Chapter 13: Viruses, Viroids, and Prions

Are viruses considered living or nonliving?

They can be seen both ways; as complex nonliving chemicals or simple living microbes depending on one’s viewpoint.

What kind of genetic material do viruses contain?

A virus has only one type of nucleic acid (either DNA or RNA, not both)

What surrounds a virus’s nucleic acid?

a protein coat, and sometimes an envelope made of lipids, proteins, and carbohydrates.

Why are viruses called “obligatory intracellular parasites”?

Because they can only reproduce inside a host cell, using the host’s machinery to make new viral parts.

How do viruses multiply?

By hijacking the host cell’s synthesizing machinery to make new viral components that spread the viral genetic material to other cells.

What does host range mean?

It’s the range or spectrum of host cells that a virus can infect and multiply in.

Do viruses infect all types of cells?

No. Most viruses infect only specific types of cells within a single host species.

What determines a virus’s host range?

The specific receptor sites on the host cell’s surface and the availability of host cell factors the virus needs to reproduce.

How is the size of a virus measured?

By using an electron microscope, since viruses are too small to be seen with a light microscope.

What is the general size range of viruses?

range from ~ 20 nanometers (nm) to 300 nanometers (nm) in length.

What is a virion?

A complete, fully developed virus particle that can infect a host cell.

What are the main components of a virion?

Nucleic acid (DNA or RNA) surrounded by a protective protein coat.

What kind of nucleic acid do viruses contain?

: Viruses contain either DNA or RNA — never both.

What forms can viral nucleic acid take?

It can be single-stranded or double-stranded, linear or circular, or divided into several separate molecules.

What is the proportion of nucleic acid to protein in viruses?

It ranges from about 1% to 50% of the total viral weight.

What is a capsid?

The protein coat that surrounds a virus’s nucleic acid.

What are capsomeres?

Subunits of the capsid, made of one or more types of protein.

What is a viral envelope?

A lipid, protein, and carbohydrate layer that surrounds the capsid in some viruses.

What are spikes on a virus?

Carbohydrate-protein complexes on the envelope that help the virus attach to host cells.

What is the shape of helical viruses?

They are hollow cylinders that surround the nucleic acid.

What is the shape of polyhedral viruses?

They are many-sided.

What are enveloped viruses like?

They are covered by an envelope, roughly spherical, and can be highly pleomorphic (variable in shape).

What are complex viruses?

Viruses with complex structures, like many bacteriophages that have a polyhedral head and a helical tail.

On what basis are viruses classified?

By the type of nucleic acid they contain and their replication strategy.

How are virus family and genus names formatted?

Family names end in -viridae, and genus names end in -virus.

What defines a viral species?

A group of viruses that share the same genetic information and occupy the same ecological niche.

Where must viruses be grown?

In living cells, because they cannot replicate on their own.

Which viruses are the easiest to grow?

Bacteriophages (viruses that infect bacteria).

What is the plaque method for growing bacteriophages?

Bacteriophages are mixed with host bacteria and nutrient agar, allowing the virus to infect the bacteria.

What is a plaque?

An area of bacterial destruction caused by viral replication around the original virus.

What does each plaque represent?

A single viral particle, and virus concentration is measured in plaque-forming units (PFU).

What do some animal viruses require for cultivation?

Whole animals.

Which animal models are used to study human AIDS?

Simian AIDS (monkeys) and feline AIDS (cats).

Can animal viruses be grown in eggs?

Yes, some animal viruses can be cultivated in embryonated eggs.

What are cell cultures?

Animal or plant cells growing in culture media outside the organism.

What happens when viruses grow in cell cultures?

They can cause cytopathic effects — visible damage to the cultured cells.

What methods are commonly used to identify viruses?

Serological tests, RFLPs (Restriction Fragment Length Polymorphisms), and PCR (Polymerase Chain Reaction).

Do viruses have enzymes for energy production or protein synthesis?

No, viruses lack these enzymes.

How do viruses multiply?

They invade a host cell and use the host’s metabolic machinery to make viral enzymes and components.

What happens during the lytic cycle of a bacteriophage?

The phage lyses and kills the host cell to release new viruses.

What is lysogeny?

When a virus incorporates its DNA into the host genome as a prophage, which can replicate with the host without killing it.

What occurs during the penetration stage of the lytic cycle?

Phage lysozyme opens the bacterial cell wall, the tail sheath contracts, the DNA enters the cell, and the capsid stays outside.

What happens during biosynthesis of a bacteriophage?

Phage DNA is transcribed into mRNA, coding for proteins and enzymes needed for viral multiplication; phage DNA is replicated, and capsid proteins are made.
Eclipse period: phage DNA and proteins exist separately inside the host.

What happens during maturation?

Phage DNA and capsids are assembled into complete virus particles.

What happens during release?

Phage lysozyme breaks down the bacterial cell wall, releasing new phages.

How are prophage genes regulated in the lysogenic cycle?

By a repressor protein coded by the prophage, keeping the virus dormant while it replicates with the host.

What are some effects of lysogeny on host cells?

Lysogenic cells are immune to reinfection by the same phage and may undergo phage conversion (expressing new traits from the prophage).

What is transduction?

A process where a lysogenic phage transfers bacterial genes from one cell to another.

Generalized transduction:

any bacterial gene can be transferred.

Specialized transduction

only specific bacterial genes are transferred.

How do animal viruses attach to host cells?

They attach to specific receptors on the plasma membrane of the host cell.

How do animal viruses enter host cells?

By receptor-mediated endocytosis or fusion with the cell membrane.

How are animal viruses uncoated?

Viral or host cell enzymes remove the capsid, releasing the viral genome.

Where does DNA virus replication occur?

Most DNA viruses release their DNA into the nucleus, where transcription and translation produce viral DNA and capsid proteins. Capsid proteins are synthesized in the cytoplasm.

Where does RNA virus replication occur?

In the cytoplasm using RNA-dependent RNA polymerase to make double-stranded RNA.

How are new animal viruses released?

after assembly, viruses are released either by:

• Budding, which forms the envelope for enveloped viruses

• Rupture of the host cell membrane for nonenveloped viruses

When was the first relationship between viruses and cancer demonstrated?

In the early 1900s, when chicken leukemia and sarcoma were transferred to healthy animals using cell-free filtrates.

What is a proto-oncogene?

A gene that encodes a protein involved in normal cell growth.

What happens when a proto-oncogene is mutated or activated?

It becomes an oncogene, which can transform normal cells into cancerous cells.

What are oncogenic viruses?

Viruses that are capable of producing tumors.

Which types of viruses can be oncogenic?

Several DNA viruses and retroviruses.

How do oncogenic viruses affect host cells?

Their genetic material integrates into the host cell DNA.

What are characteristics of transformed cells?

They contain tumor-specific transplantation antigens (TSTA), show chromosome abnormalities, and can produce tumors when injected into susceptible animals.

Which virus families include DNA oncogenic viruses?

Adenoviridae, Herpesviridae, Poxviridae, Polyomaviridae, and Hepadnaviridae.

How do RNA oncogenic viruses cause tumors?

They use reverse transcriptase to convert viral RNA into DNA, which is then integrated as a provirus into the host cell’s DNA.

What are oncolytic viruses?

Viruses that infect and lyse cancer cells, used in cancer therapy.

What is a latent viral infection?

A virus that remains in the host cell for long periods without causing disease.

Give examples of latent viral infections.

Cold sores and shingles.

What is a persistent viral infection?

A virus infection where viral levels and symptoms build up gradually over a long period; usually fatal.

What causes many persistent viral infections?

conventional viruses.

How do plant viruses enter their hosts?

Through wounds or via invasive parasites like insects.

Can plant viruses multiply in insects?

yes, some multiply in insect (vector) cells.

What are viroids?

Infectious pieces of RNA that cause certain plant diseases.

What are virusoids?

Viroids enclosed in a protein coat.

What are prions?

Infectious proteins first discovered in the 1980s.

What do prion diseases affect?

They cause degeneration of brain tissue.

How do prion diseases occur?

From an altered protein, either due to a mutation in the normal gene or contact with an altered protein.