6 viruses

virus

infect hosts and form virions; intracellular parasites

all domains susceptible

ubiquitous

non filterable

nucleic acid in protein shell →capsid of repeating protein units (capsomeres)

viral reproduction

viral genome enters → make copies of genome → transcribe/translate viral genes → assemble virions

infectivity

each species infects range of hosts

broad - rabies

narrow - hiv

animal - tissue specificity

structure

symmetrical: icosahedral or filamentous capsid; minimizes genes

enveloped: surrounds capsid and derived from host membrane

glycoprotein spikes

host recognition and attachment

filamentous virus

capsid: long protein tube with coiled genome inside

tailed virus

genome delivery devise to icosahedral head

t4 bacteriophages: helical

asymmetrical virus

influenza

lack capsid symmetry

viral genomes

‒ Small viruses: ≤ ten genes

– Large viruses: > 100 genes

zika virus: Non-segmented, single-stranded (+) RNA genome, 10000 bases

influenza virus: Segmented, single-stranded (-) RNA genome; 8 segments, 11 proteins encoded; 13,500 nt bases total

viroids

infectious rna moleculres

no capsid around rna

replicated by host rna polymerase

does not encode proteins

catalytic ability

infect plants

prions

no nucleic acid component

degenerative brain disease

misfolded brain cell protein that infects animals

produces harmful aggregates

highly resistant to physical/chemical components

classifications

genome, capsid symmetry, envelope, host range, virion size

+rna = mrna = sense rna

baltimore class

Group I (dsDNA): use host enzymes (RNA polym) to make mRNA.

Group II (+ssDNA): use a DNA polym to make complementary double strand → mRNA

Group III (dsRNA): RDRP uses negative strand of pair as a template to transcribe + mRNA.

Group IV (+ssRNA): make more of genome, uses RDRP to produce more + mRNA.

Group V (-ssRNA): carry a negative/antisense strand. use RDRP to flip negative strand to +mRNA strand before making proteins

Group VI (Retrovirus): use enzyme Reverse Transcriptase (RT) to turn RNA into DNA. DNA integrates host's genome and uses machinery to make mRNA.

life cycle

(1) Host recognition & attachment

(2) genome entry

(3) synthesis & virion assembly

(4) exit & transmission

(1) Host recognition & attachment

recognition/binding of

viral capsid proteins, enve-

lope proteins, or glycoproteins

to host cell surface proteins or

glycoproteins

(2) genome entry

entire capsid + genome

enters host cell, or only the

genome may enter

(3) synthesis & virion assembly

Genome replication, protein

synthesis; assembly of virions

(4) exit & transmission

Release from host cell 

progeny infect more host cells

bacteriophage life cycle

lytic phage: t even phages; phage replicates and kills host cell (lytic burst)

lysogenic cycle: lambda; temperate phages; do not kill host immediately; integrates into chromosome as prophage; can reactivate to be lytic; env cues when lysogeny → lytic cycle

lysis and lysogeny

virulent phages carry out lytic cycle

temperate phages: cycle in between (lambda)

slow release bacteriophages

m13 filamentous phage

phage particle reproduce and exit cell

host cells grow slowly

host defenses

genetic resistance: altered receptor proteins

restriction endonucleases: cleave viral DNA sequences lacking methylation on cytosine nucleotides

crispr

bacterial immune system

 Host cleaves phage DNA, inserts fragment into chromosome as a “spacer”

 Phage re-infectionCRISPR sequences transcribed; crRNAs target phage DNA

animal virus

bind to receptor proteins that determine viral tropism

replication depends on genome form: dna can use host replication machinery; rna possess dept polymerase to transcribe mrna; retrovirus use reverse transcriptase to copy sequence into dna for host chromosome

enter as virions: undergo uncoating (genome released from capsid)

use ribosome to form protein: Assembly of new virions; Enveloped viruses: envelope proteins are inserted into the plasma membrane or organelle membrane; Release of virions from host via lysis or budding

dna viruses

• Genome replication in

nucleus

• Synthesis of viral proteins

outside the nucleus

• Entry of viral proteins into

nucleus

• Viral assembly in nucleus

rna viruses

Genome replication,

synthesis of viral proteins,

and viral assembly, all occur

outside the nucleus

genome of rna virus

(+) rna - template for translation

(-) rna - mrna synthesis

retrovirus - dna synthesis

retrovirus rna

enveloped RNA virus; has 2 identical (+) RNA strands

infects helper t cells

uncoating releases rna genome and enzumes (Reverse transcriptase): forms provirus, may remain latent; provirus can be transcribed into rna and translated into proteins; synthesized glycoproteins insert into membrane and exit by budding after virus assembly

animal/plant host defense

genetic resistance: hosts experience mutations

rna interference: widespread; complimentary rna to genome → shuts down viral gene expression

immune system: innate immunity (interferon produced by infected cells and induces production of antiviral proteins → protect from infection); adaptive immunity antibody to virus

viral ecology

limit host pop density, recycle nutrients, increase diversity, gene transfer

marine: nutrient cycling

viral shunt

viral shunt

viral infection convert hosts to detritus (rich in molecules)