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Microbiology > Viruses > Flashcards

Viruses Flashcards

1
Q

What is a virus?

A

Ultramicroscopic infectious agent that replicates itself only within cells of living hosts; many are pathogenic; a piece of nucleic acid (DNA or RNA) wrapped in a thin coat of protein.

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2
Q

Where did viruses come from?

A
  • Regressive evolution: degenerate life-forms which have only retained essential genetic information for their parasitic way of life.
  • Cellular origins: sub-cellular macromolecules which have escaped their origins inside cells.
  • Independent entities: evolved from self-replicating molecules from the RNA world. Separate from cellular organisms.
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3
Q

What is the RNA world hypothesis?

A
  • A world filled with life based on RNA predates the current world of life based on DNA and protein.
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4
Q

Describe an example where a virus is not ‘bad’.

A
  • HERV-derived proteins, such as syncytin, have become necessary for normal human placental function.
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5
Q

List a few characteristics of viruses.

A
  • Not cells
  • A genome surrounded by a protective protein coat
  • Dependent on host for cellular machinery
  • Intracellular parasite
  • Formed form assembly of newly synthesised components made in host cell
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6
Q

What are viruses made of?

A
  • Contain genome with DNA or RNA, ss or ds, packaged in a protective protein coat or capsid
  • May also have an envelope made of cell membrane modified to contain virus proteins
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7
Q

How has international committee on taxonomy of viruses grouped viruses?

A
  • Grouped based on genome (nucleic acid type, fragmentation, organisation, sequence), strategy for replication (reverse transcription, integration, site of replication, morphology (envelope..)
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8
Q

What are 4 types of distinguishing structures a virus may possess?

A
  • Icosahedral protien coat, helical protein coat, viral envelopes, complex symmetry
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9
Q

What are the features of an icosahedral coat?

A
  • Repeating units of protein (capsomers) or may be subunits of protomers (all together is called capsid)
  • 12 vertices and 20 equilateral triangles
  • 2, 3 and 5 fold axes of symmetry
  • usually pentons and hexons detected
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10
Q

What are the number type of symmetry a virus can have? Give an example of a virus with a particular axes of symmetry.

A
  • 2-fold, 3-fold, 5-fold (odd numbers?)

- Icosahedral adenovirus capsid has 3 rotational symmetry of axes. (causes respiratory illness or conjunctivitis)

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11
Q

What are the features of a helical protein coat? Give a few examples of a viruses with this feature.

A
  • rod shaped coat consisting of repeating units
  • a single protomer associates with nucleic acid in a spiral or helical arrangement
  • ebola virus, rabies virus, measles virus
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12
Q

What are the features of a viral envelope? Give a few examples of viruses with this feature.

A
  • Icosahedral or helical nucleocapsid surrounded by a membrane
  • A host derived lipid from intracellular membrane ( nuclear, endoplasmic reticulum, golgi or plasma membrane
  • Contains virus-encoded proteins or glycoproteins (spikes)
  • May be flexible (pleiomorphic)
  • influenza virus, herpesvirus, measles virus, SARS-CoV virus
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13
Q

Which virus has the most complex symmetry?

A
  • Poxviruses are the largest and most complex with more than 100 different proteins in the virion and the genome is 130 kbp dsDNA
  • Vaccinia virus and bacteriophage also have complex symmetry
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14
Q

What are the forms of viral nucleic acid a virus can have?

A
  • ss DNA or RNA
  • ds DNA or RNA
  • Sometimes can be divided into segments
  • ssRNA can be +ve sense (like mRNA) or -ve sense (complementary to mRNA)
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15
Q

What is virus classification based on? (not the same as taxonomix groupings)

A
  • Based on epidemiologic/pathogenic criteria
  • Grouped into enteric viruses or respiratory viruses or arboviruses or sexually transmitted viruses or hepatitis viruses
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16
Q

What are features of enteric viruses? What are a few examples of these viruses?

A
  • Replicate primarily in and are localised to the intestinal tract
  • Acquired by ingestion of material contaminated with faeces
  • rotavirus, calicivirus, astrovirus, some adenoviruses
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17
Q

What are features of respiratory viruses? What are a few examples of these viruses?

A
  • Replicate primarily in and are localised to the respiratory tract
  • acquired by inhalation of droplets
  • rhinovirus, coronaviruses, adenoviruses, orthomyxovirus
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18
Q

What are features of arboviruses? What are a few examples of these viruses?

A
  • Infects insects that ingest vertebrate blood
  • replicate in the insect and are transmitted by bite
  • orbivrus, flaviviruses, togaviruses
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19
Q

What are features of sexually transmitted viruses? What are a few examples of these viruses?

A
  • include some herpesviruses and papillomaviruses that cause lesions in the genital tract
  • also certain retroviruses and hepatitis viruses that are transmitted during sexual activity but can cause generalised disease
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20
Q

What are features of hepatitis viruses? What are a few examples of these viruses?

A
  • principal target organ is the liver

- hepatitis A and E (spread via enteric route) and B, C and D (spread by blood or sexually)

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21
Q

What can virus receptors be made of? Give a few examples.

A
  • protein (ICAM-1 for most rhinoviruses; ACE2 for SARS-CoV-2)
  • carbohydrate (sialic acid for influenza virus)
  • Viruses can use 2 different receptors on the same host cell (initial attachment and closer attachment)
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22
Q

What are two ways in which a virus can enter the host cell?

A
  • fusion or endocytosis

- Viruses can uncoat at plasma membrane or within endosome or after endocytosis, uncoat at the nuclear membrane

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23
Q

How does fusion occur between virus and host cell?

A
  • The viral envelope and the cell membrane must mix and form a pore for the nucleocapsid to be released.
24
Q

Which virus uncoats at the plasma membrane?

A
  • HIV contents are released directly into cytoplasm.
25
Q

Which virus enters through endocytosis?

A
  • Alphaviruses (ross river virus) initially taken into the endosome before fusion
  • Release from endosome is triggered by the low pH of the vesicles
26
Q

How is the viral genome amplified in the host cell?

A
  • Nucleic acid replication produces new viral genomes for incorporation into progeny virions
  • mRNA is transcribed from viral DNA and codes for viral proteins that are translated by host
27
Q

Where do DNA and RNA viruses replicate?

A
  • DNA virus replicates in nucleus (except pox)

RNA viruses replicate in cytoplasm (except flu)

28
Q

What special enzyme is used to copy DNA to DNA in us?

A
  • DNA-dependent DNA polymerase (DpDp)
29
Q

What special enzyme is used to transcribe DNA to mRNA in us?

A
  • DNA-dependent RNA polymerase (DpRp)
30
Q

What can’t our special enzymes do? What is significant about this?

A
  • copy RNA to RNA
  • copy RNA to DNA
  • all RNA viruses must encode for their won RNA-dependent polymerase (RdRp) to copy RNA to RNA or RNA to DNA as we don’t have those machinery
31
Q

What do RdRps require for replication?

A
  • require a template, always generate a copy 5’ to 3’, generally form a dsRNA replication intermediate
32
Q

How does -ive sense viruses replicate within host? Why is it different from +sense viruses?

A
  • They cannot be recognised by host genome (like influenza) so its RdRp must be carried in with the virus particle.
  • It needs to do this to first copy it’s negative sense genome into mRNA
  • This is also true for dsRNA viruses who need to initially make the ss mRNA for translation
33
Q

Which virus uses reverse transcriptase and what does it do?

A
  • An enzyme required by Retroviridae
  • It can copy RNA to DNA, digest RNA and copy DNA to DNA
  • Enables viruses like HIV to integrate their RNA genome into the host DNA chromosome
34
Q

What is required for post-translational cleavage of polyproteins of viruses?

A
  • virus-encoded proteases but also some host-encoded proteases
35
Q

Where does glycosylation of envelope glycoproteins of virsuses occur?

A
  • in rough endoplasmic reticulum and golgi vesicles of host cell
36
Q

What do all non-enveloped animal viruses have?

A
  • An icosahedral structure
37
Q

What are the strategies for assembly and release of non-enveloped animal viruses?

A
  • Spontaneous assembly of capsid proteins around the nucleic acid genome
  • Proteolytic cleavage to induce final conformation in the capsid proteins of mature infectious virions. (carried out of host or viral prteases)
  • Accumulation of virions in cytoplasm or nucleus wait until cell lyses to be released.
38
Q

What are the strategies for assembly and release of enveloped viruses?

A
  • Release may take place by budding from cell surface
  • membrane surrounding nucleocapsid bulges out and becomes ‘nipped off’ to form new enveloped virion
  • Some enveloped viruses use the cellular secretory pathway to exit (through golgi vesicles)
39
Q

What are some possible effects of animal viruses on cells?

A
  • Transformation of normal cells to tumor cells
  • Lytic infection
  • Persistent infection
  • Latent infection
40
Q

What must viruses do to cause infection?

A
  • gain entry into body
  • multiply and spread
  • target appropriate organ
41
Q

What must viruses do to be maintained in nature?

A
  • spread into the environment (cough, sneeze) or
  • taken up by an arthropod vector or needle or
  • passed congenitally
42
Q

What are ways a respiratory virus can be transmitted?

A
  • droplets produced by coughing, sneezing, talking
  • direct contact with infected individual
  • contact with contaminated surface, touch mouth or eyes or nose
43
Q

How does a virus enter through the alimentary tract (gastrointestinal tract)

A
  • ingested virus is swallowed or infect oropharynx and be carried elsewhere
  • Constant movement of contents in intestinal tract allows some virus to contact specific receptors
  • These viruses are usually acid and bile resistant and generally do not have an envelope (membrane bilayer is easily destroyed by acid/bile)
  • HIV can infect rectal route
44
Q

Which alimentary viruses infect the mouth or oropharynx?

A
  • Herpes simplex virus 1 (cold sores): acquired by direct contact of infected saliva with damaged skin
  • Epstein-Barr virus (infectious mononucleosis): acquired by direct contact of saliva with oropharynx (kissing)
45
Q

What are the mechanisms of spread of virus in the body?

A
  • Local spread on epithelial surface eg. herpes
  • Sub-epithelial invasion and lymphatic spread
  • Sub-epithelial invasion and neuronal spread eg. rabies
  • Spread via bloodstream (viraemia)
46
Q

What is disseminated and systemic infection?

A
  • Spread beyond primary site

- many organs infected eg. ebola

47
Q

How does viraemia work? Give a few examples.

A
  • Virus can be free in plasma eg. arboviruses, neurotropic viruses
  • either produced by infected vascular endothelium or released in large amounts from secondary lymphoid tissues (liver, spleen)
  • Neutralised by developing Ab response and removed by macrophages (1-2wks)
  • Virus can be cell-associated eg. measles spread by monocytes
  • can persist from months to years if viral genomes latent to avoid CTL attack
48
Q

What are ways virus can be shed into the environment?

A
  • Respiratory, faeces, skin, blood, urine, milk, genital secretions
49
Q

What is a cytocidal virus? Give an example.

A
  • disease may result directly from the cell death caused by viral replication
  • influenza virus infection
50
Q

What is a non-cytocidal virus? Give an example.

A
  • cells may lose ability to perform particular functions

- SARS-CoV-2 and ACE2

51
Q

What are consequences of immune response?

A
  • Immunopathology (fever, inflammation, enlarged lymph nodes, killing of infected cells)
  • Immunosuppression (virus grows in cells of immune system eg. HIV)
52
Q

What does cytokine release syndrome increase?

A
  • immune activation, IL-6 and others, vascular permeability, inflammatory monocytes and neutrophils, liver/kidney damage, hypoxia, mechanical ventilation, death
53
Q

What are the outcomes of virus infection?

A
  • fatal (man is not natural host, high mortality rates) eg. ebola
  • full recovery (completely cleared by immune system) eg. influenza
  • recovery but permanent damage (virus cleared but left with symptoms) eg. cancer
  • persistent infection (virus not cleared and can resurface to cause disease)
54
Q

Why can we get some diseases repeatedly?

A
  • ineffective immunity eg. warts
  • effective immunity but multiple serotypes of virus eg. rhinovirus
  • constantly evolving virus eg. influenza, HIV
55
Q

What are ways which viral genomes repeatedly change?

A
  • mutation (nucleic acid copying errors)
  • If 2 viruses infect same cell, recombination (exchange of nucleic acid sequence) or reassortment ( swapping of segments for viruses that have segmented genomes)
  • Antigenic drift resulting from RNA copying errors which give rise to new seasonal epidemic strains (influenza)
  • Antigenic shift resulting from genetic reassortment give rise to pandemic of influenza
56
Q

What is an example of a latent infection?

A
  • varicella-zoster virus (chickenpox and shingle)

- Immunodeficiency virus (AIDS)

Microbiology (8 decks)

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