Chapter 13: Characterizing and Classifying Viruses, Viroids,and Prions

Viruses, Viroids, and Prions

Acellular disease causing agents, which lack cell structure and connot metabolize, grow, reproduce, or respond to their environment.

Virus

a tiny infectious agent with nucleic acid surrounded by proteinaceous capsomeres that form a coat called a capsid. It exists in an extracellular state and an intracellular state.

Virion

is a complete viral particle, including a nucleic acid and a capsid, outside of a cell.

Viruses

Their genomes include either DNA or RNA. Their genomes may be dsDNA, ssDNA, dsRNA, or ssRNA. They may exist as linear or circular and singular or multiple molecules of nucleic acidm depending on the type.

Lack a cell membrane

Bacteriophage (or phage)

a virus that infects a bacterial cell.

Virions

can have a membranous envelope or be naked.

How are viruses classified?

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

International Committee on Taxonomy of Viruses (ICTV)

have recognized viral family and genus names. With the exception of three orders, higher taxa are not established.

Virus replication

depends on random contact with a specific host cell type

Five Stages of Virus replicaiton

Attachment

Entry

Synthesis

assembly

release

After attachement between virion and host cell is made

-the nucleic acids enters the cell.

with phages only the nucleic acid enters the cell, with animal viruses the entire virion often enters the cell, where the capsid is then removed in a process called uncoating.

Once inside the host cell

-viral nuceic acid directs synthesis of more viruses using metabolic enzymes and ribosomes of the host cell.

-assembly of synthesized virions occurs in the host cell, typically as capsomeres surround replicated or transcribed nucleic acids to form new virions.

The realease of new Virions from host cell

-new virions are released from the host cell either by lysis of the host cell (such as in phages or animal viruses) or by the extrusion of enveloped virions through the hosts cytoplasmic membrane (called budding), a process seen only with certain animal viruses. If budding continues over time, the infection is persistent. An envelope is derived from a cell membrane.

Iysogeny or Lysogenic cycle

Termperate (lysogenic) bacteriophages enter a bacterial cell and remain inactive

-These inactive phages are called prophages and are insterted into the chromosome of the cell and passed to its daughter cells.

Lysogenic conversion

results when phages carry genes that alter the phenotype of a bacterium.

At some point in the generations that follow, a prophage may be excised from the chromosome in a process known as induction. At that point the prophage again becomes a lytic virus.

Latency

An animal virus remains inactive in a cell, possibly for years. A latent virus is also know as a provirus. A provirus that has become incorporated into a hosts chromosome remains there.

dsDNA viruses

act like cellular DNA in transcription and replication. (with the exception of hepatitis B virus)

Retrovirus

-HIV

-ss+RNA viruses that carry an enzyme, reverse transcriptase, to transcribe DNA from their RNA.

-ssRNA Viruses

carry an RNA-dependent RNA transcriptase for transcribing mRNA from the -RNA genome so that protein can then be translated . Transcription of RNA from RNA is not found in cells.

When dsRNA (double stranded RNA) functions as a genome..

one strand of the RNA molecule functions as the genome, and the other strand functions as a template for RNA replication.

Neoplasia

uncontrolled cellular reproduction in a multicellular animal. Environmental factors or oncogenic viruses may cause neoplasia.

Tumor

Mass of neoplasmic cells

Benign/ Malignant

harmless/invasive (cancer)

Metastasis

spreading of malignant tumores.

Culturing viruses in the lab

viruses must be cultures inside whole organisms, in embryonated chicken eggs, or in cell cultures because viruses cannot metabolize or replicate alone.

Culturing bacteriophages

cultured on a lawn of bacteria on an agar plate. The phages lyse the bacteria creating clear areas called plaques on the bacterial lawn. Plaque assay enables the estimation of phage numbers.

Two types of virus cell cultures

-diploid cell cultures- lasts about 100 generations

-Continous cell cultures derived from cancer cells last longer.

Viroids

small circular pieces of RNA with no capsid that infect and cause disease in plants.

Prions

infectious protein particles that lack nucleic acids and replicate by converting similar normal proteins into new prions. Diseases caused my prions are spongiform encephalopathies, which involve fatal neurologic degeneration.

Are viruses alive?

Outside of cells, viruses do not appear to be alive, but within cells, they exhibit lifelike qualities such as the ability to replicate themselves.

Which of the following is not an acellular agent?

rickettsia

A virus that is specific for a bacterial host is called a

phage

A naked virus

has no membranous envelope

When a eukaryotic cell is infected with an enveloped virus and shreds viruses slowly over time, this infection is

called a persistant infection

Another name for a complete virus is

virion

Which of the following can be latent

HIV, chickenpox, herpes

Which of the following is not a criterion for specific family classification of viruses?

lipid composition (type of nucleic acid present, enveloped structure, capsid type are)

A clear zone of phage infection in a bacterial lawn is a

plaque