viruses Flashcards
what are viruses
simple acellular infection pathogens
what are viruses incapable of (compared to bacteria)
self-replication
- host
metabolic activity
what can viruses infect
every category of life
how big are viruses
20-200nm
- viruses that infect humans
too small for light microscopes
how is gene information stored in viruses
either RNA or DNA
either linear or circular
RNA - +ve sense (mRNA) or -ve sense (convert to +ve first)
what is the nucleocapsid
genome + capsid
- may also contain essential or accessory enzyme to facilitate initial replication
naked/ envlopes
what is a virion
fully assembled infection extracellular particle
what is the capsid comprised of
capsomeres
- several different polypeptides grouped together
what shapes can the capsid be
distinctive symmetry
helical/ icosahedral
give an example of an icosahedral virus
Hep B
what characterises an enveloped virus
phospholipid bilayer
- derived from the host cell
Golgi apparatus
ER
nuclear membrane
what is the envelope of an enveloped virus comprised of
lipids, virus proteins and glycoproteins
what are the host cell membrane proteins replaced by in viruses
virally encoded glycoproteins
- spike proteins
what are the spike proteins on influenzas called
Hemagglutinin and Neuraminidase
where are viral attachment proteins found
alongside capsid in naked viruses
anchored in the phospholipid bilayer of enveloped viruses
what is viral tropism
ability of a given virus to productively infect a
particular cell - cellular tropism
tissue - tissue tropism
host species - host tropism
what are the modes of transmission for a virus
oral transmission
droplet transmission
direct inoculation
sexual transmission
trans-placental
direct skin contact
how are orally transmitted viruses spread
contaminated food + drink; saliva
give an example of an orally transmitted virues
Hep A/ norovirus
how are droplet transmitted viruses spread
inhalation
give an example of a droplet transmitted virus
influenza
coronavirus
measles
how do directly inoculated viruses spread
injections; trauma; insect bites
give an example of directly inoculated virus
yellow fever/ zika
give an example of a sexually transmitted virus
HIV
give an example of a virus spread by skin-skin contact
herpes
give an example of a single stranded +ve sense virus
ebola
what symptoms characterise a common cold
blocked or running nose, coughing and sneezing, headaches, sore throat
what symptoms characterise bronchoilitis
starts similarly to a cold but progresses to coughing/ wheezing/ difficulty breathing
what symptoms characterise croup
harsh brassy/ barking cough
give examples of respiratory illnesses
common cold, bronchiolitis, croup
give 3 examples of coronaviruses
SARs-CoV
MERS-CoV
SARS-CoV-2
what is a zoonose
a virus coming from an animal
list the types of influenza
A B C D
what types of influenza affects humans
A B
what are the symptoms of human influenza
fever, chills, headache, muscle pain, extreme fatigue, dry cough, sore throat, blocked nose
give three examples of human herpesviruses
measles
mumps
rubella
what type of virus is HIV
single stranded +ve sense RNA retrovirus
what CD4 count constitutes AIDS
220 cells/ mm3
list some common opportunistic infections
Epstein-Barr virus
Fungal infections - oral candidiasis, Pneumocystis pneumonia
HHV-8 virus (Kaposi’s sarcoma)
cytomegalovirus (CMV) retinitis
cryptosporidium
give two ways that viruses enter a host cell
macropinocytosis, membrane fusion
describe micropinocytosis
taches to the host cell it can become engulfed by the host cell membrane
describe membrane fusion
virus inserts itself into the host cell
what are the stages of viral replication
attachment
penetration
uncoating
replication
assembly
virion release - lysis/ budding
describe the attachment phase of viral replication
viral proteins on the capsid or phospholipid envelope interact with specific receptors on the host cellular surface
- specificity determines the host range (tropism) of a virus
describe the penetration stage of viral replication
the process of attachment to a specific receptor can induce conformational changes in viral capsid proteins (/ lipid envelope)
- results in the fusion of viral + CMs
some DNA viruses can also enter the host cell through receptor-mediated endocytosis
describe the uncoating stage of viral replication
the viral capsid is removed and degraded by viral enzymes or host enzymes, releasing the viral genomic nucleic acid
describe the replication stage of viral replication
after the viral genome has been uncoated, transcription or translation of the viral genome is initiated
- this stage differs greatly between DNA and RNA viruses + viruses with opposite polarity
- culminates in the de novo synthesis of viral proteins and genome
describe the assemebly stage of viral replication
after de novo synthesis of viral genome and proteins, which can be post-transcriptionally modified, viral proteins are packaged with newly replicated viral genome into new virions that are ready for release from the host cell
- also called maturation
describe the lysis approach to the virion release stage of viral replication
results in the death of an infected host cell
viruses that cause this are referred to as cytotoxic
give an example of a virus that uses lysis
variola major (smallpox)
describe the budding approach to the virion release stage in viral replication
typically enveloped viruses (e.g. influenza A)
acquisition of the viral phospholipid envelope
viruses that cause this are referred to as cytopathic
describe what can happen after virion release
some viral proteins remain within the host’s cell membrane
- acts as a potential target for circulating antibodies
- residual viral proteins that remain within the cytoplasms of a host cell can be processed and presented at CSM on MHC I
describe the RNA replication of dsRNA viruses
one strand is transcribed by viral polymerase into mRNA
describe the RNA replication of +ve sese ssRNA viruses
can be used directly as mRNA
describe the RNA replication of -ve sense ssRNA viruses
viral polymerase converts into +ve sense mRNA strand, which can then be used as mRNA
describe the RNA replication of retroviruses
-ve sense ssRNA -> -ve sense ssDNA (viral transcriptase enzyme) -> dsDNA -> enters nucleus -> integrated into host genome -> mRNA (host polymerase)
describe HIV replication
1- Nucleocapsids containing viral genome and enzymes enters the target cell
2- Viral reverse transcriptase catalyses reverse transcription of ssRNA to form RNA-DNA hybrids
3- To yield HIV dsDNA the viral RNA template is partially degraded by ribonuclease H and the second DNA strand is synthesized
4- Viral dsDNA is translocated into nucleus and integrated into the host genome by the viral integrase enzyme
5- Transcription factors transcribe the proviral DNA into genomic ssRNA
6- Which is exported to cytoplasm
7- In cytoplasm, host-cell ribosomes catalyse synthesis of virus precursor proteins
8- Which are cleaved into viral proteins by viral proteases
9- HIV ssRNA and proteins assemble beneath the host-cell plasma membrane
10- Forming virion buds from it
11- Maturation occurs either in the forming buds or after budding from the host cell
During maturation, HIV proteases cleave the polyproteins into individual functions HIV proteins
what 3 mechanisms cause viral genome variation
mutation
recombination
gene switching/ reassortment
do RNA or DNA viruses mutate faster
RNA
do ss or ds mutate faster
ss
what is antigenic drift
small mutations within antigenic genes that allow viruses to to evade host immune systems
describe gene recombination/ reassortment
2 strains of a virus infect the same host
recombination -> exchange of entire genes between two strands
reassortment -> exchange of entire segments of a genome between strains
- both result in antigenic shift
how does influenza A exhibit antigenic drift / shift
8 segments of -ve sense ssRNA
- susceptible to insertion, deletion and substidution (drift)
able to co-infect multiple species (with B) (shift)
zoonotic spread of new chains
