Chapter 13 Viruses, Viroids, and Prions

Virus

Contain a single type of nucleic acid, either DNA or RNA.

Contain a protein coat

Multiply inside living cells

Acellular

Obligatory Intracellular Parasites

Absolutely require living host cells in order to multiply

Host Range

The spectrum of host cells the virus can infect.

Bacteriophages

Also known as a phage. Viruses that infect bacteria

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.

Capsid

The protein coat of a virus that surrounds the nucleic acid

Capsomere

A protein subunit of a viral capsid

Envelope

An outer covering surrounding the capsid of some viruses

Spikes

A carbohydrate-protein complex that projects from the surface of

certain viruses

Nonenveloped Viruses

Viruses whose capsids are not covered by an envelope

General Morphology

Helical Viruses

Polyhedral Viruses

Enveloped Viruses - Enveloped Helical Viruses, Enveloped Polyhedral Viruses

Complex Viruses

Helical Viruses

Resemble long rods that may be rigid or flexible. The viral nucleic acid is found within a hollow, cylindrical capsid that has a helical structure. The viruses that cause rabies and Ebola hemorrhagic fever are helical viruses

Polyhedral Viruses

Many animal, plant, and bacterial viruses are polyhedral, or many-sided, viruses. The capsid of most polyhedral viruses is in the shape of an icosahedron,a regular polyhedron with 20 triangular faces and 12 corners. The capsomeres of each face form an equilateral triangle. An example of a polyhedral virus in the shape of an icosahedron is the adenovirus. Another icosahedral virus is the poliovirus.

Complex Viruses

A virus with a complicated structure, such as a bacteriophage

Viral Species

Group of viruses sharing the same genetic information and ecological niche (host range). Designated by descriptive common names, such as human immunodeficiency virus (HIV), with subspecies (if any) designated by a number (HIV-1)

What is the chemical composition of a capsid?

Protein

What is the nucleic acid in a virus?

DNA or RNA

What is the chemical composition of a capsomere?

Protein/peptide

What is the value of a capsid to a virus?

Protects the nucleic acid from nucleases and promotes the virus's attachment to susceptible host cells

What is the result of the lytic cycle?

Lysis of the host cell and release of phages

How does lysogeny differ from the lytic cycle?

In lysogeny, phage genes are in the host chromosome and phages are not being made

Of what is a viral envelope composed?

Phospholipids from the host plasma membrane; may include viral encoded peptides

Oncolytic

Viruses may selectively infect and kill tumor cells or cause an immune response against tumor cells.

Phage Therapy

Using bacteriophages to treat bacterial infections

One-step Growth Curve

Theoretical. Shows how many virus particles come out ("yield per cell") from one going in. Also shows period of time between these events ("eclipse period").

Lytic Cycle

A mechanism of phage multiplication that results in host cell lysis. Five (5) stages - attachment, penetration, biosynthesis, maturation, and release

Lysogenic Cycle

Stages in viral development that result in the incorporation of viral DNA into host DNA. If the bacteriophage does not lyse its host cell, it becomes integrated into the bacterial genome in a harmless form (provirus), lying dormant for one or more generations. The virus may stay integrated indefinitely. However, either spontaneously or as a result of environmental factors, the provirus can reemerge and enter a lytic cycle.

Phage Lysozyme

Enzyme that breaks down a portion of the bacterial cell wall

Eclipse Period

Period during viral multiplication when complete, infective virions are not yet present

Lysis

Rupture, breaks open

Lysogeny

A state in which phage DNA is incorporated into the host cell

Lysogenic Conversion

The acquisition of new properties by a host cell infected by a lysogenic phage

Prophage

DNA of a lysogenic phage that has integrated into the host cell chromosome

Phage Conversion

Host cell may exhibit new properties due to lysogenic phage where DNA of a lysogenic phage that has integrated into the host cell chromosome

Specialized Transduction

The process of transferring a piece of cell DNA adjacent to a prophage to another cell. Part of the lysogenic cycle.

Multiplication of Animal Viruses

Attachment, Entry (receptor-mediated endocytosis or fusion), Uncoating, Biosynthesis, Maturation, and Release

Receptor-mediated Endocytosis

A type of pinocytosis in which molecules bound to proteins on the plasma membrane are taken in by infolding of the membrane

Fusion

The merging of plasma membranes of two different cells, resulting in one cell containing cytoplasm from both original cells

Budding

The process by which the envelop develops around the capsid

Uncoating

The separation of the viral nucleic acid from its protein coat once the virion is enclosed within the vesicle

Sarcoma

Cancer of connective tissue

Adenocarcinomas

Virus induced cancers of glandular epithelial tissue

Oncogenes

Cancer-causing alterations to cellular DNA affect parts of the genome; a gene that can bring about malignant transformation

Oncogenic Viruses

(or oncoviruses) A virus that is capable of producing tumors

Transformation

(1) The process in which genes are transferred from one bacterium to another as "naked" DNA in solution. (2) The changing of a normal cell into a cancerous cell.

Latent Infection

A condition in which a pathogen remains in the host for long periods without producing disease

Chronic Viral Infection

An illness that develops slowly and is likely to continue or recur for long periods

Cellular Prion Protein

Normal host glycoprotein called PrPC

Persistent Viral Infection

A disease process that occurs gradually over a long period

Viroids

Infectious RNA; short pieces of naked RNA with no protein coat

Another term for a lysogenic phage is?

Temperate phage

The type of virus implicated in the cause of AIDS is a(n)?

Retrovirus

What part of the phage is absorbed in the host cell?

Tail

How does a phage form a hole in the cell wall which allows the phage to drive the tail core through the cell wall?

Phage lysozyme

What is the usual size range of viruses?

30 to 300 nm

Which two viruses make DNA from an RNA template?

Hepadnavirudae and Retroviridae

Prions cause disease by

Altering normal proteins

What type of infectious agent causes potato spindle tuber disease?

Viroid

Viruses possess genetic material comprised of

DNA or RNA

Members of the adenoviridae cause

The common cold

An example of a latent virus infection is

Shingles

Viruses that use RNA as a template for transcribing DNA include

Retroviridae

A method by which a virus enters an animal host cell?

Endocytosis

A bacterial virus?

Phage

A term meaning cancer-causing?

Oncogenic

Observable changes in a virus-infected cell?

Cytophatic effect

RNA to DNA?

Reverse Transcription

The ability of a virus to infect an organism is regulated by

The host species

The type of cells

The availability of an attachment site

Cell factors necessary for viral replication

The lytic cycle of a virus

Ends with lysis and death of the host cell

Viruses that alter cell DNA to make the host cell cancerous are called

Oncogenic viruses

A viral infection that occurs gradually over a long period of time and is usually fatal is

Persistent viral infection

When viral DNA that has inserted itself into the bacterial host chromosome it is known as?

Prophage

Transformed Cell

A cell that has been genetically changed to a tumor cell and that passes the change to its daughter cells

Antigenetic Shift

Major changes in the viral genome; genetic material from different strains is combined in a process called reassortment. Reassortment takes place when a host is simultaneously infected with two different strains