Virology Flashcards
What are the characteristics of viruses?
Filterable agents
Obligate intracellular parasites
Genomes may be RNA or DNA but not both
Have a naked capsid or an envelope morphology
Reproduce by component assemblies not by division
What are the characteristics of the virion structure?
Nucleocapsid is a nuclei acid inside a protein coat (capsid)
Nucleocapsid might be surrounded by membrane(envelope)
Viral size correlates to genome size and virion complexity
The genome is DNA or RNA, single stranded, double stranded, linear or circular
RNA genomes can be sense, anti-sense or ambisense
RNA genomes can even be segmented
What is a capsid?
A rigid structure which helps withstand harsh conditions
Assembled from individual proteins
Individual proteins associate progressively to larger units
Individual proteins associate into subunits
Subunits->protomers->capsomeres->pro-capsid or capsid
Can form around genome or as an empty shell (pro-capsid)
May contain enzymes needed for replication
May have attachment proteins that facilitate infection
Have symmetrical structures (helical or icosahedral)
What are the characteristics of the envelope?
One or more lipid bilayer,typically only one
Exception is poxvirus which has 2 layers
Lipid bilayer are taken from host cellular membrane
Envelope is added as the nucleocapsid buds out
Susceptible to drying acids, bile detergents and lipids
What are the characteristics of an enveloped virus?
Does not survive the gastrointestinal tract
Transmission require close personal or sexual contacts or organ transplants and blood transfusions
Spread in large droplets, secretions
CMI response is needed for control
Elicits hypersensitivity to cause immunopathogenesis
What are the characteristics of a naked virus?
Released after lysis of the infected cell
Stable (temperature, drying, detergents)
Resistant to poor sewage treatment, GI conditions
Spread easily through fomites, hand to hand, dust, small droplets
Antibody may be sufficient for immuno protection
What is the general aspect of viral propagation ?
Steps are similar for all type of viruses
The susceptible host cell serves as a factory and provides substrates, energy and biosynthetic machinery
Virus must encode components not available in host
Viral replication has early and late phases
Uncoating in host abolishes virus identifiable structure
Has an eclipse period which is the period from uncoating to the synthesis of new virions when the capsid is removed and loses its identity until the new particles are produced
What are the key steps in viral propagation?
Recognition of target cells Attachment Penetration Uncoating Macromolecular synthesis Assembly Release
What are the characteristics of recognition and attachment?
Entry via respiratory droplets (rhinovirus), food or water (HAV)
may be direct transfer (HIV) or arthropod (West Nile)
Host receptors and virus attachment proteins (VAPs) determined which cell is infected
Receptors and VAPs interaction induces internalization
Naked viruses enter by receptor mediated endocytosis or viropexis
Viropexis is when viral hydrophobic proteins help viral genome slip in
Enveloped virus fuse with cell membrane to deliver nucleocapsid
What are the characteristics of uncoating?
Occurs in cytoplasm
Nucleic acid genome released after digestion of capsid by cellular proteolytic enzymes
Genome of DNA virus must be delivered to the nucleus
Exception is the poxvirus
Poxvirus too big to replicate in the nucleus
What are the characteristics of the macromolecular synthesis?
Virus must produce all proteins and replicate its genome once inside
Most DNA viruses use cell’s RNA polymerase to make mRNA
Poxvirus encode all enzymes necessary for genome replication and transcription since unable to enter nucleus
RNA viruses as well
Host cells can’t do replication/transcription in cytoplasm
mRNA for nonstructural proteins is transcribed first and constitue the early genes
Only few copies of early genes are required
Transcription of late genes which are structural and other viral proteins is trigger by genome replication
What are the characteristics of DNA viruses?
All DNA viruses replicate in the nucleus except poxvirus
Small DNA viruses (parvovirus) use host DNA polymerase and replicate only in growing cells
Larger and complex viruses encode their own viral polymerase which are faster but error prone
Large viruses can replicate in growing and non growing cells
Nucleotide analogs serve as antiviral drugs
Limiting rate of replication is the DNA, RNA polymerase and dNTP pool
What are the characteristics of RNA viruses?
Can be either a +RNA (direct mRNA) ready to be translated or -RNA (mRNA template) which has to be transcribed first as mRNA before transcription
dsRNA in a virally infected cell is an inducer of innate responses
Replicate in cytoplasm except orthomyxo and retrovirus
Must provide RNA polymerase that are absent in host cytoplasm
Retroviruses carry a reverse transcriptase (RT)
RT helps synthesize circular cDNA in the cytoplasm
cDNA travels to the nucleus and integrated into host chromatin
Viral genome becomes part of cellular genome
What are the characteristics of viral assembly?
It starts once the necessary pieces are made
Sites of assembly is dependent on site of genome replication and if it is naked or enveloped virus
Most DNA viruses except poxvirus assemble in the nucleus
Virion proteins are shipped into the nucleus for assembly of DNA viruses
RNA viruses and poxvirus assembled in the cytoplasm
Enveloped viruses need membrane and glycoproteins
Viral glycoproteins are delivered to cellular membranes
Viruses acquire region of cell membrane containing GPs
What are the characteristics of virion release?
Released is done after cell lysis for naked virus or by budding for enveloped virus
Infected cell is not lyse during release of enveloped virus
Release of enveloped viruses is slow and continuous
