Introduction To Virology

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Characteristics of Viruses

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-"Filterable agents" when bacteria can't be
-Not technically "living"- don't grow/not active outside a host
-Obligate, intracellular parasites
-Viral components must self-assemble
-Infect every living organism
-Tiny (size reflects the complexity of the organism)
-Genomic material (DNA or RNA) + protein

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Viral Classification

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CAN NOT be classified based on genetic relatedness
highest classification= order/family
Names chosen based on: characteristics, diseases it's associated with, places where it was first isolated

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Classification (Taxonomy)

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Order (Virales)
Family (Viridae)
Subfamily (virinae)
Genus (virus)
Species

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Viral Structure-Virion

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virion= the virus particle
-nucleic acid genome
-packed into a capsid (protein coat) that CAN be surrounded by a membrane (envelope)
-the envelope is the package/protection/delivery vehicle during transmission; mediates attachment via VAPs (viral attachment proteins)
-Capsid/Nucleic acid binding proteins may associate w/ genome to form nucleocapsid

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Viral Structure- Genome

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RNA or DNA.

Front 6

RNA Genome

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Positive Sense- serves as mRNA b/c it is immediately translatable by host cell ribosome (infectious virus!)

Negative Sense- must be transcribed to mRNA before translation; must carry RNA polymerase with them

-LACKS proofreading capability; mutation rate much higher

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Viral Structure- Capsid

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-Rigid structure that can protect from harsh environment

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Viral Structure- Envelope

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-Membrane w/ lipids, proteins, glycoproteins
-Acquired via budding thru host membrane
-Readily disrupted by drying,acid conditions, detergents
-Need to remain wet
-Transmission: fluids, respiratory droplets, blood/tissues

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Capsid (Naked) Viruses

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-usually resistant to drying, acids/detergents
-easily pass fecal-orally
-the capsid is assembled from individual proteins
-repetitive structure serves as a PAMP
-helical/icosahedral- most energy efficient shape
-release from cell via lysis
-capsid protects genome and contains VAP

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Enveloped Viruses

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-Contain few host proteins
-Round/pleomorphic shape
-Capsid lies under envelope
-Viral glycoproteins can serves as VAPs (e.g.. hemagglutinins which bind to RBCs)
-ALL negative sense RNA is ENVELOPED

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Viral Replication

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ALL viruses have same major steps:
-Recognition of target cell
-attachment
-penetration
-uncoating
-macromolecular synthesis (nonstructural proteins first, replication of genome, late mRNA/structural protein synthesis, posttranslational modification)
-assembly of virus
-budding (if encapsulated)
-release

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Viral Replication (2)

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ANY Process NOT provided by host cell must be coded for by virus

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Viral Replication Phases- Early

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Recognition of target, attachment, penetration, release of genome (uncoating), early macromolecule synthesis

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Viral Replication Phase- Late

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Start of genome replication --> viral assembly/release

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Eclipse Period

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-begins in early phase; INTACT virus has NOT assembled
-ends when NEW virus has been assembled

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Latent Period

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includes eclipse period- and ends w/ release of new viruses. extracellular infectious virus not detected during this time

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Burst Size

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Yield of infectious virus per cell

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Viral Replication- Recognition and Attachment to Host

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1. Host's receptors for the virus can be proteins or carbs
2. This dictates the host range and tissue tropism of virus

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Viral Replication- Penetration

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VAP/host cell interaction initiate internalization of vision
1. Naked viruses enter via endocytosis
2. Enveloped viruses- fuse membranes w/ host cell

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Viropexis

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DIRECT penetration of membrane by a virus

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Viral Replication- Uncoating

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a variety of methods are used to release the genome to cytoplasm/nucleus

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Viral Replication-Macromolecular Synthesis

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Synthesis of viral mRNA/protein and the generation of identical copies of the virus

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DNA Viral Macromolecular Synthesis

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USe host cell machinery and replicate in the nucleus

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RNA Viral Macromolecular Synthesis

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Replicated in the cytoplasm; mRNA coded by RNA viruses may not contain cap or polyA tail

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Infectious Virus

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A virus whose naked genome can initiate viral replication upon entry into cell (DNA viruses and +-sense RNA viruses)

Front 26

Properties of DNA Viruses

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-Many DNA viruses establish persistent infections
-Replication happens in nucleus
-Complex viruses can code for own transcriptional activators
-genes can overlap, have introns
-replication of viral DNA requires a primer
-some larger DNA viruses can code for their own polymerase
--limitations: availability of DNA polymerase/DNA substrates and viruses can develop mechanisms to speed up cell replication (ie. interfere w/ p53/RB gene product)

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RNA Virus Properties

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1. Genome is mRNA or template for mRNA
2. DS-RNA intermediate occurs at some point (can serve as a viral PAMP)
3. Genome must code for an RNA-Dep, RNA-pol
4. RNA degrades relatively quickly- so polymerases work FAST and are ERROR PRONE
5. +-sense RNA viruses are infectious upon cell entry
6. - sense RNA is a template for production of mRNA therefore must carry polymerase w/ them into genome

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SEE HANDWRITTEN NOTES FOR THE REST!!

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handwritten