Lecture Exam 2 Viruses, influenza, and metabolism

Are viruses considered alive or dead?

They are described as active or inactive rather than alive or dead because

they are noncellular infectious particles rather than independent organisms

What does it mean that viruses are "obligate intracellular parasites"?

They cannot multiply unless they invade a specific host cell and hijack its

genetic and metabolic machinery

What are the basic components that make up a virus?

o Capsid: The protective protein shell surrounding the nucleic acid.

o Nucleic Acid Core: Contains either DNA or RNA (never both).

o Nucleocapsid: The capsid and nucleic acid combined.

o Envelope: A modified piece of the host cell membrane surrounding the

capsid (found only in enveloped viruses, absent in naked viruses).

o Spikes: Glycoprotein projections on the surface that allow viruses to dock

with specific host cell receptors

What is a virion?

A fully formed, intact virus particle capable of establishing an infection in a

host cell

How do capsomeres assemble, and what are the primary structural shapes?

Capsomeres are identical protein subunits that self-assemble into helical

(continuous helix), icosahedral (20-sided geometric figure), or complex (non-

symmetrical, e.g., bacteriophages) capsids

What is the difference between positive-sense and negative-sense RNA?

Positive-sense RNA (+ssRNA): Genomes ready for immediate translation

into viral proteins by host ribosomes.

o Negative-sense RNA (-ssRNA): Genomes that must first be converted into a

complementary positive strand before protein synthesis can occur

Where do DNA vs. RNA viruses typically replicate in animal cells?

DNA viruses: Generally replicate in the host cell nucleus.

o RNA viruses: Generally replicate in the host cell cytoplasm

What specialized enzymes do certain viruses carry?

Polymerases: Synthesize DNA or RNA.

o Replicases: Copy RNA genomes.

o Reverse Transcriptase: Synthesizes DNA from an RNA template (e.g.,

retroviruses)

What are the general stages of animal virus replication?

1. Adsorption: Specific attachment to host cell receptors.

2. Penetration: Entry into the cell via endocytosis or membrane fusion.

3. Uncoating: Enzymatic removal of the capsid to release nucleic acid into the

cytoplasm.

4. Synthesis: Replication of viral nucleic acid and production of viral proteins.

5. Assembly: Packaging of viral nucleic acid into capsids.

6. Release: Departure from the host cell via budding (exocytosis) or cell lysis

What defines host range?

The specific spectrum of hosts/cells a virus can infect, dictated by exact

receptor matching:

▪ Restricted: Infects very specific cells (e.g., Hepatitis B only in human

liver cells).

▪ Intermediate: Infects a moderate range (e.g., Poliovirus in primate

intestinal/nerve cells).

▪ Broad: Infects diverse tissues across species (e.g., Rabies in various

mammalian cells)

What are Cytopathic Effects (CPEs)?

Microscopic virus-induced changes to host cells, including cell shape

alteration, formation of inclusion bodies (compacted viral

masses/damaged organelles), or syncytia (fusion of multiple host cells into

a multinucleated giant cell

What is the difference between the Lytic Cycle and the Lysogenic Cycle?

Lytic Cycle: Rapid replication ending in host cell destruction and lysis.

o Lysogenic Cycle: Temperate phage DNA integrates into the host

chromosome as an inactive prophage, replicating passively with the host

until induction triggers entry into the lytic cycle.

What is lysogenic conversion?

When a bacterium acquires new traits (such as toxin production) from a

temperate phage prophage (e.g., diphtheria, cholera, and botulinum toxins)

What are prions and how do they differ from viruses?

Prions are infectious protein particles containing no nucleic acid. They cause

spongiform encephalopathies (e.g., Creutzfeldt-Jakob Disease, Scrapie,

BSE) marked by protein fibril deposition and brain degeneration

What are satellite viruses?

Dependent viruses that require helper viruses (or cell disruption) to replicate

(e.g., Adeno-associated virus)