Ch 13 viruses, viroids, and prions

when was the tobacco mosiac disease transfer from plant-to-plant (virus discovery)

1886

when was it known that the causative agent of tobacco mosaic disease could pass through the pores of a filter (virus discovery)

1892

when was the tobacco mosaic virus purified, enabling the study of its structure using electron microscopy (virus discovery)

1935

why were viruses initially distinguished from other infectious agents because theyre….

small and obligate intracellular (needs a host cell) microbes

how do we now distinguish viruses from cellular lifeforms based on their structure

single type of nucleic acid (DNA or RNA)
protein coat
multiply within host cells using host machinery
responsible for synthesis of structures that transfer viral nucleic acid to other cells

what is host range/ tropism

the spectrum of host cells a virus can infect

• all organisms are susceptible

• some viruses infect different species than other viruses

• only infect certain tissues/ cells

what is the host range primarily determined by

the ability of the virus to attach to the host cell and reproduce

what does attachment involve for the host range

viral proteins and a receptor on the host cell

how does viral size vary

most are way smaller than or the same size as bacteria

giant viruses are..

susceptible to smaller viruses (virophages)

what is a virion

complete, fully developed, infectious viral particle, found outside a host cell- made of nucleic acid surrounded by a protein coat

what are differences of virus genome’s

DNA or RNA

singe-stranded or double-stranded

circular or linear

segmented or non-segmented

what is a capsid

protein coat that surrounds a virus’s genome- present in all viruses

what is an evelope

lipid layer that covers the capsid - not in all

what are characteristics of an evelope

formed from plasma membrane when virus exits host cell

spike proteins on outside

what are the viral structures

helical

polyhedral

complex

helical structure

nucleic acid within a hollow, cylindral capsid

polyhedral structure

many-sided, often icosahedral (20 triangular faces)

complex structure

lacks symmetry (ex: bacteriophage)

what is a bacteriophage

virus that infects bacteria

forms plaques

what are plaques

zones of bacterial cell lysis

infects the cell to replicate and lyses the cell to further infect other cells

culturing animal viruses

sometimes only able to in living animals

what are other ways viruses can be grown

embryonated eggs and cell cultures

cell culture for viruses

plant/ animal cell grown in media in the lab and cell lines

primary cell lines

derived from tissue, survive only a few gens

continuous cell lines

derived from cancerous cells (immortal)

ex: HeLa cells

why have bacteriophages been studied

its a model of virus replication

some phage undergo what kind of life cycle

lytic, which converts bacterial cell to make more phages (lysis)

what the lytic life cycle

attachment

penetration

biosynthesis

maturation

release

lytic- what is the attachment stage (first)

phage attaches by the tail fibers to a receptor of the bacterial cell

lytic- what is the penetration stage (second)

dna is injected into the bacterial cell

lytic- what is the biosynthesis stage (third)

production of phage dna and proteins

lytic- what is the maturation stage (fourth)

assembly of phage particles

lytic- what is the release stage (fifth)

phage lyse the bacterial cell and release into the environment

what is the lysogenic life cycle

phage genome integrates into bacterial genome to prophage and is passed down as the bacterial cell vertically divides

how do some phages choose between lytic and lysogenic cycles

temperate phage- depends on the signal based on the bacteria

animal- what is the attachment (1) stage

virus binds to receptor on host cell

animal- what is the entry (2) stage

virus enters into host cell through injection, receptor-mediated endocytosis, fusion (enveloped virus)

animal- what is the uncoating stage (3)

loss of capsid, releases nucleic acid into host cell

animal- what is the biosynthesis stage (4)

production of nucleic acid and proteins- depends on baltimore classification

animal- what is the maturation stage (5)

nucleic acid and capsid proteins assemble

animal- what is the release stage (6)

leaves the host cell by rupture or budding (enveloped virsuses)

what is budding in release of animal viruses

pushes against the membrane- membrane breaks and makes the envelope

how do dna genome viruses replicate

dna polymerase

how do rna genome viruses replicate

rna-dependent rna polymerase

what does the structure of the viral genome impact the performance of …

transcription

the manner by which viruses synthesize mRNA is the basis of the baltimore classification system…

dna vs rna genome

ds vs ss genome

(+) vs (-) sense genome

what are some rna viruses considered

retroviruses

what is the meaning of a retrovirus

rna genome but they convert their genome into a dna version to integrate into the host cell genome

what enzyme does the retrovirus use

rna-dependent dna polymerase

what does rna-dependent dna polymerase do

reads rna and synthesizes dna

what is meant by latent virus

virus can lay dormant in a host for an extended period of time- doesnt cause symptoms

what are examples of latent viruses

herpes simplexvirus (cold sores)
varicella virus (chicken pox, shingles)

what is meant by chronic/ persistent infections

gradually increases over time (HIV)

what are prions

misfolded proteins that induce misfolding of host proteins

characteristics of prions

doesnt contain dna/ rna

initiates a chain reaction

affects brain/ neurological function

when was prion first discovered

1982 from disease in sheep called scrapie

how does amplification of prions occur

conversion- convert an existing protein into the prion form

where are PrPC located

membrane of cells in the brain

what are normal host protein PrpC

C for cellular and primarily alpha helices

what is the prion form PrpSc

Sc for scrapie and formation of beta pleated sheets

what happens when PrpC comes into contact with PrpSc

PrpC is converted into PrpSc (chain rxn)

what do PrpSc proteins form when aggregated

amyloids

what happens when amyloids accumulate in infected tissue 

causes damage and cell death

how can prions be sporadic

spontaneous conversion of PrpC to PrpSC

how can prions be acquired

ingestion of prions from infected animals

why are prion diagnosis challenging

long incubation period before symptoms show and need analysis of brain tissue

why are there no treatments available

prion structure is too simple

cant target naturally existing proteins

not easily degraded

how do you prevent prion

ban animal-product feeds

control imported animal products from foreign countries

careful removal of nervous system tissue by slaughterhouses

how do you decontaminate prions

incineration