Microbiology - 3 - Viruses Flashcards
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
- only has:
Note:
- virus = OBLIGATE requirement for intracellular growth!
- dependence on host cell structural & metabolic components
- aka = obligate intracellular parasite
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
- more suseptible to environmental factors
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
- 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
- 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
- ALL NAKED CAPSIDS + some envelope
What are the different viral genomes?
Genomes can be:
- RNA or DNA
- single standed or double stranded
RNA can be:
- (+) sense = viral mRNA
- (-) sense = complementary/antisense mRNA
- double stranded (+)::(-) or anti-sense (+)::(+) / (-) :: (-)
- linear
DNA can be:
- linear
- circular
Exception:
- retrovirus = 2 identical copies of its genome; diploid RNA
How are viruses classified?
Generally, viruses w/ similar structures exhibit similar replication strategies
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
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
What are the stages of a virus infecting a host cell?
Stages of virus infection of Host Cell
- adsorption or attachment to host cell
- penetration or entry
- uncoating –> release genome
- production
- assembly
- release from cell
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
- usually MONOCISTRONIC
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
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)
- examples:
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
-
viral DNA/RNA –> incorporate into host genome
Mechanisms of virus-induced cancer:
-
Direct
- virus = express viral oncogenes that transform cell
-
Indirect
- chronic viral infection –> inflammation & mutations –> tumor formation
- short circuit & overload system –> unregulated cell division
