Chapter 13: Viruses, Viroids, and Prions

Viroid

Small, circular piece of RNA that infects plants.

Prions

Infectious proteins.

Virus

Acellular, infectious agent that can infect plants, animals, and bacteria.

Viral Characteristics

No nucleus, no cytoplasm, no organelles, obligate intracellular parasite, only contains one type of nucleic acid (either DNA or RNA).

Virion

What we call the virus when it is outside the cell; extracellular state.

Capsid

Protein coat that surrounds the nucleic acid of the virus.

Nucleocapsid

The viral genome (DNA or RNA) plus the capsid of a virus.

Envelope

Viruses may or may not acquire an outer phospholipid bilayer from the host cell of which disguises the virus from host cells.

Naked virus

Virus without an envelope.

Spikes

Glycoproteins on the envelope of the virus that aid in entry into the host cell.

Helical

Ribbon-like capsid shape of some viruses.

Polyhedral

Many-sided geometrical capsid shape of some viruses.

Icosadhedral

A polyhedral capsid shape of some viruses of which has exactly 20 sides; most common type of polyhedral shape.

Complex

Capsids of many different shapes other than helical and polyhedral.

Host range

The spectrum of hosts a virus can infect.

Viral specificity

The specific types of cells a virus can infect.

Steps of Viral Replication

1) Viral attachment, 2) Entry, 3) Synthesis, 4) Assembly, 5) Release

Bacteriophages

Viruses that infect bacteria.

Lytic Replication

Viral replication that ends in release of virus or host cell lysis.

Lysis

Rupture or splitting of a host cell to release viral particles.

Lysogenic Replication

Viral replication that is "dormant"; viral DNA enters and becomes part of the host cell DNA.

Prophage

When a bacteriophage becomes incorporated into host cell DNA.

Induction

Process in which a bacteriophage may be excised from the host cell DNA.

Mechanisms of Animal Viral Entry

Direct penetration, membrane fusion, or endocytosis.

Mechanisms of Animal Viral Release

Enveloped viruses - mostly by budding; Naked viruses - mostly by exocytosis or lysis of host cell.

Latency

When an animal virus becomes dormant inside host cells.

Provirus or latent virus

When an animal virus becomes incorporated into host cell DNA; cannot be excised from host DNA.

DNA Replication

Strands of DNA are used as templates to make new strands of DNA.

Transcription

A strand of DNA is used to make a complementary strand of RNA (mRNA).

Translation

mRNA is "decoded" to make protein

DNA viruses

Host uses viral DNA, transcribes it to make viral mRNA, followed by translation to make new viral proteins.

(+) Sense RNA viruses

The viral RNA acts like mRNA and thus is translated by the host cell to make new viral proteins.

(-) Sense RNA viruses

The viral RNA acts like DNA and thus is transcribed to make viral mRNA, followed by translation to make new viral proteins.

Retroviruses (+) sense RNA viruses

Produce the enzyme, reverse transcriptase, which can reverse transcription of mRNA thus creating viral DNA.

Scrapie

A neurological disease caused by prions of which is found in sheep.

Variant CJD

"Mad cow disease" in humans; a neurological disease in humans caused by prions of which is thought to be caused by feeding on remains of infected cattle.

Prp protein

Normal protein found on the plasma membrane of many mammalian cells, especially the brain; abnormal folding of these proteins can result in clumping, neuronal death, and holes in the brain tissue (prion infection).

Tumor

Mass of neoplastic cells.

Cancers

Malignant tumors.

Oncogenes

Cancer-causing genes.