ch 13. Viruses, Viroids, and Prions

Obligatory intracellular parasites

require living host cells to multiply

Bacteriophages

viruses that infect bacteria

Animal virus

receptor sites are typically on the plasma membrane

virion

complete, fully developed viral particle

capsid

protein coat made of capsomeres (subunits)

Envelope

lipid, protein, and carbohydrate coating on some viruses

spikes

projections from outer surface

Helical viruses

hollow, cylindrical capsid that is helical (rabes; Ebola)

polyhedral viruses

many sided; Most are an icosahedron (20 triangular facets and 12

corners); adenoviruses and poliovirus

enveloped viruses

roughly spherical

complex viruses

complicated structures (bacteriophage)

Viral species

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

plaques (bacteriophages)

clearing on a lawn of bacteria on the surface of agar

plaque forming units (PFU)

Each plaque corresponds to a single virus; can be expressed

ELISA

virus is detected and identified by its reaction

with antibodies

lytic cycle

phage causes lysis and death of the host cell (Tequatrovirus)

Lysogenic cycle

phage DNA is incorporated in the host DNA, phage conversion, specialized transduction

Attachment

phage attaches by the tail fibers to the host cell

Penetration

phage lysozyme opens the cell wall; tail sheath contracts to force the tail core and DNA into the cell (similar to a hypodermic syringe)

Biosynthesis

production of phage DNA and proteins; Host cell protein synthesis is halted

Maturation

assembly of phage particles

Release

Phage lysozyme breaks the cell wall

prophage

inserted DNA phage

Phage conversion

the host cell exhibits new properties, encoded by the prophage DNA ( production of diphtheria toxin by Corynebacterium diphtheriae)

Specialized transduction

Specific bacterial genes transferred to another bacterium via a phage (Changes genetic properties of the recipient bacteria)

Entry (animal virus)

receptor-mediated endocytosis or fusion

uncoating

(separating the viral nucleic acid from its capsid) by viral or host enzymes; or envelope if present

Biosynthesis: production of nucleic acid and proteins

production of nucleic acid and proteins

Maturation:

nucleic acid and capsid proteins assemble

ssRNA; + (sense) strand

Viral RNA serves as mRNA for protein synthesis

ssRNA -(antisense) strand

Viral RNA is transcribed to a + strand to serve as mR NA for protein synthesis

dsRNA

double stranded RNA

Sarcoma

cancer of connective tissue

Adenocarcinomas

cancers of glandular epithelial tissue

Proto-oncogenes:

genes that encode proteins involved in stimulating normal cell growth; Mutated proto-oncogenes become oncogenes

Oncogenes

transform normal cells into cancerous cells

Transformation:

cells acquire distinct properties leading to cancer

Oncogenic viruses

become integrated into the host cell’s DNA and induce tumors

A transformed cell harbors

a tumor-specific transplantation antigen (TSTA) on the surface and are often irregularly shaped

Oncolytic viruses:

infect and kill tumor cells or cause an immune response against tumor cells

Latent virus

remains in asymptomatic host cell for long periods; all herpesvirus, may reactivate due to changes in immunity

Varicellovirus

Shingles

persistent viral infections

occurs gradually over a long period; is generally fatal (measles virus)

Plant viruses:

enter through wounds or via insects; plants cells are protected from diseases by their cell walls; wounds cause diseases

Viroids

short pieces of naked RNA

virusoids

viroids enclosed in a protein coat; only cause disease when plant cell is coinfected with a virus; need another virus

prions

Proteinaceous infectious particles

PrP^c:

normal cellular prion protein, on the cell surface

PrP^sc:

scapie protein; accumulates in brain cells, forming plaques

Disease is caused by the conversion of a normal host glycoprotein

PrP^c

an infectious misfolded form

PrP^sc