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MedST IB: Biology of Disease (BoD) > 2.0 Virology > Flashcards

Flashcards in 2.0 Virology Deck (73)
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What is the size range of a virus?

Most are 20-700nm

(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?

1) Nucleic acid (genome)
2) Capsid

Some have:
3) Lipid membrane (envelope)


Structure of capsids:

Composed of capsomers (repeating protein units)
Symmetrical (can be helical or icosahedral


Structure of lipid membrane:

Phosopholipid membrane (aquired from host)
Embedded viral protein
May be glycosylated


How do virions increase the coding capacity of their small genomes?

1) Densely packed genes
2) Overlapping reading frames
3) Splicing
4) Few non-coding regions


What are the different ways to measure viruses?

1) Electron micrograph
- Useful for quantification
- No info on virulence

2) Polymerase chain reaction
- Useful for diagnosis
- No info on virulence

3) Immunological evidence of infection
- Detection of adaptive immune response
- Too slow for diagnosis (useful epidemiologically)
- Can give some info on virulence

4) Plaque assay
- Preffered method to measure infectivity


What are the stages of viral replication?

1) Adsorption and penetration
2) Eclipse phase
3) Assembly and release


What is the mean burst size?

Average yield of virus particles per cell
- Varies greatly


How does HIV bind to target cell?

gp120 (envelope glycoprotein) on HIV binds to CD4 + chemokine co-receptor

CD4 = only on T-cells


How does influenza bind to target cell?

Haemagglutin (HA) (envelope glycoprotein) on influenza binds to sialic acid

Sialic acid = on most cells


How does EBV bind to target cell?

gp340 (envelope glycoprotein) on EBV binds to CD21


How does HIV penetrate the target cell?

Binding of gp120 to CD4 → conformational change in virus gp120/gp41 → virus envelope fuses with plasma membrane


How does infleunza penetrate the target cell?

Binding of HA to sialic acid → endocytosis → endosome is acidified

↓ pH → rearrangement of HA → viral envelope is pulled closer to vesicle membrane → disruption → fusion


What happens in the eclipse phase?

No virus particle present in host cell

Virus has disassembled
Genome is being replicated
Virus proteins are being made


What are the steps for -ve ssRNA and dsRNA viral replication?


1) Viral genome transcribed to +ve sense RNA
- Enzyme = viral RNA dependent RNA polymerase
2) +ve sense RNA can be used as mRNA or to make new viral genome

Exception to this is influenza (occurs in nucleus - uses host RNA pol II)


What are the steps for +ve ssRNA viral replication?


1. Translation
- Translated proteins include RNA dependent RNA polymerase
2. Virus genome is replicated into complimentary (-ve sense) RNA (RNA dependent RNA polymerase)
3. Second stage of replication is to copy -ve sense to +ve RNA (RNA dependent RNA polymerase)
4. These can then be packaged into new virions


What are the steps for retrovirus replication?

1. Virus genome is copied by reverse transcriptase
- This is an RNA dependent DNA pol
- Packaged within the virus particle
- Creates dsDNA intermediate
2. dsDNA intermediate is integrated into host genome
- Provirus
3. mRNA is transcribed by host DNA dependent RNA pol II
- provirus = template
4. Full length transcripts can be translated
5. Or packaged into new virus capsids in the cytoplasm


What are the steps for ds DNA viral replication?


1. Virus genome is transported into nucleus
2. Transcription
- Uses host DNA-dependent RNA pol
3. mRNA translation
- Occurs in cytoplasm
4. Some proteins are transported back to nucleus
- e.g. DNA pol and capsid proteins
5. In nucleus viral DNA is replicated and progeny genomes are packed into new capsids


How are poxviridae virions an exception to ds DNA viral replication?

Replication occurs in cytoplasm

They carry their own enzymes (DNA dep. RNA pol + capping/polyadenylating enzyme)

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)

2) Late genes → structural proteins of virion

(Early proteins = low amounts, late proteins = large amounts)


What are the mechanisms virions use to make different proteins (poly-protein processing)?

1) Post-translational cleavage (using specific proteases)

2) Segemental genome (influenza)

3) Splicing (e.g. HIV → gp160 → gp120 + gp41)


What are the two mechanisms of viral release from a cell?

1) Lysis of cell
2) Budding of enveloped virus


What viruses show latency?

1) Retroviruses
2) Herpesviruses


What modifications to host cells can viruses induce?

1) Subversion of cellular metabolism to make only viral proteins
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


What viruses cause lytic infections?

1) DNA viruses
2) Non-enveloped RNA
3) Viruses that cause host-cell shut off


What viruses cause non-lytic infections?

1) Enveloped RNA
2) Retroviruses


What viruses cause cell transformation?

1) HPV → wart/cervical cancer (16 +18)
2) Rous sarcoma virus → sarcoma in chickens


What viruses have the following portals of entry?

Respiratory tract:
Alimentary canal:
Genital tract:
Insect bite (blood):

Oropharnx: HSV, CMV, EBV

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