Microbiology - 3 - Viruses Flashcards Preview

Foundations > Microbiology - 3 - Viruses > Flashcards

Flashcards in Microbiology - 3 - Viruses Deck (10):
1

What is a virus? What are its structural features?

Basics: 

  • set of genes packaged into a protein-containing coat (capsid)
    • either DNA or RNA

Structural features: 

  • comple virus particle = virion
  • do NOT have nucleus, cytoplasm, mitochondria, etc
    • only has:
      • nucleic acid
      • surface proteins (spikes)
    • some have:
      • outer lipid bilayer = envelope

Note: 

  • virus = OBLIGATE requirement for intracellular growth! 
    • dependence on host cell structural & metabolic components 
    • aka = obligate intracellular parasite

2

What are the differences between the enveloped and naked capsid viruses? 

Nakes capsid virus 

  • genome = condensed w/ defined external capsid
    • resistant & survive well in outside world
    • acid/bile resistant --> can go thru GI tract
    • released from cell by LYSIS
    • easily spread!

Enveloped virus

  • nucleocapsid wrapped in lipid bilayer envelope
    • more suseptible to environmental factors
      • drying, gastric enzymes
    • environmentally labile
    • released by BUDDING
      • must stay moist to spread

Both Function

  • protect nucleic acid genome
  • aid in entry into cell
  • package viral enzymes essential for early step of infection process

Tropism

  • localized to site of inoculation/particular organ/tissue/cell type
    • HPV (warts)
  • Receptor recognition = basis
    • CD4+ cells = infected by HIV
    • CD155 = receptor for polio
    • NAR = receptor for rabies

Viral entry

  • 2 basic mechanisms for enveloped virus:
    • both involve fusion of viral envelop w/ cell membrane
    • end result = free nucelocapsid into cytoplasm
  1. Direct fusion
    • HIV-1, HSV, RSV, measles; NO NAKED CAPSIDS
    • tendency to have infected cell fuse w/ uninfected cells
      • viral envelop gets incorporated into the plasma membrane
    • Steps: 
      • spikes bind to host receptors
      • adsorption
      • membrane fusion
      • nucleocapsid released into cytoplasm
  2. Receptor mediated endocytosis
    • ALL NAKED CAPSIDS + some envelope
      • esp: influenza, rubella, rabies, SARS
    • viropex
      • internalizing GFH + nutrients 
    • STEPS: 
      • spikes bind to host receptors
      • adsorption --> endosomal vesicle (low pH)
      • recycled receptors
      • membrane fusion (lysis)
      • nucleocapsid release into cytoplasm

3

What are the different viral genomes?

Genomes can be:

  1. RNA or DNA
  2. single standed or double stranded

RNA can be:

  1. (+) sense = viral mRNA
  2. (-) sense = complementary/antisense mRNA
  3. double stranded (+)::(-) or anti-sense (+)::(+) / (-) :: (-)
  4. linear

DNA can be:

  1. linear
  2. circular

Exception

  • retrovirus = 2 identical copies of its genome; diploid RNA

4

How are viruses classified?

Generally, viruses w/ similar structures exhibit similar replication strategies

A image thumb
5

What are the 5 modes of human viral transmission? 

5 modes of viral transmission: 

  • Respiratory
    • Influenza A
  • Fecal-oral
    • Enterovirus
  • Blood-borne
    • Hepatitis B
  • Sexual
    • HIV
  • Animal
    • Rabies

Notes: 

  • attachment/adsorption by host depends on: 
    • intermolecular forces
    • specific interactions btw nucleocapsid or envelope & host cell membrane receptor

6

What are the lytic and lysogenic cycles?

Lytic cycle: (Productive)

  • new virus produced
  • viral genetic material = latent state indefinitely

Lysogenic cycle: (Non-productive)

  • no new virus produced
  • cell survives & divides
  • viral genetic material = latent state indefinitely

7

What are the stages of a virus infecting a host cell?

Stages of virus infection of Host Cell

  1. adsorption or attachment to host cell
  2. penetration or entry
  3. uncoating --> release genome
  4. production 
  5. assembly
  6. release from cell

A image thumb
8

What are viral transcription and translation?

TRANSCRIPTION

Only (+) ssRNA can be translated --> proteins

  • if (-)ssRNA --> BYORP (byo RNA polymerase)
    • provided by virus

Retrovirus: 

  • (+)ssRNA --> (-)ssDNA
    • via viral reverse transcriptase 
  • (-)ssDNA --> (+)ssRNA

TRANSLATION

  • viral mRNA --> translated in host cytoplasm --> viral proteins
    • usually MONOCISTRONIC
      • single coding region

9

What are latent, lytic, and persistent infections?

Lytic infections: 

  • virus goes thru round of replication
  • new viral particles released via LYSIS
    • polio, flu

Persistent infections: 

  • virus goes thru round of replication
  • cell remains alive & viral particles released slowly over time
  • infected carrier can by asymtomatic yet infectious
    • hepatitis B

Latent infections: 

  • virus remains quiescent
  • viral genome may be in cytoplasm or host genome
  • replication takes place ONLY when triggered
    • HSV & cold sores

Notes:

  • ALL = involve normal host cells where cell metabolism & regulatory processes are disrupted

 

10

What is the relationship btw viruses and cancer? How can a virus transform a normal cell --> malignant cell?

Viruses can "transform" host cell --> cancerous

  • regulated cell growth & contact inhibition = LOST
    • cell continue to divide & form random aggregation
  • Some cells --> invasive & can form tumors
    • examples: 
      • warts on hands/feet = benign
      • genital warts = cervical cancer (HPV)

Cancer-inducing viruses: 

  • can be either DNA or RNA viruses
  • examples: 
    • human T-cell lymphotropic virus
    • Epstrein-Barr (mono)
    • HPV 16 & 18
    • HBV & HCV

Mechanisms involved = vary btw viruses

  • ALL involve interference w/ nomral regulation of division & response to external growth promoting/inhibiting factors
    • viral DNA/RNA --> incorporate into host genome
      • insertion of oncogene
      • alter expression of host oncogene
      • exposure to carcinogens

Mechanisms of virus-induced cancer: 

  1. Direct
    • virus = express viral oncogenes that transform cell
  2. Indirect
    • chronic viral infection --> inflammation & mutations --> tumor formation
  • short circuit & overload system --> unregulated cell division