CH 6 (Part 1) - Viruses, Viroids, & Prions

What domains are susceptile to viruses?

Cell types in ALL taxonomic domains are susceptible to virus infection.

Discovery of viruses

(Ivanovsky & Beijerinck) tobacco mosaic virus; “non-filterable particles”

• tobacco plant infection by virus - found virus in filtrate (tiny - 20 nm)

Virus size range

20 to 900 nm

(0.02 to 0.90 um)

Viruses

acellular; they are obligate (they must do it) intracellular parasites. ALL viruses consist of nucleic acid (DNA/RNA) in a protein shell ⟶ a capsid; comprised of repeating protein subunits, capsomeres.

Process common to ALL VIRUSES but can have variations

What is needed from the host?

• DNA/RNA polymerase (can vary based on DNA/RNA virus)

• Ribosomes

• tRNA

• Nucleotides (*varies)

• Other: viruses lack a metabolism; rely on host

host range

Each species of virus infects a particular group (range) of host species

• Broad: rabies virus

• Narrow: HIV only infects humans

trophism

Animal viruses ⟶ tissue specificity

• how many types of cells in a host species can be infected?

• Broad: Ebola

• Narrow: HIV (infects T-helper cells)

• based on: glycoproteins or capsid proteins on its surface recognize those of the host cells.

Symmetrical viruses

icosahedral or filamentous capsid

• geometric pattern

Simplicity of capsid structure

minimizes the number of genes

Enveloped viruses

an envelope surrounds the capsid; derived from the host membrane

• “Naked” viruses LACK envelopes

Naked viruses

lack an envelope

Glycoprotein spikes

Function in host recognition & attachment or other function

• BOTH naked and enveloped can have spikes

Filamentous viruses

The capsid ⟶ long tube of protein,with genome coiled inside; vary in length

Ex: Ebola, M13, TMV

Tailed viruses

addition of a genome delivery device tothe icosahedral head.

• Ex: T4 bacteriophages: helical “neck” & tail fibers

• complex viruses - many structures

Asymmetrical viruses

lack capsid symmetry —→ uneven shape

• influenza viruses (as well as others)

Coronavirus

• enveloped viruses

• nucleocaspid proteins - protects genome (stuck to genome)

Viral Genomes

• Small viruses: ≤ 10 genes

• Large viruses: > 100 genes

• average virus has 11,000 nucleotides

Zika Virus Genome

• Non-segmented, single-stranded (+) RNA genome

• 10,794 ntbases long

Influenza Virus Genome

• Segmented, single-stranded (-) RNA genome; 8 segments

• 11 proteins encoded;13,500 ntbases total

• mutation and reassortment of segmented genomes ⟶ lead to variants that are infectious in humans

Viroids

are RNA molecules without a capsid surrounding the RNA; infect plants. They are not viruses.

• Are replicated by host RNA polymerase

• The RNA does not encode for proteins; 300-400 nucleotides long

• Some have catalytic ability.

EFFECT: alter gene expression in affected plant

Prions

proteins that infect animals; they have NO nucleic acid component.

• Cause degenerative brain diseases; transmitted in food, prepared from infected animals

• misfolded form of a normal brain cell protein

• converts a normal protein into the prion version, creating a chain reaction; produces harmful aggregates in the cell. (VERY slow process)

• Are highly resistant to physical/chemical agents

Important roles in ecosystems (Viral Ecology)

limit host population densities; recycle nutrients; increase host diversity; gene transfer

• Marine Ecosystems: cycling of nutrients

viral shunt (Viral Ecology)

viral infection of hosts convert them to detritus, rich in organic & inorganic molecules.

• marine ecosystems

Weird viruses

• Very large viruses

• can have some metabolism - like a normal cell (but not always

• virus that can infect other viruses

International Committee on Taxonomy of Viruses; based on:

Genome

Capsid symmetry

Envelope

Host range

Virion size

1971: Baltimore classification

genome (RNA or DNA) & route used to express messenger RNA (mRNA)

• + RNA = mRNA = “sense” RNA —→ codes for protein (final step)

Group I: ds DNA

± DNA —→ host RNA polymerase —→ + mRNA

• use host DNA polymerase to make copies of genome

Group II: + ss DNA

+ DNA —→ host DNA polymerase —→ ± DNA —→

host RNA polymerase —→ + mRNA

• use host DNA polymerase to make copies of genome

RDRP

RNA-Dependent RNA Polymerase

• VIRAL enzyme, not found in cells

Group III: ds RNA

± RNA —→ viral RDRP —→ + mRNA

• use RDRP to make copies of genome from

Group IV: + ss RNA

+ ssRNA —→ viral RDRP —→ - RNA —→ viral RDRP —→

+ mRNA

• more efficient to have >1 copy of RNA so to make a bunch, you need to make a template ( - RNA )

Group V: - ss RNA

- RNA —→ viral RDRP —→ + mRNA

• use RDRP to make copies of genom

Group VI: Retroviruses

+ RNA —→ RT —→ - DNA —→ RT —→ ± DNA —→ host RNA polymerase —→ + mRNA

• need double strand DNA to integrate into host chromosome

• NOT lysogenic b/c they integrate into host chromosome and start replicating actively