2.0 Virology Flashcards
What is the size range of a virus?
Most are <b>20-700nm</b><br></br><br></br>(can be smaller)
What is the smallest virus?
Foot and mouth disease virus (FMDV) - 20nm
What is the largest virus?
Mimivirus - 700nm
What is the structure of virions?
<b>1) Nucleic acid (genome)</b><br></br><b>2) Capsid</b><br></br><br></br>Some have:<br></br>3) Lipid membrane (envelope)
Structure of capsids:
Composed of <b>capsomers</b> (repeating protein units)<br></br>Symmetrical (can be <b>helical</b> or <b>icosahedral</b>
Structure of lipid membrane:
Phosopholipid membrane (aquired from host)<br></br>Embedded viral protein<br></br>May be glycosylated
How do virions increase the coding capacity of their small genomes?
1) Densely packed genes<br></br>2) Overlapping reading frames<br></br>3) Splicing<br></br>4) Few non-coding regions
What are the different ways to measure viruses?
<b>1) Electron micrograph</b><br></br>- Useful for quantification<br></br>- No info on virulence<br></br><br></br><b>2) Polymerase chain reaction</b><br></br>- Useful for diagnosis<br></br>- No info on virulence<br></br><br></br><b>3) Immunological evidence of infection</b><br></br>- Detection of adaptive immune response<br></br>- Too slow for diagnosis (useful epidemiologically)<br></br>- Can give some info on virulence<br></br><br></br><b>4) Plaque assay</b><br></br>- Preffered method to measure infectivity
What are the stages of viral replication?
1) Adsorption and penetration<br></br>2) Eclipse phase<br></br>3) Assembly and release
What is the mean burst size?
Average yield of virus particles per cell<br></br>- Varies greatly
How does HIV bind to target cell?
<b>gp120</b> (envelope glycoprotein) on HIV binds to <b>CD4</b> + chemokine co-receptor<br></br><br></br>CD4 = only on T-cells
How does influenza bind to target cell?
<b>Haemagglutin (HA)</b> (envelope glycoprotein) on influenza binds to <b>sialic acid</b><br></br><br></br>Sialic acid = on most cells
How does EBV bind to target cell?
<b>gp340</b> (envelope glycoprotein) on EBV binds to <b>CD21</b>
How does HIV penetrate the target cell?
Binding of gp120 to CD4 → conformational change in virus gp120/gp41 → <b>virus envelope fuses with plasma membrane</b>
How does infleunza penetrate the target cell?
Binding of HA to sialic acid → <b>endocytosis</b> → endosome is acidified <br></br><br></br>↓ pH → rearrangement of HA → viral envelope is pulled closer to vesicle membrane → disruption → fusion
What happens in the eclipse phase?
<b>No virus particle present in host cell</b><br></br><br></br>Virus has disassembled<br></br>Genome is being replicated<br></br>Virus proteins are being made
What are the steps for -ve ssRNA and dsRNA viral replication?
<b>OCCURS IN CYTOPLASM</b><br></br><br></br><b>1) Viral genome transcribed to +ve sense RNA</b><br></br>- Enzyme = viral RNA dependent RNA polymerase<br></br><b>2) +ve sense RNA can be used as mRNA or to make new viral genome</b><br></br><br></br>Exception to this is influenza (occurs in nucleus - uses host RNA pol II)
What are the steps for +ve ssRNA viral replication?
<b>OCCURS IN CYTOPLASM</b><br></br><br></br><b>1. Translation</b><br></br>- Translated proteins include RNA dependent RNA polymerase<br></br><b>2. Virus genome is replicated into complimentary (-ve sense) RNA </b>(RNA dependent RNA polymerase)<br></br><b>3. Second stage of replication is to copy -ve sense to +ve RNA </b>(RNA dependent RNA polymerase)<br></br><b>4. These can then be packaged into new virions</b>
What are the steps for retrovirus replication?
<b>1. Virus genome is copied by reverse transcriptase</b><br></br>- This is an RNA dependent DNA pol<br></br>- Packaged within the virus particle<br></br>- Creates dsDNA intermediate<br></br><b>2. dsDNA intermediate is integrated into host genome</b><br></br>- Provirus<br></br><b>3. mRNA is transcribed by host DNA dependent RNA pol II</b><br></br>- provirus = template<br></br><b>4. Full length transcripts can be translated</b><br></br><b>5. Or packaged into new virus capsids in the cytoplasm</b>
What are the steps for ds DNA viral replication?
<b>OCCURS IN NUCLEUS</b><br></br><br></br><b>1. Virus genome is transported into nucleus</b><br></br><b>2. Transcription</b><br></br>- Uses host DNA-dependent RNA pol<br></br><b>3. mRNA translation</b><br></br>- Occurs in cytoplasm<br></br><b>4. Some proteins are transported back to nucleus</b><br></br>- e.g. DNA pol and capsid proteins<br></br><b>5. In nucleus viral DNA is replicated and progeny genomes are packed into new capsids</b>
How are poxviridae virions an exception to ds DNA viral replication?
Replication occurs in cytoplasm<br></br><br></br>They carry their own enzymes (DNA dep. RNA pol + capping/polyadenylating enzyme)<br></br><br></br>Viral DNA alone is not infectious
In viral replication, what do early genes code for and what do late genes code for?
1) Early genes → nucleic acid replication (+ modification of host cell)<br></br><br></br>2) Late genes → structural proteins of virion<br></br><br></br>(Early proteins = low amounts, late proteins = large amounts)
What are the mechanisms virions use to make different proteins (poly-protein processing)?
<b>1) Post-translational cleavage</b> (using specific proteases)<br></br><br></br><b>2) Segemental genome</b> (influenza)<br></br><br></br><b>3) Splicing</b> (e.g. HIV → gp160 → gp120 + gp41)
What are the two mechanisms of viral release from a cell?
1) Lysis of cell<br></br>2) Budding of enveloped virus
2) Herpesviruses
2) Cell stimulation
3) ↑ dNTP pool
4) Membrane modifications
5) Cytopathic effect (CPE)
6) ↓ host cell signalling (↓ innate immunity)
7) Lytic/non-lytic infections
8) Cell transformation
2) Non-enveloped RNA
3) Viruses that cause host-cell shut off
2) Retroviruses
2) Rous sarcoma virus → sarcoma in chickens
Oropharnx:
Respiratory tract:
Alimentary canal:
Conjunctiva:
Skin:
Genital tract:
Blood:
Insect bite (blood):
Respiratory tract: Influenza, measles, mumps, rubella, VZV, adenovirus, rhinovirus
Alimentary canal: Poliovirus, Hep A, rotavirus
Conjunctiva: HSV
Skin: HPV, HSV, Rabies
Genital tract: HIV, HSV, HPV
Blood: Hep B, HIV
Insect bite (blood): Yellow fever, dengue
2) Unusual structure (RNA 5' triphosphate)
2) IRF (interferon response factors)
TNFα + TNFβ
Released from infected cells
↑ antiviral state in adjacent cells
↑ MHC I
Type II
TNFγ
Released by T cells and macrophages
↑ Th1 responses
↑ Inflammation
Type III
TNFλ
Important for epithelial cells
2) NFkB/IRF3 go to nucleus + ↑ IFNβ transcription
3) IFNβ secreted → binds to Type I IFN receptor on adjacent cells
4) This stimulates JAK-STAT pathway
5) JAK-STAT → + ISGF-3 → + ISRE → ↑ proteins that make cells resistant to viruses
2) Releasing IFN binding factors to prevent their effect
3) Inhibit JAK-STAT pathway
4) Block Interferon stimulated genes (ISGs) + block action
1) Poliovirus
2) Rotavirus
3) HIV
4) Hep B
5) Rabies
Motor neurons → paralysis
2) Rotavirus
Gut epithlia → diarrhoea
3) HIV
CD4 cells → immunodeficiency
4) Hep B
Hepatocytes → acute hepatitis
5) Rabies
Purkinje cells (cerebellum) → hydrophobia
1) Hep B
2) Measles
3) HSV-1/HSV-2
4) VZV
Hepatocytes → chronic hepatitis
2) Measles
Neurons
3) HSV-1/HSV-2
Neurons → cold sore/genital warts
4) VZV
Neurons → chicken pox/shingles
1) Hep B
2) HPV 6,11
3) HPV 16, 18
4) EBV
5) RSV
2) HPV 6,11 → wart
3) HPV 16, 18 → cervical/penile cancer
4) EBV → B cells → Burkitt's/nasopharyngeal CA
5) RSV → chicken sarcoma
2) Route of entry
3) Age + sex
VZV/EBV/HBV different in different aged people
HBV = worse prognosis in males
4) Physiological state (of host)
Does not spread to other tissues
Outcome = independent of specific immune response
2) Influenza
3) Gastroeneritis
More severe
Outcome = dependent on specific immune responses (esp. CTLs)
2) Chicken pox
3) Measles
4) FMDV
2) Lymph
3) Nerve tracts (rabies)
1) HSV
2) VZV
3) CMV
4) EBV
1) Blood
2) Skin
3) Alimentary canal
4) Respiratory system
5) Saliva
6) Genital tract
7) Breast milk
8) Placenta
(e.g. picornaviridae spread feco-orally because stable in water)
- Glycoprotein
- 17 different types
2) Neuraminidase (NA)
- Glycoprotein
- 9 different types
3) M2
- Ion channel
2) PB2
3) PA
2. Virion is internalised by endocytosis
3. Vesicle with virion is acidified ⟶ conformational change in HA ⟶ fusion of virus membrane with endosomal membrane
4. Nucleocapsid enters the cytosol and is transported to nucleus (where replication takes place)
Each monomer is composed of HA1 + HA2 subunits
H1
- Globular head
- Binds to sialic acid
H2
- Stalk between head and viral membrane
- Has fusion peptide at N-terminal
↓pH → H1 moves → exposure of fusion peptide
∵ sialic acid is present over the cell surface, if there was no NA, the virus would just bind back to the cell and be unable to escape
The NA also removes sialic acid residues from the HA and NA proteins on virions, to prevent aggregation of virions
Drugs that inhibit NA have developed and marketed (tamiflu and Relenza)
- Gradual amino acid mutations in HA
Antigenic shift
- Radical change to HA
- Caused by acquiring new HA from another virus
- Due to reassortment in cell infected by two different viruses
α helix
Highly conserved
Binds copper
Glycosylated
High levels in CNS
β sheet
Resistant to protease degradation
↑ misfolding of normal protein (acts as template for PrP to misfold)
2) Misfolded protein causes normal protein to misfold
3) Amino acid sequence of protein can influence ease of conversion to misfolded form
Cannibal → Kuru
Infected animal food → BSE in cattle
Eating infected animals → nvCJD
2) Sporadic cases
3) Familial cases
Co-receptors are needed (CCR5 + CXCR4)
- Nucleoside analogue
- Used by reverse transcriptase → chain termination
2) Protease inhibitors
Target HIV aspartate protease (needed to cleave gag capsid proteins to mature forms)
Anti-HIV
Contains no 3'hydroxyl
Selective to HIV reverse transcriptase
• But is phosphorylated by HSV thymidine kinase
• The nucleotide triphosphate is incorporated into newly synthesised viral DNA by the HSV DNA polymerase
• Incorporation terminates chain growth
• It inhibits the neuraminidase so that influenza virus cannot be released from the infected cell
• The virus also clumps to itself because HA and NA are glycoproteins.
• During virion assembly the polyprotein is cleaved by the virus-specific protease to yield the capsid protein and the reverse transcriptase
• Several protease inhibitors have been designed that inhibit the HIV protease, but not host proteases
• These drugs prevent the completion of virus assembly
