Viruses, Viroids, Prions & Related Topics – Exam Review

What is the smallest known archaeal genome discovered to date?

Sukunaarchaeum mirabile with ~238 Kbp and extreme genome reduction.

Which archaeal lineage did Sukunaarchaeum mirabile help expand?

A previously overlooked deep-branching lineage within the Archaeal Tree of Life related to DPANN.

Why is Sukunaarchaeum mirabile described as ‘dedicated to self-replication’?

It lacks metabolic pathways and retains only genetic information processing and replication machinery, relying on a host for metabolites.

Name two key structural differences between typical bacteria and viruses.

Viruses lack ribosomes & ATP-generating metabolism, and do not divide by binary fission; bacteria possess these.

Which filterability characteristic distinguishes viruses from bacteria?

Viruses typically pass through bacteriological filters (<0.2 µm); most bacteria do not.

What size range covers most viruses, necessitating an electron microscope?

Less than 0.2 µm (200 nm).

Give an example of a ‘giant’ virus visible by light microscopy.

Megavirus (~800 nm).

What are the two main components of a nucleocapsid?

The viral nucleic acid and its surrounding capsid (protein coat).

How does an enveloped virus differ from a naked virus?

Enveloped viruses possess a lipid bilayer envelope with matrix proteins and spikes outside the capsid; naked viruses do not.

Define ‘virus tegument.’

A protein layer (matrix) between the nucleocapsid and envelope that contains factors aiding early infection.

List the four basic viral morphologies.

Helical, polyhedral (icosahedral), enveloped, and complex.

Which classic plant virus is helical and was key to structural virology?

Tobacco Mosaic Virus (TMV).

Name the receptor-binding spike of influenza virus.

Hemagglutinin (HA).

What is the function of influenza neuraminidase (NA)?

Cleaves sialic acid residues to allow virion release from the cell surface.

Which RNA virus family includes SARS-CoV-2?

Coronaviridae.

State one reason RNA viruses mutate faster than DNA viruses.

RNA-dependent RNA polymerases lack proofreading ability.

Which two surface proteins define influenza subtypes such as H5N1?

Hemagglutinin (H) and Neuraminidase (N).

What is the antiviral mechanism of oseltamivir (Tamiflu)?

It inhibits neuraminidase, blocking release of progeny influenza virions and reducing viral load.

Within how many hours of symptom onset is Tamiflu most effective?

Within 48 hours.

What is antigenic reassortment, and why is it important for influenza pandemics?

Exchange of genome segments between different influenza strains co-infecting a host, generating novel viruses (e.g., 1957, 1968 pandemics).

Which step does Paxlovid target in SARS-CoV-2 replication?

Viral proteolysis via 3CLpro inhibition, blocking polyprotein processing.

Identify the five stages of the one-step viral growth curve.

Attachment/penetration, eclipse period, maturation, release, plateau.

Define ‘cytopathic effect’ (CPE).

Observable cellular damage (rounding, syncytia, inclusion bodies, vacuolation, plaques) produced by viral infection in culture.

What is a plaque and how is it used in virology?

A clear zone of lysed cells on a bacterial lawn; used to quantify bacteriophages as plaque-forming units (PFU).

Which in-ovo sites are commonly inoculated for viral cultivation?

Chorioallantoic membrane, allantoic cavity, amniotic cavity, yolk sac, shell membrane.

Describe the five basic steps of the lytic cycle of a bacteriophage.

Attachment, penetration, biosynthesis, maturation, release (lysis).

What is a prophage?

Phage genome integrated into bacterial chromosome during lysogeny.

Give one medical consequence of lysogenic conversion.

Corynebacterium diphtheriae gains diphtheria toxin gene from a prophage.

Differentiate generalized from specialized transduction.

Generalized: random bacterial DNA packaged; specialized: specific adjacent genes transferred with prophage during excision.

Name three outcomes of animal virus infection.

Productive lytic infection, persistent infection, or latent state; transformation into tumor cells is another possibility.

List the general steps of DNA animal virus replication.

Attachment, entry & uncoating, early transcription/translation, DNA replication, late protein synthesis, assembly (maturation), release.

Which DNA virus family uses reverse transcriptase during replication?

Hepadnaviridae (e.g., Hepatitis B virus).

Give two oncogenic DNA viruses.

Human Papillomavirus (HPV) and Epstein-Barr Virus (EBV).

What tumor antigen is expressed on the surface of virally transformed cells?

Tumor-specific transplant antigen (TSTA).

Name an RNA tumor virus and its associated cancer.

HTLV-1 (Human T-cell Lymphotropic Virus) associated with adult T-cell leukemia/lymphoma.

Define latent viral infection and provide an example.

Virus remains dormant with intermittent reactivation; e.g., Varicella-Zoster Virus causing shingles years after chickenpox.

Contrast persistent viral infection with latent infection.

Persistent: continuous low-level viral production (e.g., HIV); Latent: no production between reactivations (e.g., HSV).

What percentage of human cancers are attributed to tumor viruses?

About one in six (~16%).

Explain herd immunity in one sentence.

When a high proportion of a population is vaccinated, chain of infection is disrupted protecting even the unvaccinated.

Why is measles rare in the U.S. today?

Widespread vaccination eliminated endemic transmission.

State a key distinguishing feature of prions compared with viruses.

Prions lack nucleic acid; they are misfolded proteins causing transmissible spongiform encephalopathies.

Name two human prion diseases.

Creutzfeldt-Jakob disease (CJD) and fatal familial insomnia.

What is the proposed role of A1 astrocytes in prion disease?

Shift to neurotoxic phenotype promoted by microglia, contributing to neuronal death.

Why are plant cells generally resistant to viruses, and how do viruses overcome this?

Rigid cell walls impede entry; viruses require wounds or vectors (e.g., aphids) for transmission.

What is a viroid?

Small circular single-stranded RNA molecule infecting plants without a protein coat (e.g., potato spindle tuber viroid).

How can viroid infection alter gene expression?

Viroid RNA processed into vsRNA that hijacks RNA silencing pathways, disrupting normal mRNA regulation.

Give an example of a double-stranded DNA plant virus and its vector.

Cauliflower mosaic virus (Caulimoviridae) transmitted by aphids.

Which vector transmits Watermelon wilt virus from the Bunyaviridae family?

Whiteflies.

What is the laboratory test principle behind the COVID-19 home antigen test?

Serological detection of viral proteins via specific antibodies on a test strip (lateral flow).

Which molecular method amplifies viral nucleic acids for diagnosis?

Polymerase Chain Reaction (PCR).

Why are continuous cell lines valuable for virus culture?

They can be maintained indefinitely, providing a consistent host for viral replication.

In the influenza vaccine production flow, why are ferrets used for antigenic analysis?

Ferrets exhibit an immune response to influenza similar to humans, aiding strain selection.