CHAPTER 12 - Viruses, Viroids, and Prions (p2)

persistent viral infection

(or chronic viral infection) occurs gradually over a long period

conventional viruses

persistent viral infections are fatal and are caused by

subacute sclerosing panencephalitis (SSPE)

several years after causing measles, the measles virus can be responsible for a rare form of encephalitis called

detectable virions

in most persistent viral infections, — gradually builds up over a long period

cervical cancer, HIV, liver cancer, persistent enterovirus infections, progressive encephalitis, subacute sclerosing panencephalitis

example of persistent viral infections

caulimoviridae, bunyaviridae, virrgaviridae, rhabdoviridae, reoviridae

example of PLANT VIRUSES

insect cells

some plant viruses can multiply inside

plant virus

• cause color change, deformed growth, wilting, and stunted growth

include bean mosaic virus, wound tumor virus (corn and sugarcane) and potato yellow dwarf virus

protoplasts and in insect cell cultures

plant viruses are cultured in:

hyperplasia, hypoplasia, cell necrosis, abnormal growth patterns, discoloration

some common symptoms of plant viral diseases

hyperplasia

galls (tumors)

hypoplasia

thinned, yellow splotches on leaves

cell necrosis

dead, blackened stems, leaves, or fruit

abnormal growth patterns

malformed stems, leaves, or fruit

discoloration

yellow, red, or black lines, or rings in stems, leaves, or fruit

viroids

short pieces of naked RNA, only 300 to 400 nucleotides long, with no protein coat

virusoids

are enclosed in a protein coat

• cause disease only when the cell is infected by a virus

gene silencing

the viroid RNA is a ribozyme that cuts the continuous RNA into viroid segments; may cause disease by

introns

current research on viroids has revealed similarities between the base sequences of viroids and —

Hepatitis D

• may be caused by a virusoid

• is RNA enclosed in a protein coat and requires coinfection by Hepatitis B virus

satellite RNA

some researchers call HDV

Stanley Prusiner

proposed that prion (proteinaceous infectious particle) caused scrapie

prion

(proteinaceous infectious particle) caused scrapie, a neurological disease in sheep;

• often run in families, which indicates a possible genetic cause

proteases

scrapie can be reduced by treatment with

transmissible spongiform encephalopathies

prions cause 9 animal neurological diseases called —, that include the mad cow disease

kuru, Creutzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Scheinker syndrome, and fatal familial insomnia

the human diseases of prions are

mad cow disease

• arose from feeding scrapie-infected sheep meat to cattle

• the new (bovine) variant was transmitted to humans who ate undercooked beef from infected cattle

CJD

has been transmitted with transplanted nerve tissue and contaminated surgical instruments

infectious form, PrPSc (for scrapie protein)

Prion diseases are caused by the conversion of a normal host glycoprotein, PrPC (for cellular prion protein) into an —

chromosome 20

the gene for PrPC is located on — in humans

PrPC

recent evidence suggests that —is involved in preventing cell death

plaques

fragments of PrPSc molecules accumulate in the brain, forming — but they don’t appear to be the cause of cell damage

If an abnormal prion protein (PrPSe) enters a cell, it changes a normal prion protein PrpC to PrpSc, which now can change another normal PrP, resulting in an accumulation of the abnormal PrpSc in the cell and on the cell surface.

how can a protein be infectious