis or budding.

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Families of DNA viruses (smallest to largest)

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-Parvovirus (small, naked)
-Papovirus (small, naked)
-Adenovirus (naked, has tail fibers*)
-Herpesvirus (enveloped)
-Pox Virus (enveloped)

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RNA Viruses (smallest to largest)

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-Picornavirus (small, naked)
-Reovirus (small, naked)
-Togavirus (enveloped)
-Coronavirus (enveloped)
-Orthomyoxyvirus (enveloped)
-Rhabdovirus (enveloped)
-Paramyxovirus (enveloped)
-Filoviridae)

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MIMI

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"Mimicking Microbe)
Girus (large virus)

Contains dsDNA and RNA, had bacterial properties, gram positive.
Replicate like viruses.

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Influenza Virus

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Influenza genome has 8 segments, which code for the 16 different hemagglutinins and 9 different neuraminidases.

Hemmagglutinins are key in the attachment of the virus to receptors on cells. Neuraminidase is key in the release or spread of the virus from infected cells.

Viruses can change because of high frequency recombinations. The virus can undergo two different types of changes: Antigenic Drift (a point mutation) and Antigenic Shift (when a hemmagglutinin or neuraminidase is completely different from the one before)

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Epidemics vs Pandemics

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Epidemics are caused by Antigenic Drift (a point mutation). Pandemics are caused by Antigenic Shift (when a hemmagglutinin or neuraminidase is completely different from the one before).

Shifts are more dangerous than drifts.

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Properties of a Virus (6)

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1. Viruses are filterable agents
2. Viruses are obligate intracellular parasites
3. Viruses cannot make energy or proteins independent of the host cell
4. Viral genomes may be RNA or DNA but not both
5. Viruses have a naked capsule or envelope morphology
6. Viral components are assembled and do not replicate by "division:

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Consequences of Viral Properties (5)

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1. Viruses are not living
2. Viruses must be infectious to endure in nature
3. Viruses must be able to use host cell processes to produce their components (viral mRNA, protein, and identical copies of the genome. DNA viruses need a DNA polymerase to replicate. RNA viruses need an RNA dependent RNA polymerase, which we do not have in our cells so they either need to bring it in or encode for it)>
4. Viruses must encode for any required processes not provided by the cell.
5. Viral components must self-assemble

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Morphologies of Viruses

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1. Naked Icosahedral- has cubic symmetry, consists of 20 faces, 12 vertices, and 30 edges.

2. Envelope Icosahedral- HIV and herpes; has glycoproteins on the surface.

3. Naked Helical- no mammalian virus are naked helical****** Only plant viruses are.

4. Enveloped Helical

5. Complex Viruses

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Naked Capsid vs Enveloped Virus: Components

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Naked- protein and nucleic acid.

Enveloped- membrane, lipids, proteins, glycoproteins (when virus buds off it buds off in an area of membrane modified by the virus and containing glycoproteins).

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Naked Capsid vs Enveloped Virus: Environmental Properties

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Naked- environmentally stable to temperature, acid, protease, detergents, and drying.

Enveloped- environmentally labile- if you affect the envelope the virus is not infective anymore. Disrupted by acid, detergents, drying, and heat. Modifies cell membrane during replication.

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Naked Capsid vs Enveloped Virus: Virus Release

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Naked- is released from the cell by lysis.

Enveloped- is released by budding and cell lysis.

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Naked Capsid vs Enveloped Virus: Spread of the Virus

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Naked- can be spread easily (on formities, from hand to hand, by dust, by small droplets)

Enveloped- spreads in large droplets, secretions, organ transplants, blood transfusions. Does not need to kill the cell to spread.

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Naked Capsid vs Enveloped Virus: GI tract properties

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Naked-can survive adverse conditions of the gut

Enveloped- cannot survive the GI tract.

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Naked Capsid vs Enveloped Virus: Survival Properties

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Naked- can dry out and still remain infectious; can be resistant to detergents and poor sewage treatment.

Enveloped- must stay wet to remain infectious; if the envelope of the virus is altered it will no longer be infectious.

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Naked Capsid vs Enveloped Virus: Immune System Properties

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Naked- antibody may be sufficient for immunoprotection.

Enveloped- may need antibody and cell mediated immune response for protection and control; elicits hypersensitivity and inflammation to cause immunopathogenesis.

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Class I RNA Viruses

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ssRNA with messenger sense (positive polarity)- meaning this is functionally mRNA and can be directly translated into proteins.
-this is the only class of RNA viruses that has an infectious genome.
-it is also the only one that doesn't have transcriptase in their visions.

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Class II RNA Viruses

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ssRNA with anti-sense (or minus) polarity; cannot be infectious or make proteins

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Class III RNA Viruses

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antisense segmented

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Class IV RNA Viruses

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dsRNA positive sense and non-enveloped (Reovirus); segmented genome

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Class V RNA Viruses

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positive sense ssRNA (Retrovirus is a major example); must get transcribed into ssDNA --> dsDNA which is INTEGRATED in order to make gene products. This class of virus has a diploid genome with two identical 35S RNA units-dimer. Keep in mind the DNA step involved in replication.

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Viral Cytopathic Effect

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morphological change in host cell caused by viral infection

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Hemadsorption

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the unique viral ability to absorb red blood cells

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Latent Infection

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virus remains dormant and non-replicating for long period of time.

Replication may be triggered by reoccurrence of the viral infection, going from a latent infection to a primary infection type.

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Abortive Infection

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virus begins to infect the his cell but is somehow inhibited from completing infection and replicating.

Can become active later via continued exposure to virus or a change in host cell environment

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Steps in Viral Replication

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1. Recognition of the target cell
2. Attachment
3. Penetration
4. Uncoating
5. Macromolecular Synthesis
6. Assembly of Virus
7. Budding of enveloped viruses
8. Release of Virus

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Virion Production Phases

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Eclipse Phase- period of time from the 1st attachment to the appearance of INTRACELLULAR virus.

Latent Phase- includes period of time from 1st attachment to the appearance of EXTRACELLULAR virus (longer).

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Viral Attachment

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viruses contain viral attachment proteins (VAP) that line up to cell receptors. The matching of viral attachment proteins with viral receptors on cells is a common way for viruses to enter cells.

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Properties of RNA Viruses

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DNA viruses are more stable.

Retroviruses and influenza replicate in nucleus.

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Properties of DNA Viruses

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DNA is not transient or labile
-viral genomes remain in the infected cell
-many DNA viruses establish persistent infections* (permanent)

DNA genomes reside in the nucleus (except for pox viruses)

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Viral Pathogenesis

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depends on virus steps and the site of entry.

You get attachment, penetration, and replication at the local sites. If it enters the blood (systemic) or lymph nodes, or nerve you can get different forms of infection.

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Transmission Mechanisms

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can be horizontal or vertical- these are different mechanisms of how various transmissions occur with the infection of a virus and the effect upon the cell- whether its lytic, abortive, or transforming.