General Virology (Dustin) Flashcards
General description of viruses
- Infectious genetic information: either DNA or RNA
- Obligate intracellular parasite: independently cannot reproduce, make energy, or synthesize proteins
- 20-300nm
5 structural aspects of viruses
- Capsid: protein shell enclosing the nucleic acid genome
- Capsomeres: outer covering proteins on the capsid
- Nucleocapsid: protein-nucleic acid complex representing packaged form of viral genome
- Viral nucleic acids
- (optional) Envelope: host lipids, carbs + viral peptides (peplomers). Much more protective.
How are nucleocapsids categorized?
By symmetry:
- Helical: proteins bound periodically to viral nucleic acid, winding it into a helix. Flexible or rigid.
- Cubical: icosahedral pattern. Capsomeres arranged in 20 triangle faces.
- Complex: pox virus
6 properties of viruses used for classification
- RNA or DNA
- Single vs Double-Stranded
- Circular or Linear nucleic acids
- Capsid symmetry
- Enveloped or not
- Size (nm)
5 features of viral nucleic acids
- RNA or DNA
- SS or DS
- Circular or Linear
- Segmented or not
- Usually only single copy of genome (haploid) except retrovirus are diploid
What is a viroid?
Smallest infectious agent of virus. Naked, closed circles of SS RNA, 300-400 nucleotides. Form diseases in plants but not known in humans
What is a prion?
Infectious protein particle, capable of self-reproduction. 50-100 kDa.
Examples: scrapie, CJD, BSE, Kuru
How do viruses propagate / what are the steps?
- Attachment: with specific receptor on host cell (specific to cells of various species or tissues)
- Penetration: receptor-mediated endocytosis, membrane fusion (if enveloped), or bacteriophage mode (attach tail and use lysozyme to degrade cell wall)
- Uncoating: viral nucleic acid freed
- Eclipse period: intense synthetic activity
- Assembly: viral genomes and capsids are put together
- Release: either by rupture (non-enveloped) or budding (enveloped, cell may survive)
Molecular bases of the biosynthesis of viruses: productive infection
Need mRNA to make structural proteins for (new) progeny viruses.
- RNA viruses: positive strand ones work as mRNA and synth proteins right away, neg strand ones must first be converted to pos RNA by RNA-dependent RNA polymerase
- DNA viruses: DNA transcribed to mRNA
- Retrovirus: viral RNA -> DNA by reverse transcriptase, then from DNA -> mRNA
Latent and persistent viral infections
Risks of latency?
Persistent: virus remains for a long period of time either intact or in the genome
Latent: after initial infection, virus is persistent by laying dormant (“lysogenic viral life cycle”). Infection can recur at some point. Different than chronic infection (virus is constant problem). Latency is typical feature of herpes virus family.
Besides recurring, latent viruses can cause uncontrolled cell proliferation / neoplasia (HPV). Latency in some cells infected with HIV is important as it maintains a reservoir that it is not yet possible to eliminate.
Congenital viral infections: 6 important examples
- Rubella: classic triad of cataracts, heart defect, sensory-neural deafness
- CMV: most common. Often causes mental retardation and various other CNS or organ problems
- Varicella-Zoster: rare but great risk of CNS and eye defects
- Neonatal Herpes Simplex: HSV1 or 2, infection during delivery. Can become disseminated.
- HIV: somewhat low chance of transmission but severe consequences
- Parvovirus B19: often causes abortion
Two classes of tumor viruses and most important examples of viruses that cause tumors:
Common function:
- DNA viruses: HPV 16 and 18, EBV, HBV, HHV-8
- RNA viruses (retroviruses with ss positive-sense RNA): Human T-lymphotropic virus, HCV
Both types integrate into host genome
What is the difference between acute and slowly transforming tumor viruses?
Acute: overactive oncogene (v-onc) that alters the infected cell as soon as v-onc is expressed.
Chronic: not v-onc, but cause transformation long after infection by insertional mutagenesis. Usually cause leukemia
Some specifics on genes/proteins by which these types of viruses can cause neoplasia, and what types of neoplasia they cause:
- HPV
- EBV
- HPV: encode proteins that bind/inactivate cell growth regulatory proteins (e.g. p53, RB). HPV 16 and 18 cause cervical carcinoma.
- EBV: stimulates cell growth (B-mitogen) and inducing Bcl-2 oncogene, preventing apoptosis. Has c-myc oncogene, causing t(8:14). Causes Burkitt lymphoma, Nasopharyngeal carcinoma, Hodgkin lymphoma
3 broad mechanisms of any viral tumor formation
the same as all tumor formation, answer is easy but just putting it for completeness sake
- Activation of oncogenes: provide stimulating growth gene
- Elimination of tumor suppressor genes / proteins that limit cell growth
- Inhibiting apoptosis / blocking apoptotic signals