Virus Structure and Classification (2B) - Bio & Biochem

1. What is a complex virus capsid morphology?

   Capsid architecture that is neither purely helical nor icosahedral. Seen in viruses like bacteriophages, allowing specialized functions such as tail-mediated injection.

2. What is the Baltimore classification system based on?

   Genome type and the pathway used to generate mRNA. Organizes viruses into groups reflecting nucleic acid nature and mRNA synthesis mechanisms.

3. Which Baltimore group is single-stranded RNA reverse-transcribing viruses (retroviruses)?

   Group VI. Reverse transcriptase converts RNA to DNA, allowing integration into the host genome.

4. Which two macromolecule types can serve as a viral genome?

   DNA or RNA. Viruses lack other macromolecules for genomes, relying solely on these nucleic acids for replication and protein coding.

5. What is the viral nucleocapsid?

   Viral genome plus its surrounding capsid. Forms the core infectious particle, essential for genome protection and delivery into host cells.

6. Which capsid symmetry is characterized by a roughly spherical appearance?

   Icosahedral symmetry. Features 20 equilateral triangular faces, providing efficient spherical enclosure for many viral genomes.

7. What is the key structural difference between enveloped and nonenveloped viruses?

   Enveloped viruses have a lipid bilayer; nonenveloped do not. This distinction influences viral stability, transmission routes, and susceptibility to environmental factors.

8. What are capsomeres in viral structure?

   Repeating protein subunits that assemble to form the capsid. These identical units self-assemble into symmetrical capsid structures, optimizing viral stability and efficiency.

9. Identify the typical bacteriophage structures used for attachment and genome delivery.

   Head (capsid), tail sheath/tube, base plate, tail fibers. Facilitate precise attachment to bacterial surfaces and efficient genome injection for infection.

10. What is a viral envelope?

    Host-derived lipid bilayer surrounding some nucleocapsids. Acquired during viral budding, it enhances stability and facilitates fusion with host membranes.

11. What is a bacteriophage?

    A virus that infects bacteria. Specializes in bacterial hosts, exploiting prokaryotic machinery for replication and lysis.

12. Which Baltimore group is positive-sense single-stranded RNA ($+\text{ssRNA}$) viruses?

    Group IV. Genome acts directly as mRNA, enabling rapid translation by host ribosomes upon entry.

13. Which Baltimore group is double-stranded DNA (dsDNA) viruses?

    Group I. Utilize host transcription machinery to produce mRNA from dsDNA templates, mimicking cellular processes.

14. Which Baltimore group is double-stranded DNA reverse-transcribing viruses (pararetroviruses)?

    Group VII. Involve reverse transcription of pregenomic RNA to DNA during replication cycle.

15. Which genome type must carry an RNA-dependent RNA polymerase in the virion to make mRNA after entry?

    $-\text{ssRNA}$. Cannot serve as mRNA directly, necessitating polymerase for positive-sense strand synthesis.

16. Which property primarily determines viral tropism at the entry step?

    Viral binding to specific host-cell receptors. Receptor interactions ensure selective entry, restricting infection to compatible cell types.

17. Which Baltimore group is double-stranded RNA (dsRNA) viruses?

    Group III. Employ virion-encoded polymerases to transcribe mRNA from segmented or non-segmented dsRNA.

18. Which viral surface proteins mediate host-cell attachment and entry?

    Viral glycoprotein spikes (peplomers). These structures recognize and bind host receptors, initiating infection through membrane penetration.

19. What is the capsid of a virus?

    Protein coat that encloses and protects the viral genome. Composed of protein subunits, it safeguards the genetic material during transmission between host cells.

20. Which Baltimore group is single-stranded DNA (ssDNA) viruses?

    Group II. Convert to dsDNA intermediates using host enzymes for subsequent replication and transcription.

21. Which capsid symmetry is characterized by a rod-like or filamentous appearance?

    Helical symmetry. Capsomeres coil around the genome, forming elongated structures suited for certain RNA viruses.

22. Identify the mRNA equivalence of a $+\text{ssRNA}$ viral genome upon entry.

    $+\text{ssRNA}$ can function directly as mRNA. Permits immediate translation by host machinery, bypassing initial transcription requirements.

23. What is viral tropism?

    Specificity of a virus for particular host cells or tissues. Influenced by host factors, it dictates infection patterns and disease manifestations in organisms.

24. Which Baltimore group is negative-sense single-stranded RNA ($-\text{ssRNA}$) viruses?

    Group V. Virion-packaged polymerase transcribes complementary positive-sense RNA for protein synthesis.

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