Viral Structure and Function Flashcards
Define the basic properties of viruses
- Submicroscopic, obligate intracellular (molecular) parasites.
- Virus particles themselves are not alive and do not grow or undergo division.
- In an appropriate host cell, the genome is replicated and directs the synthesis of viral components that will be assembled to form progeny viruses.
- Particles are produced from self-assembly of newly-synthesized components within the host cell.
List the strategies viruses employ for survival
- DNA/ RNA genomes are in capsids (small proteinaceous particles)
- Genome contains all the information needed to start and finish an infectious cycle.
- They establish a relationship in a population of hosts that ranges from benign to lethal. However simple this is, the diversity of solutions for carrying it out is immense with variation in particle architecture; size, nature and topology of genomes; protein coding strategies; cell/tissue/host tropism and pathogenesis
Describe two means of classifying viruses
- The classical system: Viruses grouped according to shared physical properties.
• DNA vs. RNA
• Symmetry of the capsid (helical or icosahedral)
• Naked or enveloped
• Dimensions of the virion and capsid - Baltimore system: Categorizes viruses based on how they produce mRNA.
- Based on the Central Dogma: DNA → RNA → protein (*All viruses are parasites of the host mRNA translation machinery and therefore, must produce mRNA to decode their genomes.)
Describe basic methods for studying viruses
- electron microscopy
- animal models
- sequence analysis
- cell culture
- serology
- other molecular techniques
main structural characteristics of virus particles
- genetic material (DNA vs. RNA)
- capsids (symmetry - helical or icosahedral)
- envelopes
what does the capsule do?
protects the genomes and serve as specific nucleic acid genome recognition, packaging and delivery device
- delivery of genome
- mediates ineractions with the host
- assemble from components made during infection
forms of virus symmetry
- symmetrical arrangement of many identical/similar proteins to ensure maximal contact and non-covalent bonding between them
- highly conserved structural motifs, not necessarily the protein sequences, are found in capsid proteins
- helical or isohelical symmetry (possible due to organized self-assembly of the subunits)
- –each subunit non-covalently bonds with its identical neighbors to form identical bonds -> repetitive structure
- –between non-identical -> clumps/aggregates
- –helical capsules = rotational symmetry with identical subunits and irregularly shaped proteins arranged around a circumference -> disc
- –icosahedral capsules = genome surrounded by subunits arranged in a “hollow, quasi-spherical structure”
Enveloped vs. non-enveloped
- Enveloped
- –Envelopes add a layer of complexity to virus, lipid bilayers acquired during assembly of viral particles (by budding through membrane of host cell) and have viral glycoproteins (role: entry and host range determination, assembly and egress, evasion from vertebrate immune system) embedded in the membrane. Important part of mechanism by which virus escapes from infected cell. - Naked
- –Escape from infected cell via lysis (terminal)
seven basic virus genomes
- dsDNA
- gapped circular dsDNA
- ssDNA
- dsRNA
- ss(+) RNA
- ss(-) RNA
- ss(+) RNA with DNA intermediate
dsDNA: protein expression strategies, rep strategies
Protein expression strategies: must transcribe mRNA using the (-) strand of the DNA genome as a template (gaps must be filled BEFORE genes can be transcribed – host RNA polymerase II in the nucleus)
—exception: poxviruses replicate in the cytoplasm and encode their own RNA pol
Replication strategies: replicate in the nucleus using cellular factors [can use viral polymerases and accessory factors] (except poxvirus which have replication factors in their genomes)
gapped circular dsDNA: protein expression strategies, rep strategies
Protein expression strategies: must transcribe mRNA using the (-) strand of the DNA genome as a template (gaps must be filled BEFORE genes can be transcribed – host RNA polymerase II in the nucleus)
Replication strategies: replicate in the nucleus using cellular factors [can use viral polymerases polymerases and
ssDNA: protein expression strategies, rep strategies
Protein expression strategies: must transcribe mRNA using the (-) strand of the DNA genome as a template (gaps must be filled BEFORE genes can be transcribed – host RNA polymerase II in the nucleus)
Replication strategies: formation of a ds intermediate in the nucleus – use as template of genomic ssDNA
dsRNA: protein expression strategies, rep strategies
Protein expression strategies: + sense mRNA must be transcribed from genome in order to have gene expression. These viruses must have RdRp with them in cell (animal cells don’t have the ability to do this and genomes is not “ribosome ready”)
Replication strategies: Transcription and genome replication are integrated, use anti-genome as template, production of anti-genome and replicated genome is performed by RdRp.
ss(+) RNA: protein expression strategies, rep strategies
Protein expression strategies: **use RNA-dependent RNA polymerase (RdRp)
-cellular ribosomes directly translate and RdRp mediates amplification of mRNA copy number/ production of sub-genomic mRNAs
Replication strategies: **transcription and genome rep are highly integrated
-RdRp produces anti-genome which is used as the template for genome to replicate
ss(-) RNA: protein expression strategies, rep strategies
Protein expression strategies: **use RNA-dependent RNA polymerase (RdRp)
-package RdRp in the virus particle because the genome is not “ribosome-ready”
Replication strategies: **transcription and genome rep are highly integrated
-RdRp produces anti-genome which is used as the template for genome to replicate
