Microbio 1: Properties Of Viruses

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First FDA approved antiviral

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Amantadine

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Size of Virus

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around 100nM

100x smaller than bacteria

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Definition of Virus

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Sub-microscopic entities consisting of a single nucleic acid surrounded by a protein coat and capable of replication only within the living cells of bacteria, animals or plants

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Capsid

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Protective protein coat that surrounds nucleic acid

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2 Phases of Life Cycle

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Extracellular phase (metabolically inert) and Intracellular phase (progeny production occurs)

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Lifestyle of Virus

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Obligate intracellular pathogens

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Minimum thins tatt a virus must make

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1. DNA or RNA genome
2. A polymerase that replicates the genome
3. One or more capsid proteins that form a core around the genome and provides protection (Capsid)

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Viruses may also produce

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1. A lipid Membrane
2. Virally encoded Spike proteins (attachement)
3. proteins that modulate the host in some way

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Enveloped Virus

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Surrounds self with membrane it steals from the cell

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Types of Viral Genomes

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DNA: Double stranded
Single stranded

RNA

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Implications of RNA genome

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1. Usually do not proof-read --> more mutated than DNA viruses
2. Replicate quickly, particualrly +RNA viruses, since genome is in effect mRNA

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Do RNA viruses cause chronic or acute infections

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Acute

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Do DNA viruses cause chronic or acute infections

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Chronic

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4 strategies for viruses to conserve genetic space

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1. Make polyprotein then cleave
2. RNA splicing
3. Overlapping reading frames (and splicing)
4. Ribosomal frame shifting (ribosome slip)
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A - or + sense RNA virus must carry its own polymerase?

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A - sense virus must carry its own RNA polymerase, since cells cant copy RNA's

+ sense viruses serve as mRNA so can encode a polymerase

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2 Main Capsid Shapes

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Icosahedral and Helical

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Capsid Function

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1. Packaging and Condensation
2. Protection of Nucleic acid
3. Transport Nucleic Acid
4. Provides Specificity for Attachment
5. Metastable- undergoes changes that result in delivery of genetic material into cell

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Do all or some helical viruses have membrane envelope

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ALL

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Do all or some icosahedral viruses have membrane envelope

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Some

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How do viruses acquire membrane

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Budding

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Spike Proteins

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A viral glycoprotein that is transported to the cell surface acts as a cellular membrane protein. Binds to viral receptors and mediate fusion b/w virus and cellular membrane

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Enveloped Viruses transmitted by...

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Bodily fluids

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Non-enveloped viruses transmitted by...

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fecal/oral, bodily fluids, and other......
are very stable in the environment.

Harder to kill.

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Steps in life cycle of virus

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1. Attachment
2. Penetration
3. Uncoating
4. Translation, Transcription, and replication
5. Assembly
6. Release

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Viral Tropism

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Ability of a virus to infect some cell types and not others. Dictated by presence of receptors.

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How do you measure the amount of virus in a solution or sample:

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Plaque forming units PFUs

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Eclipse Phase

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The period of time between the start of infection and when the first progeny viruses are released

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Roles of Viral Receptor

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1. Bind virus ot cell (capsid- non-envelopd... spike protein- enveloped)
2. Can acts to tether the virus
3. Can act to initiate entry
4. Can trigger conformation change in capsid
5. May send signals to cell

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3 ways to overcome barrier of cell membrane

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1. Membrane Fusion (enveloped)
2. Pore Formation (non-enveloped)
3. Membrane Lysis (non-enveloped)

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3 mechanisms for viral mutation

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Simple Mutation (RNA viruses higher error rate)
Recombination - Exchange of information between two genomes, Common in DNA viruses
Reassortmen- viruses with segmented genomes.

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How do viruses selectively package own DNA or RNA, and not cellular molecules

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Viral genomes have packaging signals that are recognized by capsid proteins

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Viral Release Mechanisms

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Buddding: Enveloped viruses
Lyse cell: Non-Enveloped viruses

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Burst Size

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Average number of viral particles released from an infected cell