Chapter 13 Study Quiz

What are viruses?

Minuscule, acellular infectious agents that contain either DNA or RNA and require a host to replicate.

What are the key traits of viruses?

No metabolism, no growth, no independent reproduction, acellular, use host's cellular machinery.

What is a virion?

The extracellular form of a virus, consisting of a nucleic acid and protein coat (capsid); some have envelopes.

What happens during the intracellular state of a virus?

Capsid is removed; virus exists as nucleic acid inside the host cell.

What forms can viral genomes take?

DNA or RNA, single or double-stranded, linear, segmented, or circular.

How do viruses differ in host range?

Some are specific to one cell type or species; generalists infect many types of cells or hosts.

What is a capsid?

A protein coat that protects the viral genome and helps with attachment to host cells.

What are capsomeres?

Protein subunits that make up the capsid.

What are the basic shapes of viruses?

Helical, polyhedral, and complex.

What is a viral envelope?

A phospholipid bilayer with proteins, acquired from the host cell during replication or release.

What are glycoprotein spikes on viruses used for?

Host cell recognition and attachment.

Are enveloped viruses more or less stable than naked viruses?

Less stable—envelopes are fragile and sensitive to environmental conditions.

How are viruses classified?

By nucleic acid type, presence of an envelope, shape, and size.

What is the lytic replication cycle?

Virus replication that ends in host cell lysis; involves attachment, entry, synthesis, assembly, and release.

What are temperate phages?

Bacteriophages that can enter a lysogenic cycle instead of immediately lysing the host.

What is lysogeny?

A viral replication strategy where the virus integrates into host DNA and remains dormant as a prophage.

What is lysogenic conversion?

A change in the phenotype of a host due to expression of a prophage gene.

How do animal virus replication cycles differ from bacteriophages?

Due to presence of envelopes, eukaryotic cells, and lack of cell wall.

What methods do animal viruses use to enter host cells?

Direct penetration, membrane fusion, or endocytosis.

What is direct penetration?

Viral capsid attaches and sinks into the cytoplasmic membrane thus creating a pore.

What is membrane fusion?

Viral envelope and the host’s cytoplasmic membrane fuse.

What is the process of entry into animal cells called endocytosis.

attachment of the virus to receptor molecules on cells surface triggers cell to endocytize the entire virus.

How do DNA viruses replicate in animal cells?

Most replicate in the nucleus; proteins are synthesized in the cytoplasm.

How are dsDNA viruses synthesized in animals?

Similar to the replication of cellular DNA

How are ssDNA viruses synthesized in animals?

These cells do not use ssDNA so the DNA must fold back to form ddDNA in order to be synthesized

How do RNA viruses replicate in animal cells?

Replication occurs in the cytoplasm; strategies depend on whether RNA is +ssRNA, –ssRNA, dsRNA or retrovirus.

What are retroviruses?

+ssRNA viruses that use reverse transcriptase to convert RNA into DNA.

What is the function of reverse transcriptase?

To synthesize DNA from an RNA template in retroviruses.

How are new animal viruses assembled and released?

DNA viruses assemble in the nucleus, RNA viruses in cytoplasm; enveloped viruses release by budding, naked viruses by exocytosis or lysis.

What are latent viruses (proviruses)?

Animal viruses that remain dormant in host cells and may integrate into host DNA.

Can proviruses be removed from host DNA?

No—they become a permanent part of the genome.

Why is it difficult to treat viral diseases?

Because viruses use the host's cellular pathways, disrupting viruses risks harming host cells.

What is the relationship between viruses and cancer?

Viruses can carry oncogenes or interfere with tumor suppressor genes, causing neoplasia and metastasis.

What are examples of cancers caused by viruses?

Burkitt’s lymphoma, Hodgkin’s disease, Kaposi’s sarcoma, cervical cancer (HPV).

What are protooncogenes?

Genes that promote normal cell growth; when mutated or overactive, can cause cancer.

What environmental factors contribute to oncogene activation?

UV light, radiation, carcinogens, viruses.

How are viruses cultured in the lab?

In mature organisms, embryonated eggs, or cell cultures.

What are viral plaques?

Zones of bacterial cell lysis on agar plates used to estimate phage concentration.

Why are embryonated chicken eggs used to culture viruses?

They're large, sterile, and nutrient-rich—ideal for growing viruses.

What are cell cultures?

Cells grown in the lab; used to support viral replication.

What are the two types of cell cultures?

Diploid (normal) and continuous (from tumors; immortal).

Are viruses alive?

Controversial: they replicate and evolve but lack independent metabolism and cell structure.

What are viroids?

Tiny, circular ssRNA molecules that infect plants; lack a capsid and do not encode proteins.

How do viroids cause disease?

They bind to plant mRNA, which is then degraded by host enzymes, disrupting gene expression.

What are prions?

Infectious proteins with abnormal folding (β-sheets) that induce misfolding in normal proteins.

What is cellular PrP?

A normal brain protein with alpha-helical structure, involved in neural function.

What is prion PrP?

The misfolded, disease-causing form of PrP with β-pleated sheets.

What is templating in prion diseases?

Prion PrP acts as a template to convert cellular PrP into prion form.

What are prion diseases?

Spongiform encephalopathies causing brain damage, e.g., mad cow disease, scrapie, CWD, vCJD, kuru.

How are prion diseases transmitted?

Ingestion, transplantation, or contact with infected tissue.

Can prions be destroyed by cooking?

No—prions resist standard sterilization and heat treatments.

How are prions inactivated?

By incineration (482°C), autoclaving in sodium hydroxide, or specialized enzyme treatments.

Is there a treatment for prion diseases?

No—currently there is no standard cure.