Viruses Flashcards
Virus Classification is based on (3 factors)
Host cell (what they infect), Genome type (what they contain), Virion Structure (shape)
Adenovirus structure
Icosahedral, dsDNA, naked
Influenza virus structure
Helical, segmented RNA, enveloped
Herpesvirus structure
Icosahedral, dsDNA, enveloped
Poxvirus structure
complex, dsDNA, enveloped
Retrovirus (HIV) structure
icosahedral, (+) ssRNA (2 copies), enveloped
Define Virus
capsid encoding organism, obligate intracellular parasites, associated with all life forms. Viruses can only replicate in cells and cannot be cultured and isolated on agar plates or nutrient media
Capsid
protein shell that encapsulates the nucleic acid genome
Virion
particle encoded by a virus genome. Genome, Capsid and sometimes an envelop (optional) make up a virion.
3 basic gene modules of a virus
- capsid proteins, 2. replicon which encodes polymerase and proteins to copy genome, 3. multifunctional proteins that interact with the host (host interacting factors).
Virus strategy for survival (3 parts)
- genome encodes capsid to protect it outside cell, 2. genome has info for infecting cell, replicate genome, survive in cell, assemble virions, 3. transmission to new host
What happens to virions outside host cell
Virions are inert and lose infectivity over time
Requirements for Virus Replication (Name 6)
right host (tropism), cells with right receptors (susceptible), appropriate intracellular environment (permissive), biosynthesis machinery, abundant building blocks (nucleotides, amino acids, ATP, etc), and time to finish replication
Steps in Virus Replication
- Recognition of the target cell, 2. Attachment, 3. Entry: Penetration or Fusion, 4. Uncoating, 5. Transcription of mRNA, 6. Protein Synthesis, 7. Replication of the genome, 8. Assembly of virions, 9. Egress: lysis, budding, exocytosis
Single Step Virus Growth Curve
Eclipse: no virus is recovered during the replication and assembly phase, Maturation and release: virus particles are made and can infect other cells
Burst size
number of infectious viral progeny from a single round of replication
Virus vs Virion
Virus is living, Virion is inert (not alive)
Replicon
enzymes that are needed to copy virus genome
Virion Components
Genome segments, nucleocapsid, capsid/core, tegument, matrix, envelope, glycoprotein, spike/fiber
Virion Structure (shape - name 3)
Helical, Icosahedral, Complex (neither)
RNA-dependent RNA polymerase (RDRP)
low fidelity (no proofreading), highly efficient enzyme encoded all RNA viruses (except retroviruses. they encode reverse transcriptase) to copy their RNA genome into mRNA. does its job in the cytoplasm with replication occurring on cell membranes.
(-)RNA and dsRNA viruses must package RDRP in virion whereas (+)RNA viruses may or may not package RDRP in the virion.
(+) strand for RNA
sense (coding) strand, can directly make mRNA
(-) strand for RNA
antisense (template) strand, template for mRNA. Needs RDRP to make mRNA.
RNA genome types
dsRNA, (+) ssRNA, (-) ssRNA
Common features of RNA viruses
RNA is the genetic material and template for protein synthesis, dual purpose of replication is to copy genome and make mRNA, diverse strategies have evolved to accomplish these dual goals.
Alternative methods of RNA virus genetic diversity
- Rapid evolution by recombination, 2. Re-assortment of genome segments
Poliovirus
enterovirus (affects gut) family, (+) ssRNA genome, linear mRNA molecule, infects GI epithelial cells, fecal-oral transmission, gets around through sanitation problems particularly in water, infects only humans
muscle weakness could lead to paralysis
Rotavirus
reovirus, dsRNA, segmented, naked, icosahedron
Influenza Virus
orthomyxovirus, (-) ssRNA, segmented, enveloped
Human Immunodeficiency Virus
retrovirus, (+) ssRNA, 2 copies per capsi, enveloped
Rotavirus life cycle
- Genome is segmented, one gene each, 2. RDRP in the virion first transcribes mRNA, 3. After viral proteins are translated, new virions and genome segments are synthesized in the cytoplasm, 4. Virions assemble and then bud into the rough ER, 5. Egress is via exocytosis (membrane vesicles carry virions out) or by cell lysis, 6. Virions mature in gut lumen, then infect more enterocytes or are shed in profuse diarrhea
Influenza virus life cycle
- Genome is segmented, (-) ssRNA, 2. Genome segments traffic to the nucleus for transcription and replication by RDRP, 3. Viral proteins and genome segments accumulate at the plasma membrane, 4. Virions assemble and egress by budding, 5. Neuraminidase (N antigen) releases virions from sialic acid on cell surface, 6. Virions are shed in respiratory droplets (coughs and sneezes)
HIV life cycle
- After the virion fuses with the plasma membrane, the Reverse Transcriptase enzyme (RT, included in the virion) converts the (+) ssRNA genomes into dsDNA, 2. The dsDNA genomes integrate into the host chromosome for life, 3. Host RNA Pol II transcribes mRNA from the integrated genome, which also serves as the genome that is packaged into new virions, 4. Viral proteins and 2 genomes bud from the plasma membrane, 5. Virion maturation occurs outside the cell when the viral protease cleaves the capsid proteins, forming the final trapezoidal shape
Rotavirus clinical features
Causes severe gastroenteritis: Profuse watery diarrhea, Dehydration, Malabsorption, Affects infants and children (adults are usually asymptomatic), >600,000 deaths annually, mostly in developing world, Peak incidence during winter
Influenza clinical features
Acute respiratory illness, mainly during the winter
“Uncomplicated” - upper and/or lower respiratory tract involvement with fever, headache, myalgia, and weakness
“Complicated” - pneumonia caused by influenza, bacterial pneumonia, Mixed viral and bacterial pneumonia, Muscle involvement: myositis (pain) and rhabdomyelitis (breakdown)
HIV clinical features
- P. carinii pneumonia, 2. Esophageal candidiasis, 3. Wasting, 4. Kaposi’s sarcoma, 5. Disseminated M. avium infection, 6. Tuberculosis, 7. Cytomegalovirus disease, 8. HIV-associated dementia, 9. Recurrent bacterial pneumonia, 10. Toxoplasmosis
Issues with (+) RNA
collisions can occur between RDRP and ribosomes. Translation occurs first when RDRP is scarce. (-) RDRP synthesis occurs later when RDRP is abundant.
Herpes simplex type 1
Primary infection is mouth or eye sores, reactivation is cold sores, treatment is acyclovir
Herpes simplex type 2
Primary infection is genital sores, reactivation is genital sores, treatment is acyclovir
Varicella-zoster virus
Primary infection is chicken pox, reactivation is shingles (herpes zoster), treatment is acyclovir, vaccines, foscarnet
highly contagious aerosol transmission, latent in the dorsal root ganglia, appears as a distinctive rash, complications include inflammation of liver, brain, lung tissue, and bacterial infection of lesions
Epstein-Barr virus
Primary infection is mononucleosis, reactivation is lymphoma, treatment is oncotheraphy
transmitted by saliva, results in fever, fatigue, sore throat, and swollen lymph glands. no antivirals or vaccines available
Cytomegalovirus
Primary infection is like mononucleosis, reactivation is systemic disease, treatment is ganciclovir (antiviral), no vaccines available
fever, lassitude (lack of energy), lymphadenopathy (diseased lymph nodes), rash, no sore throat, can result in congenital CMV disease which can cause hearing loss
Human herpesvirus 6 or 7
Primary infection is roseola, reactivation is systemic disease, no treatment available
transmitted in saliva, fever followed by rash, no treatment or vaccines available
