intro to viruses Flashcards
What is a virus
- small infectious agent that replicates by infecting the cells of a host organism
- 20-300 nm
- composed of nucleic acid (DNA or RNA) surrounded by a protein coat
- most viruses can only infect specific cell types (tropism)
- cant replicate outside cells
what are the main virus shapes
- icosahedral
- complex
- helical
what is the mature virus particle composed of
- nucleic acid (gene)
- surrounded by a protein coat (capsid)
- surface of the virus may have embedded proteins which help the virus to enter cells
- some viruses have lipid envelopes
describe the composition of enveloped viruses
- derive envelope from cell membranes by budding
- plasma membrane
- nuclear membrane
- internal organelles
eveloped viruses are typically less stable than non-enveloped (to heat, pH extremes, detergents)
describe the virus genome
- either RNA or DNA
- single strand or double strand
- circular, linear or segmented DNA
- if single stranded can be positive or negative sense (positive sense sequence can code directly for virus protein, negative sense reverse complement to positive sense, does not typically code for protein
what are the functions of virus proteins
**structural proteins encode: **
- capsid
- nucleocapsid
-envelope glycoproteins
non-structural proteins
- mediate replication of genome (i.e. polymerases)
- may have role in suppressing the immune system or changing the cellular environment
Accessory proteins
- have special functions, usually in restricted cell types
list the stages of the simple virus replication cycle
- viral entry
- uncoating
- transcription
- translation
- viral assembly and release
what do viruses need to replicate
viruses must infect cells which support viral replication in order to multiply.
- must be able to enter cells (i.e. have appropriate receptors
- host cell physiology must be appropriate (supplies replicative enzymes, ATP, nucleotides, other proteins)
describe how polymerase enzymes work as a viral replication strategy
- replicate viral genetic material to produce mRNA (protein) and genomic nucleic acid (provided by the cell or virus)
if suitable polymerase not provided by cell OR not provided in compartment in which the virus replicates, virus must supply polymerase - most DNA viruses replicate in the nucleus
- most RNA viruses replicate in the cytoplasm
How do double stranded DNA viruses replicate?
- encode own DNA polymerase so can control replication of genome independent of cellular DNA replication
- Transcription of DNA viruses by cellular (DNA dependant) RNA polymerase
How do single stranded DNA viruses replicate?
- ssDNA viruses require active host DNA polymerase to replicate genome i.e. produce dsDNA (these viruses can productively infect only dividing cells where DNA polymerase is active)
- then use host RNA polymerases to produce mRNA
how do retroviruses work
- viral RNA is reverse transcribed
- the cDNA integrates into into the chromosome of the host cell by viral integrase enzyme
- the integrated DNA then produces RNA and protein using normal cell machinery
integration into host genome is permanent (for life of cell)
can disrupt normal genome activity and lead to tumour formation
list the different disease outcomes of viral infection
clinical disease
- cell death and tissue damage (+/- secondary bacterial infection)
- host response to infection
- cancer
sub-clinical infection
- transmission to susceptible animals
- undetected effects (fertility problems)
- latent viruses
can cause problems when trying to control disease in a population
what factors affect the clinical outcome of viral infection
- virus strain
- virus load
- genetics (immune system genes, etc.)
- age
- pre-existing immunity
- physiological status
- concurrent infection
- overcrowding
- ventilation
describe the mechanisms behind how viruses change their genetics
- spontaneous mutation
- mistakes made during genome replication
- RNA viruses = higher rate of mutation since enzymes cannot proofread
- very high replication rate of some RNA viruses means very large numbers of mutant viruses - gene transfer between viruses (or cell)
- transfer of genetic information between two, usuallt related, viruses which jave infected the same cell
- production of hybrid genomes
- recombination (relatively rare, occurs between viruses with related DNA/RNA sequence or between virus and host nucleic acid which can lead to viruses acquiring host sequences) or reassortment (can occur for viruses with a segmented genome simple exchange of genes can occur when two different viruses infect a cell leading to rapid evolution of virus variants)
