Chapter 13: Viruses, Viroids, and Prions

What are the two ways that viruses can multiply?

1. Lytic cycle: host cell dies 2. Lysogenic cycle: host cell will remain alive

Viral species

A group of viruses sharing the same genetic information and ecological niche (host range)

Bacteriophages

Also called phages; viruses that infect bacteria

Prion

An infectious protein; causes neurological diseases; spongiform encephalopathies: large vacuoles develop in brain; runs in families, but not purely inherited; mad-cow, kuru

Herpes viridae

Around 100 types; cold sores; herpes

How are viral species designated?

By descriptive common names

How are viruses classified?

By their nucleic acid and by differences in the structures of their coats

True or False? Most viruses can infect many types of cells.

False. Most viruses can only infect specific types of cells of only one host species.

True or False? Viruses contain both DNA and RNA.

False. Viruses only contain a single type of nucelic acid, either DNA or RNA.

Polyhedral viruses

Many-sided virus; shape of an icosahedron; 20 triangular faces, 12 corners

What is the difference between primary cell lines and continuous cell lines?

Primary cell lines are derived from tissue slices and tend to die out after only a few generations. Continuous cell lines occur when viruses are routinely grown in a lab. These transformed cells can be maintained through and indefinite number of generations (immortal cell lines)

Capsid

Protein coat surrounding and protecting the nucleic acid

Capsomeres

Protein subunits that make up the capsid; in some viruses the proteins composing the capsomeres are of a single type, in others they are composed of several different types

Poxviridae

Pus-filled lesions; small pox

How do some viruses escape antibodies?

Regions of their genes that code for their surface proteins are susceptible to mutation

Helical viruses

Resemble long rods; may be rigid or flexible; viral nucleic acid found within a hollow cylindrical capsid

Cytopathic effect (CPE)

Cell deterioration

What are 3 methods commonly used for culturing animal viruses?

1. In living animals 2. In embryonated eggs 3. In cell cultures

What are the stages of multiplication of animal viruses?

1. Attachment 2. Entry 3. Uncoating 4. Transcription and translation of a portion of viral DNA 5. Synthesis of capsid proteins 6. Capsid proteins migrate into nucleus of host cell and maturation occurs 7. Release

What are the 5 steps of the lytic cycle?

1. Attachment: Phage attaches to host cell 2. Penetration: Phage injects its DNA into the host cell 3. Biosythesis: Cut up host DNA, direct synthesis of viral components 4. Maturation: Viral components are assembled into virons 5. Release (lysis): Host cell is lysed and new virons are released to infect other cells

What different variations can a virus's nucleic acid have?

1. DNA or RNA 2. Single-stranded or double-stranded 3. Linear or circular 4. Size - few thousand up to 250,000 nucleotides

What are 3 ways viral growth can be signaled in an embryonated egg?

1. Death of the embryo 2. Embryo cell damage 3. Formation of typical pocks or lesions on the membrane

What are the 4 ways multiplication of animal viruses different from the basic bacteriophage multiplication?

1. Differ in their mechanism of entering the cell 2. Synthesis and assembly of new viral components are different due to the differences between prokaryotic cells and eukaryotic cells 3. Mechanisms of maturation and release are different 4. Effects on the host cell differs

What are 2 reasons why one would want to culture a virus in an embryonated egg?

1. Fairly convenient 2. Inexpensive

Why have cell cultures replaced embryonated eggs as the preferred type of growth medium for many viruses?

1. Generally very homogeneous collections of cells 2. More convenient to work with

What are the 4 kinds of general morphology of viruses?

1. Helical 2. Polyhedral 3. Enveloped 4. Complex

What are the 5 steps of the lysogenic cycle?

1. Phage attaches to host cell 2. Phage DNA penetrates host cell 3. Phage DNA circularizes (at this point it can enter the lytic or lysogenic cycle) 4. Once it has entered the lysogenic cycle it is called a prophage and can stay in this stage for many generations 5. Eventually it will enter back into the lytic cycle

What are 3 reasons one would need to culture a virus in a living animal?

1. Some animal viruses can only be cultured in living animals 2. Most experiments to study the immune system's response to viral infections must be performed in virally infected live animals 3. May be used as a diagnostic procedure for identifying and isolating a virus from a clinical specimen

Virion

A complete, fully developed, infectious viral particle composed of nucleic acid and surrounded by a protein coat outside of a host cell, and is a vehicle of transmission from one host cell to another

Plant viruses

Can cause color change, deformed growth, wilting, stunted growth; plant viruses must enter through wounds or be assisted by other plant parasites because plant cells are protected by cell wall; infected plants can spread to other plants by pollen

Enveloped viruses

Capsid covered with an envelope; roughly spherical

Spikes

Carbohydrate-protein complexes that project from the surface of the envelope; these can help some viruses attach to a host

Adenoviridae

Causes acute respiratory diseases, like the common cold

Complex viruses

Complexly structured virus; particularly bacterical viruses; some bacteriophages have capsids to which additional structures are attached

Envelope

Covers the capsid; usually consists of some comnination of lipids, proteins, and carbs; not all viruses have envelopes

Diploid cell lines

Developed from human embryos and can be maintained for about 100 generations; widely used for culturing viruses that require a human host

How does one culture a virus in an embryonated egg?

Drill a hole in the shell, inject a viral suspension or suspected virus-containing tissue into the fluid of the egg; several membranes in egg so virus is injected into the one most appropriate for its growth

Viroid

Infectious RNA; plant disease

Host Range

The spectrum of host cells a virus can infect

What is the structure of the capsid determined by?

The viral nucleic acid

What determines a virus's host range?

The virus's requirements for its specific attachment to the host cell as well as the availability within the potential host of cellular factors required for viral multiplication

How are cell culture lines started?

Treat slice of animal tissue with enzymes that separate the individual cells, suspend cells in solution that provides osmotic pressure, nutrients, and other growth factors, normal cells will adhere to container and reproduce to form a monolayer

True or False? Viruses contain a protein coat that surrounds the nucleic acid.

True.

True or False? The capsid usually makes up most of the mass of the virus

True. Especially in smaller viruses

True or False? Most drugs that interfere with viral multiplication also interfere with the functioning of the host cell.

True. These drugs are toxic for clinical use.

When are viruses considered alive?

Viruses are only considered alive when they are in their host cells.

How do viruses multiply?

Viruses must take over the metabolic machinery of the host cell in order to multiply.

Nonenveloped viruses

Viruses whose capsids are not covered by an envelope; the capsid of a nonenveloped virus is what protects the nucleic acid from nuclease enzymes in biological fluids and promotes the virus's attachment to susceptible host cells

Papovaviridae

Warts and tumors; papilloma virus can cause uterine cancer

When a host has been infected by a virus, the host immune system is stimulated to produce ___________, the interaction between the host's ___________ and virus proteins should ____________the virus and stop the infection.

antibodies; antibodies; inactivate

For a virus to infect the host cell, the ____________ of the virus must chemically interact with the specific _____________ on the surface of the cell

outer surface; receptor sites

The receptor site can be ___________ or _____________. For animal viruses, the receptor sites are on the ________________ of the host cell.

part of the cell wall of the host; part of the fimbriae or flagella; plasma membrane of the host cells

Antibodies

proteins that react and potentially inactivate the surface proteins of the virus