Viral Structure and Function Flashcards
Basic properties of viruses
- obligate intracellular bacteria
- not alive; do not undergo division
- virus replicates and reassembles w/in a host cell
Viral strategies for survival
- house DNA or RNA genomes in small proteinaceous particles (capsids)
- genome contains info to carry out infectious cycle
- establish relationships w/population of hosts from benign to lethal
Major classification systems of viruses
- classical system: viruses grouped according to shared physical properties
- baltimore system: based on the central dogma (based on how they produce mRNA)
Criteria for classification via classical system
- nature of genetic material (DNA vs. RNA)
- symmetry fo the capsid (helical vs. icosahedral)
- naked vs. enveloped
- dimensions of the virion and capsid
Methods for studying viruses (5)
- electron microscopy
- animal models
- sequence analysis
- cell culture
- serology
Key principles of viral genomes (2)
- genomes are templates for synthesis of progeny genomes; thus there is finite number of nucleic acid copying strategies
- viral genome goal inside host is to make mRNA (that will be translated by host protein synthesis machinery); genomes must provide mechanisms for synthesis of mRNA
Classes of viral genome configuration (7)
- dsDNA
- gapped circular dsDNA
- ssDNA
- dsRNA
- ss(+)RNA
- ss(-)RNA
- ss(+)RNA w/DNA intermediate
Functions of virion proteins (3)
- protect genome: recognize and package genome, assemble stable protein shell, interact w/ cell membrane if envelope formation needed
- deliver genome: bind receptors, uncoat genome, induce fustion w/host cell membrane, direct transport of genome to appropriate site
- mediate interactions w/host
General characteristics and types of capsid proteins
- symmetrical arrangement of identical/highly similar proteins
- NON-COVALENT bonds (allows for stable assembly and ready reversal during genome delivery)
- sub-units are self-assembled by engaging in “identical” bonding contacts w/neighbors
- helical capsids
- icosahedral capsides
Helical capsid properties
- multiple identical subunits arranged using rotational symmetry
- irregularly shaped proteins arranged around a circumference of a circle to form a disk
Icosahedral capsid properties
- hollow, quasi-spherical structure
- 20 traingular faces w/12 vertices
- faces w/2-,3-, and 5-fold symmetry
Properties of enveloped viruses
- envelope=lipid bilayers acquired during assembly of viral particles; usually have viral glycoproteins w/in membrane
- usually acquired through budding through a membrane of host
- don’t necessarily kill host cells in the course of replication
General stages of “one-step growth curve” experiment
- every cultured cell is infected w/virus
- eclipse period (~0-12hrs): virus particles have broken down after penetrating cells; no detectable virus
- latent period: time between infection and release of new infections virus particles from the cell
Processes taking place during latent period
- attachment of virus to cell: virus-attachment protein binds to receptor
- entry of virus into cell; uncoating of genome: energy-dependent, endocytosis, fusion (enveloped viruses)
- viral gene expression
- genome replication
- assembly of new viruses
- egress of new virus particles from cell
DNA virus gene expression strategy
- must transcribe mRNA using (-) strand of the DNA genome
- uses host RNA pol II to fill gaps/replicate DNA and produce mRNA
RNA virus gene expression strategies
-use viral enzyme RNA-dependent RNA pol (RdRp) for production of mRNA and replication of RNA genomes
(+)RNA viruses gene expression strategy
genome may be directly translated by host ribosomes; RdRp is coded by and then replicates genome
(-) RNA viruses gene expression strategy
- must transcribe (+)sense mRNA to express genes
- package RdRp w/in virus particle
Retrovirus ((+)RNA w/DNA intermediate) gene expression strategy
- copy ssRNA into dsDNA via virus particle reverse transcriptase
- dsDNA incorporated into host genome
- host RNA pol II transcribes host genome into mRNA
Viral genome replication strategies
- dsDNA: nuclear or cytoplasm
- ssDNA: nucleus. ds intermediate=template for ssDNA synthesis
- gapped circular dsDNA=viral reverse transcriptase to take viral mRNAs –> DNA
- RNA: RdRp. genome replicated using antigenome as a template
Viral assembly strategies
- icosahedral:
a. capsid assemble around genome
b. genome fed into preformed capsids - helical: genome coated w/capsid protein during synthesis
Viral egress strategies
- naked: lysis
- enveloped: budding through a. cytoplasm –> extracellular environment or b. cytoplasm –> golgi/ER –> secreted from cell
