L22: Intro to Medical Virology Flashcards
(31 cards)
Characteristics of a virus
Acellular infectious agent with nucleic acid surrounded by protein coat; obligate parasite; 20-300 nm
Virion components
Simplest viruses just have capsid and genome, more complex have envelope (lipid layer with special proteins) with matrix between; genome can either be DNA or RNA
Segmented vs. contiguous viral genomes
Segmented need ALL of DNA to have complete genome
Types of viral genomes
ssDNA, dsDNA, ssRNA (- or + sense), dsRNA
(+)ssRNA
In correct orientation to direct cell synthesis; can generate template for ribosome
(-)ssRNA
In opposite orientation (inert in cells); use RNA as template
Properties of RNA viruses
CELLS CANNOT REPLICATE RNA –> need RNA polymerase; labile and transient, replicate in cytoplasm, must carry polymerases (except +RNA genome), prone to mutation
Capsid structures
- -Helical (slinky)
- -Icosahedral (regular, closed)
- -Complex (variation of icosahedral)
Naked capsid viruses
Environmentally stable to temperature, acid, proteases, detergents, drying; released by lysis; spread easy, retain infectivity even when dry
Envelope viruses
Environmentally labile, disrupted by acid, detergents, drying, heat; modifies cell membrane during replication and are released by budding; must stay wet (can’t survive GI tract), spread in droplets, don’t need to kill cell to spread
Virus life cycle
- Attachment
- Entry
- mRNA production
- Protein and genome synthesis
- Virion assembly
- Egress
Attachment
Attachment to membrane occurs with cell receptors, fusion to membrane, allowing capsid to enter (Ex. HIV)
Entry
Receptor engagement with viral proteins that leads to an enveloped virus, pH triggers degradation of envelope and capsid is released
How do cells produce mRNA (dependent on genome)?
dsDNA: can just use transcriptional machinery
ssDNA: use cellular DNA repair enzymes to produce dsDNA
+ssRNA: retroviruses use viral reverse transcriptase to make dsDNA, others use genome just like mRNA molecule
-ssRNA: use viral RdRp to make mRNA
dsRNA: use viral RdRp to make mRNA
How can cellular DNA replication machinery be made available at all times for DNA viruses?
- -Papovaviruses stimulate cell growth and synthesis to have cell make them
- -Poxviruses encode polymerase and enzymes
How can RNA viruses make cellular RdRp?
+ssRNA: encode RdRp then make enzymes
-ssRNA: encode RdRp and enzymes right away
Viral assembly
- Individual viral proteins form into capsid subunits, which combine to form complete capsid
- Viral genome and components are selectively packaged in capsid
- Virus exits cell
Budding (to leave cell)
Viral envelope proteins forming a patch on membrane, recruiting integument proteins, protruding, then releasing into extracellular environment
Viral genetics
Can change through:
- -Point mutations (slow)
- -Recombination (DNA viruses)
- -Reassortment (ex. influenza, two segments affect cell and reassort inside cells to lead to quick, large change)
Viral infection types
- -Acute
- -Chronic
- -Latent
Acute infection
Affected by virus, immune system kicks in and virus is eradicated
Chronic infection
Continuous lower level of virus but never disappears totally (6 months or more)
Latent infection
Looks like acute but can reactivate (cycle can continue for long time)
Invasion of viruses into body
Can occur through oral transmission, droplet transmission, direct inoculation, direct skin contact, sexual transmission, or transplacental
