CHAPTER 19: VIRUSES (REPLICATIVE CYCLE OF AN ENVELOPED RNA VIRUS…
CHAPTER 19: VIRUSES
nucleic acid surrounded by a protein coat
replicate only in host cells
along with "
" are formidable pathogens in animal & plants
single-stranded or double-standed; DNA or RNA
enclosed in a protein capsid
enclosed within a membrane envelope derived form host (at times)
possibly from plasmids or transposons
Using the metabolic machine of their host.
Bacterium | Animal | Plant hosts are used.
Viruses only able to replicate in host cells.
Obligate intracellular parasites; lacking metabolic enzymes
each virus has a limited of; due to proteins on the outside of the virus.
Proteins recognize only specific receptor molecules on the host cell surface
may destroy the host cell
cause disease within the host organism
DISCOVERY OF VIRUSES
search for the cause of tobacco mosaic disease; discovery
infectious agent could neither be filtered from infected sap.
couldn't be cultivated in nutrient media
able to replicate within plants
W. Stanley crystallized the infectious particle; TMV (tobacco mosaic virus)
STRUCTURE OF VIRUSES
consists of gene inside a protein coat
other viruses a membranous envelope
: single- or double-stranded DNA/RNA
contained on a single linear/ circular nucleic acid molecule
: derived from membranes off the host cell
viral proteins & glycoproteins
; may cloak the capsids of viruses that infect animals
some viruses contain a few
- (protein shell)
built from a large # of identical protein subunits (
sometimes more complex in shape
complex capsids found among
"that which eat" root word
a virus that infects bacterium
similar capsid structures consisting of an icosahedral head & protein tail piece with tail fibers
VIRAL REPLICATIVE CYCLES
viral genome: enters the host; cell's enzymes, nucleotides, amino acids, ribosomes, ATP, and other resources.
the above items are used to replicate the viral genome and produce capsid proteins.
DNA viruses use host DNA polymerases to copy their genome.
RNA viruses use virus-encoded polymerases to replicate their RNA genome.
Viral nucleic acids and capsid proteins SPONTANEOUSLY self assemble to form new viruses with the host cell.
100's/1000's of newly formed virus particles are released
once newly formed often the host cell is destroyed in the process.
REPLICATIVE CYCLES OF PHAGES
culminates in lysis of the host cells
release of newly produced phages
eliminates the host.
VIRULENT PHAGES: only replicate by a lytic cycle
a virus replicates its genomes without killing the host.
- replicate by both lytic and lysogenic cycles
Phage Lambda injects DNA into E.Coli cell. Begin lytic cycle or
DNA may incorporate into host cell's chromosomes & begin cycle PROPHAGE
a protein represses most of the genes of the inserted phage genome coded for by a prophage gene.
reproduction- of the host cell replicates the phage DNA along with the bacterial DNA.
prophage may exit the bacterial chromosome in response to environmental stimuli--> starts the lytic cycle.
tail fibers stick to receptor site on the surface of E.Coli cell
sheath of the tail contracts and thrusts its viral DNA into cell.
Empty capsid is left behind
E. Coli cell: beings to transcribe/translate phage genes; codes enzyme that chops up host cell DNA.
nucleotides from the degraded bacterial DNA are used to produce viral DNA.
capsid proteins: assembled into phage tails/tail fibers & heads.
Viral components ---> phages particles
once released: after the manufacture of an enzyme that damages the bacterial cell wall
cell swells and burst
- cut foreign DNA; degrades phage DNA
mutations that alter bacterial receptor sites provide some protection against infection by Phages.
BACTERIA AND ARCHAEA- posses the CRISPR-Cas defense system- involes repetitive DNA sequences.
DNA of an infecting Phage integrates into the hose genome between 2- CRISPR repeat sequences.
A subquent attack by a similar phage cause TRANSCRIPTION of the CRISPR region- RNA molecules made into pieces.
REPLICATIVE CYCLE OF ANIMAL VIRUSES
CLASSIFIED BASED ON THEIR TYPE OF GENOME: DOUBLE- OR SINGLE- STRANDED FOR DNA OR RNA.
Membranous envelopes surrounds capsid of almost all animal viruses with RNA genomes.
REPLICATIVE CYCLE OF AN ENVELOPED RNA VIRUS
Glycoproteins extending from the (viral membrane) attach to receptor sites on a host cell's (plasma membrane)
capsid enters the cell and viral genomes replicates with a (viral RNA polymerase)
viral RNA directs
of viral proteins
viral glycoproteins are produced and embedded in the ER (endoplasmic reticulum membrane)
new viruses bud off within an envelope that is derived form the host's plasma membrane and bear viral glycoproteins.
Replicate within host cell nucleus and temporarily cloaked in a membrane derived from the host cell nuclear envelope.
membrane later shed and replace by one derived from the GOLGI APPARATUS.
the double-stranded DNA remains latent as a (mini chromosome in the nucleus)
initiates herpes infections in times of stress
ANIMAL CLASS IV VIRUSES- serve as mRNA.
RNA genome of class V- 1st transcribed into strand of complementary RNA.
using a viral enzyme packaged inside the capsid
serves as a mRNA and a template for making genome RNA
Viral genome (RNA) transcribed into double stranded DNA by viral enzyme (
viral DNA integrated --> a chromosome, where it is transcribed by the host cell --> into viral RNA.
HAS two acts 1: new viral genome 2: mRNA for viral proteins
: Human immunodeficiency virus- causes
: acquired immunodeficiency syndrome.
integrated viral DNA remains
within the host cell DNA.
NEW VIRUSES: Assembled with 2 copies of RNA genomes and 2 molecules of Reverse Transcriptase within a capsid.
Bud off, covered in host cell plasma membrane studded with (viral glycoproteins)
EVOLUTION OF VIRUSES
EVOLVED FROM FRAGMENTS OF CELLULAR NUCLEIC ACIDS THAT MOVED FROM ONE CELL TO ANOTHER
Plasmids: self-replicating circles of RNA found in Bacteria and yeast.
Transposons: segments of DNA that can change location within a cell's genome.
Viruses/Plasmids/Transposons: all are mobile genetic elements.
Genomes of Viruses: similar to those of host cells than genomes of viruses infecting other host.
Sequencing of Viruses: revealed genetic similarities among viruses that otherwise seem distantly related.
MIMIVIRUS: has genes for cellular machinery- debate whether it evolved before or after first cells.
VIRUSES AND PRIONS
Formidable pathogens in animals and plants.
VIRAL DISEASES IN ANIMALS
viral infections caused by viral-programmed toxins
produced by infected cells/ cells killed/ damaged by the virus/ body's defense mechanisms fighting the infection.
VIRAL DISEASES IN PLANTS
most plant viruses are "RNA VIRUSES"
spread through [vertical transmission] from a parent plant via infected seeds or cuttings.
[horizontal transmission]: from external source
[increase susceptibility to viral infections] when plant is injured. Allows insects to infect because they act as carriers of viruses.
harmless variants or derivatives of pathogens
induce the immune system- to react against the actual disease agent.
greatly reduced the incidence of many viral diseases.
use the host's cellular machinery to Replicate unlike bacteria.
some drugs found to treat and cure viral infections.
anti-viral drugs: resemble nucleosides & interfere with viral nucleic acid synthesis.
EXAMPLES: HIV/ Ebola Virus/ West Nile Virus/ Chikungunya Virus/ Zika Virus
: a general outbreak of a disease
: global epidemic; spreads rapidly
Sudden Emergence; possibly linked to the mutation of an existing virus.
->Most common in RNA viruses (higher mutation rates)
-> Dissemination- of an existing virus to more widespread population (ex. H1N1)
-> One host species to another; ex. 2009 Flu Pandemic PIGS TO HUMANS.
-> New human diseases- though to originate from third mechanisms.
INFLUENZA A/B/C - HUMANS | INFLUENZA A - ANIMALS
IF different strains of Virus
undergo genetic recombination within the animal cells
start accumulating mutations that allow the virus to infect the human cell
recombinant virus maybe highly Pathogenic.
H1N1: form of the viral surface proteins Hemagglutinin & Neurominidase (caused 1918 & 2009 flu Pandemics.
-H5N1: wild and domestic birds- possibly transmitted to humans
human mortality rate: "arian flu" greater than 50% not yet able to transfer among the people.
GLOBAL CLIMATE CHANGE: allows virus vectors (ex:mosquitos) to expand their ranges INCREASES spread of viral diseases.
: "Proteins of Infectious Agents"
protein infectious agents linked to many degenerative brain diseases.
ANIMALS: scrapie/ mad cows disease
HUMANS: Creutzfeldt-Jacob disease/ KURU.
Misfolded forms of a protein
] & apparently cause disease by:
converting normal cellular protein into misfolded prion version
[CAPS-] : a box (capsid): the protein shell that encloses a viral genome.
[-PHAGE] : that which eats (bacteriophage): a virus that infects bacteria.
[PRO-] : before (provirus): a viral genome that is permanently inserted in a host genome.
[RETRO-] : backward (retrovirus): an RNA virus that replicates by transcribing its RNA into DNA & then inserting DNA into a cellular chromosome.