Chapter 21: Viruses Flashcards
Why are viruses considered non-living?
Viruses lack cellular structure, cannot reproduce independently, and do not carry out metabolic processes. They require a host cell to replicate.
Describe the basic structure of a virus.
Capsid: Protein shell enclosing genetic material.
Genetic material: DNA or RNA (single- or double-stranded).
Envelope: Lipid membrane (optional, derived from host cell).
Spike proteins: Enable host cell attachment (e.g., hemagglutinin in influenza).
How do scientists classify viruses?
Based on:
Genetic material (DNA vs. RNA).
Capsid shape (icosahedral, helical, complex).
Envelope presence (enveloped vs. non-enveloped).
Host specificity (bacteriophages, animal/plant viruses).
What evidence supports the hypothesis of viral origins from mobile genetic elements?
Similarities: Some viruses (e.g., retroviruses) integrate into host genomes like transposons.
Reduction theory: Viruses may have evolved from cellular organisms by losing metabolic genes.
Compare the lytic and lysogenic cycles in bacteriophages.
Lytic: Virus immediately replicates, lyses host cell (e.g., T4 phage).
Lysogenic: Viral DNA integrates into host genome (prophage) and replicates with host (e.g., lambda phage).
How do retroviruses (e.g., HIV) replicate?
RNA → DNA via reverse transcriptase.
Viral DNA integrates into host genome (provirus).
Host machinery transcribes viral genes for new virions.
Explain the difference between acute and persistent viral infections.
Acute: Rapid onset, short duration (e.g., influenza).
Persistent: Long-term infection (e.g., HIV latency, herpesviruses).
What is a zoonotic virus? Provide an example.
Viruses transmitted from animals to humans (e.g., SARS-CoV-2 from bats, Ebola from primates).
How do vaccines work? Name two types of viral vaccines.
Mechanism: Stimulate immune memory without causing disease.
Types:
Live-attenuated (e.g., measles vaccine).
mRNA (e.g., COVID-19 Pfizer/Moderna vaccines).
Why are antiviral drugs (e.g., Tamiflu) less common than antibiotics?
Viruses use host cell machinery, making it harder to target without harming host cells.
What are prions, and how do they cause disease?
Prions: Misfolded proteins that induce normal proteins to misfold (e.g., Creutzfeldt-Jakob disease, mad cow disease).
Mechanism: Accumulation of prions disrupts neural function.
How do viroids differ from viruses?
Structure: Viroids are circular RNA molecules without a protein coat.
Hosts: Infect plants (e.g., potato spindle tuber viroid).
How does HIV evade the immune system?
High mutation rate: Reverse transcriptase errors create diverse viral strains.
Latency: Hides in CD4+ T cells as provirus.
Why is influenza a recurring global health threat?
Antigenic drift: Minor mutations in surface proteins (yearly epidemics).
Antigenic shift: Reassortment of RNA segments (pandemics, e.g., H1N1).
Why can’t viruses be treated with antibiotics?
Antibiotics target bacterial structures (e.g., cell walls), which viruses lack.
Design a public health strategy to reduce viral zoonosis risk.
Monitor wildlife markets for emerging pathogens.
Educate communities on safe animal handling.
Develop rapid diagnostic tools for early detection.
