Viruses Flashcards
Virus characteristics (4)
1) parasites, cannot replicate on their own, MUST have host
2) Made up of proteins and ONE type of Nucleic acid (either RNA or DNA)
3) main function of a virion (a single virus molecule) → deliver its DNA or RNA into the host cell so that it can be expressed (transcribed and translated) by the host cell
4) can infect animal cells, bacteria (bacteriophages), human cells
Examples of RNA viruses
COVID-19
HIV
Examples of DNA viruses
Hep B
Herpes (HSV-1, HSV-2)
“pox” viruses
varicella zoster (chicken pox, shingle)
Bacteria vs Viruses (4)
1) viruses are incredibly small, smaller than bacteria, need an electron microscope to see them
2) bacteria self-replicate, viruses do not
3) bacteria have cellular machinery including ribosomes, viruses do not
4) bacteria have DNA AND RNA, viruses only have one, not both
Methods to detect viruses (2)
1) antibody test
2) PCR
Viruses can be detected through antibody tests if they are
a) very common
b) rare or you get them once
b) rare or you get them once
Viruses that can be detected through antibody tests
HIV
Herpes
Ebola
West Nile
Canada and US: Dengue
Envelope of virus is made of ________
glycoproteins from host cell
Capsid
protein shell that protect the nucleic acid of the virus
important for the attachment of viruses to specific host receptors
provides structure and symmetry to the virus
consists of assembly of identical protein subunits
shapes - either:
1) Icosahedral
2) Helical
3) Spherical
Enveloped virus layers
Envelop
Capsid
Non-enveloped virus layers
Capsid
Which virus is most stable in the environment outside of host?
a) enveloped
b) non-enveloped
b) non-enveloped
can live on inanimate objects up to a week
Example of an enveloped virus
HIV
Examples of non-enveloped viruses
norovirus
poliovirus
Exception - example of enveloped virus that is stable in the environment
pox viruses
e.g. smallpox on blankets
e.g. m pox on hospital laundry affecting cleaning staff
Virus Classification System (5)
1) Nature of the nucleic acid in the Virion (RNA or DNA)
2) Symmetry of the capsid (helical, icosahedral)
3) Presence or absence of an envelope(enveloped or naked)
4) Structure, size and or Morphology of a virus
5) Tissue or organ tropism (Adenovirus, Enterovirus, Rhinovirus)
Virus Replication Cycle Steps (4)
QUIZ/MIDTERM Q**
1) Attachment
-to host cell
2) Entry and Uncoating
-remove capsid
3) Replication and Assembly
-replicates in cytoplasm or nucleus
4) Egress or Release of the Virus
-need to leave cell to infect other cells
Mechanisms of Virion Attachment (4)
1) Direct fusion
2) Endocytosis
3) Receptor-mediated entry
4) Nucleic acid translocation
Direct fusion
virus directly fuses with the host plasma membrane and the nucleic acid is released
Endocytosis
internalized into a vacuole, transported to an endosome and then the nucleic acid is released
may or may not be receptor mediated
Receptor-mediated entry
specific receptors are used by the virus to gain entry into the cell
engagement of the receptors will often lead to changes in the structures of the virus that further help with entry
Examples of viruses that use receptor-mediated entry (many)
HIV
Hep C
SARS
COVID
influenza
Nucleic acid translocation
rare
feature of non-enveloped viruses
capsid adheres to host cell membrane
partial rearrangement of the virion
nucleic acid passed directly into host cell
Types of viral lifecycles (2)
1) lytic
2) lysogenic
Lytic lifecycle
results in the host cell lysing in the end, releasing the virus so it can go on to infect other cells
usually clinically quite apparent
e.g. influenza
Lysogenic lifecycle
integrates its genetic material into the host cell’s DNA and remains dormant
e.g. HPV
Examples of a virus that is lytic AND lysogenic (2)
1) HSV-1
-lytic phase - cold sore
-lysogenetic - clinically silent
-lytic - another cold sore
2) Varicella Zoster
-lytic - chickenpox
-lysogenetic - clinically silent
-lytic - shingles
Important enzyme that viruses carry
viral polymerase
important for viral replication and transcription
Which type of virus mutates faster?
a) RNA viruses
b) DNA viruses
a) RNA viruses
poorer antiviral response
e.g. HIV, Hep C, COVID, SARS, influenza, RSV
dsDNA virus Replication
double-stranded
viral DNA is transcribed to viral mRNA by the VIRAL polymerase
mRNA is then translated to make proteins and enzymes that allow for new virus particle production
uses the host RNA polymerase to make RNA (capsid proteins, DNA polymerase)
newly created DNA polymerase can then replicate the virus DNA
Examples of dsDNA viruses (many)
Herpes viruses:
-Herpes Simplex Virus 1 and 2 (HSV-1, HSV-2)
-Cytomegalovirus (CMV)
-Varicella zoster virus (VZV)
-Epstein Barr virus (EBV)
Hepatitis B virus
Parvovirus B19
RNA Viral Replication
much simpler
some can act directly as mRNA and be read on the ribosome
some more complex i.e: must bring extra replicative enzymes with them
RNA virus examples (many)
Rotavirus
Poliovirus
West Nile Virus
Influenza Virus
Hepatitis A virus
HIV
Hepatitis C
SARS-CoV-1 and SARS-CoV-2 (COVID-19)
Latent viral infection
implies lysogeny
persistence of viral genomes, but not infectious visions, in host cells without the destruction of the infected cell
for certain viruses, viral DNA can also become integrated within host cell DNA
can lead to latent infection
virus may start to replicate at later times
Examples of latent viral infections
HSV and varicella-zoster virus established latency in sensory neuron
HIV-1 can avoid host immune responses and antiretroviral drugs through the latent infection of resting memory CD4(+) T cells
Examples of triggers for latent viral infections (5)
1) stress
2) UV light
3) hormones
4) immune suppression
5) unknown
Negative sense (-) RNA Viruses
has to be converted to a positive stranded genome prior to mRNA and protein production
virion associated RNA polymerase results in a positive sense RNA that can them be transcribed to proteins
Example of - RNA viruses
influenza viruses
Postive sense (+) RNA Virus
RNA directly to mRNA and can be used to make proteins
no intermediary steps
Example of + RNA viruses
SARS-CoV-2 and other coronaviruses
Retrovirus
RNA virus that use the reverse transcriptase enzyme to create a single strand of viral DNA complementary to the retroviral RNA
ssDNA copied to form complementary DNA
dsDNA now enters the host cell nucleus
a later infection results
Methods of detecting viruses (5)
1) Electron microscopy
2) Serology
3) Virus culture
4) Amplification of nucleic acids
5) Antigen detection
Electron microscopy
direct detection
best for: brand new viruses
very large microscope
difficult to do, time-consuming, impractical
not as sensitive as PCR
can only tell you what family of viruses the virus is from not the actual virus
Serology
detection of the immune response by the host against the infectious agent
detecting specific immunoglobulin - Host antibody response
best for:
-very narrow window of viremia: West Nile Virus, Zika
-not culturable: Hep B
-at low levels soon after infection
Virus Culture
growth in tissue culture
not all viruses can be cultured
slow (days to weeks)
requires specific cell lines and many different tissues types must be inoculated
Cytopathic effect (CPE)
effects the virus infection has on the appearance of cells in culture viewed by light microscopy after a patient specimen is inoculated onto the cell culture and the virus has had a chance to replication
fairly non-specific
Amplification of Nucleic Acids
Polymerase Chain Reaction
find a specific segment of RNA or DNA
amplifies it exponentially
Antigen Detection
direct test on a patient specimen
detecting specific antigens (antigen = antibody generator) of the particular organism you are looking for
similar to a pregnancy test
