Viral Life Cycle & Infections Flashcards
The overall infectious cycle-steps
• Attachment/absorption of virion to a receptor site on the host cell
• Entry/penetration of virion or its genome into the host cell
• Uncoating (for animal viruses) capsid separates and releases viral genome
• Biosynthesis of new nucleic acids and viral proteins by the host cell’s
enzymes and ribosomes (transcription, translation, replication)
• Assembly of new viral particles within the host cell
• Release of newly formed virions from host cell
Two broad phases of the viral infectious cycle
Early phase:
• Attachment to delivery of the genome
• “Early proteins” synthesized soon after infection and are usually need to take control of the host cells and for viral genome replication (ex. RdRp)
Late phase:
• “Late proteins” are the virus structural proteins such as capsomeres
• Final steps of the infectious life cycle → assembly and release
Two different cycles for bacteriophages
Lytic cycle: ends with the lysis and death of the host cell
-virulent phages
Lysogenic cycle: the host cell remains alive; the phage DNA is incorporated into the host DNA
-temperate phages
Attachment/absorption
Cannot diffuse passively, must attach to a receptor
-Bacteriophages:
• Chemical attraction between viral protein and cell proteins on cell wall
• Some use fibers at the end of their tails to attach to host cell surface
-Animal viruses:
• glycoprotein spikes (enveloped virus)
• attachment molecules on capsid (naked virus)
• host range/specificity depends on viral and host surface molecular interaction
• can use this to develop drugs to prevent viral infection
Ex. ACE 2 receptor
Entry/Penetration
- Bacteriophages:
• release lysozyme (protein enzyme) to weaken the bacterial cell wall
• phage’s tail pierces through the cell wall
• Nucleic acid from capsid passes through the tail and enters the bacterial cell
• Only nucleic acid enters the host cell - Animal vinses:
• Not understand well, 3 different mechanism
1. Direct penetration:
Naked virus, capsid attaches and sink into membrane, create a pore, let genome enter (ex. Polivirus)
2. Membrane fusion: Envelop and host membrane (ex. Measles)
3. Endocytosis (phagocytosis): enveloped and naked virus (ex. Adenovirus(naked), HCV (enveloped))
Uncoating
Only for animal viruses
- The seperation of the viral nucleic acid from its capsid
- different types
• host cell lysosomal enzyme action
• acidification of the content of the endosome - Some take place in the cytoplasm, some in the nucleus
Biosynthesis
- Bacteriaphages:
• use host nucleotides and several enzyme
• use host cell’s ribosome and amino acid - Animal virus = Baltimore classes
Assembly
- Bacteriosynthesis:
not completely understand, but the assemble is spontaneously - Animal viruses:
• spontaneously
• most DNA viruses assemble in the nucleus
• most RNA viruses develop solely in the cytoplasm
• number of viruses produced depends on type of virus, size me initial health of host cell
Release
- Bacteriaphages: lysis, lead to host cell death.
- Animal viruses:
• Budding: enveloped, acquire a portion of the cellular membrane
• Exocytosis or lysis: naked viruses, similar manner but don’t need membrane
Lysogenic life cycle
-bacteriophage:
produce prophage, can shift to lytie cycle under
some trigger
- animal viruses:
• latent viruses or proviruses: animal viruses remain
dormant in host cell
• May be prolong for years with no viral activity
• ex. HIV → provirus, Herpes simplex virus (ds DNA): undergo lytic
or stery dormant. Can exist as an episome
• episome: separate from host chromosomal DNA and establish latency
Requirements for a successful infection
- Enough virus
- Cell susceptible and permissive
- Local antiviral defense absent or overcome
Abortive infection
virus enters a host cell but is unable to complete its replication cycle successfully
Acute infection
Short-term, intense infection
Chronic infection
long-lasting, virus continuously produces low levels of new particles and host immune system may struggle to clear infection
Persistent infection
long-lasting, virus may periodically reactive and replicate, leading to recurrent symptoms
Latent infection
Virus remains dormant or inactive in host for long time
Viral tropism
The specificity of a virus for a particular cell type, tissue, and/or host
• Cellular tropism:
Virus replicates in one cell type but not another
• Tissue tropism:
Virus replicates in a particular tissue or organ but not another
• Host tropism:
Virus replicates in one host species but not another
Viral spread within the body
-Viruses may remain localized and contained by tissue structure, cell susceptibility and immune system
-systemic infection:
Some viruses spread beyond the primary site of infection and lead to systemic infection, where many organs become infected
Ex.SARS-CoV-2 enter lung and then infect ACE2-expressing cells
Viral pathogensis
• Effects of viral replication on the host
• Effects of host response on virus and host
Virus transmission methods
- Direct transmission from person to person
- Droplet or aerosol infection
- Fecal-oral route
- Sexual contact
- Transfusion of contaminated blood
- Hand-mouth, hand-eye, or mouth-mouth contact - Animal to animal, human as accidental hosts
-by bite
-by droplet or aerosol infection form rodent-contaminated quarters 住所 - Transmission by an arthropod vector
Ex. Mosquito
Transmission type
Horizontal transmission
Vertical transmission
Germ line transmission
Nosocomial transmission
Horizontal transmission
between members of same species
Vertical transmission
Transfer of infection between parent and offspring
Ex. Breast feeding
Germ line transmission
Agent is transmitted as part of the genome
(ex.proviral DNA)
Nosocomial
When an individual is infected while in hospital or health care facility
Zoonosis
• Any disease or infection that is naturally transmissible from animals to humans
• Animals play an essential role in maintaining zoonotic infections in nature
• Zoonoses may be bacterial, viral, or parasitic.
• Ex. Rabies, Avian flu
viral diseases emerging
• Environmental changes
(flood, drought, deforestation, climate change)
• Human behavior
(drug use, outdoor recreation, sexual activity)
• Social, political and economic factors
(poverty, war)
• Travel and commerce
(international air travel)
• Health care
(blood transfusions, immunosuppressive drugs)
• Microbial adaptation
(changes in virulence, development of drug resistance, microbe mutation)
• Public health measures
(inadequate sanitation, vaccine/antivirals accessibility and vector control measures)
