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Chapter 19: Viruses (Concept 19.2: Viruses replicate (Viral Genetic…
Chapter 19: Viruses
Concept 19.2: Viruses replicate
viruses are obligate intracellular parasites
can only replicate within a host cell
Host range: each particular virus can infect cells of only a limited number of host species
identify host cells by a lock and key fit
viruses have broad host ranges
West Nile infects humans, mosquitoes, birds, and horses
Viral infection of multicellular eukaryotes is limited to certain tissues
colds=upper respiratory tract
HIV= receptors present only on certain types of immune cells
General Features of Viral Replicative Cycles
Viral infection begins when the virus binds to and enters the host cell
Genome entry depends on virus type and host cell type
proteins it encodes can commandeer the can reprogramming the cell so that viral proteins are made
the host cell provides:
nucleotides
enzymes
ribosomes
tRNAs
amino acids
ATP
RNA viruses use virally encoded RNA to replicate genomes
Hundreds or thousands of viruses leave the host cell damaged or destroyed
Lytic Cycle
A phage replicative cycle that culminates in death of the host cell
virulent phage: replicates only by the lytic cycle
last stage of infection
successive cycles can destroy an entire bacterial population in a few hours
Lysogenic Cycle
many phages coexist w/ their host cells
allows replication of the phage genome
Temperate Phage: capable using both modes of replicating w/in a bacterium
Prophage: genome that's been inserted into a specific site on a bacterial chromosome
capable of regenerating active phages that lyse their host cells
Bacterial Defense Against Phages
Lysogeny is one reason why bacteria have not seen extinction
mutants have surface proteins that aren't
restriction enzymes restrict a phages ability to replicate within the bacterium
CRISPR-Cas System
The nucleases, CRISPR-associated proteins cut the phages
Replicative Cycles of Animal Viruses
can replicate only inside host cells
key variables are the vital genome and presence or absence of an membranous envelope
Many animal viruses have both
Viral Genetic Material
Broadest variety of RNA genomes found among animal viruses
Class IV viruses
serve as mRNA; translated into viral protein immediately
All virals that use an RNA genome as a template for mRNA transcription require RNA to DNA synthesis
RNA animal viruses w/ most complicated replicative cycles are retroviruses (class VI)
Reverse transcriptase an RNA template into DNA
HIV
HIV is an enveloped virus like others
contain two identical molecules of single-stranded RNA and two molecules of reverse transcriptase
Provirus: viral genome that is permanently inserted into a host genome
never leaves the host genome
Viral Envelopes
the viral envelope is usually derived form the host cell's plasma membrane
Herpesviruses leaves copies of viral DNA as mini chromosomes
Evolution of Viruses
an isolated virus is biologically inert
infect every form of life
Plasmids: small, circular DNA molecules found in bacteria and in unicellular eukaryotes
replicate independently of the bacterial chromosome
transferred between cells
Transposons: DNA segments that can move from one location to another within a cell's genome
Plasmids and Transposons are mobile genetic elements
Concept 19.1: A virus consist of a nucleic acid surrounded by a protein coat
infectious particle consisting of little more than genes packaged in a protein coat
Discovery of viruses: Scientific Inquiry
Tobacco Mosaic Disease
stunts the growth of tobacco plants
leaves are a motted, mosaic coloration
1883: Adolf Mayer transmitted the disease from plant to plant w/ its sap
Ivanowsky passed the sap through a filter designed to remove bacteria
Structure of Viruses
smaller than a ribosome (20nm)
largest have a diameter 1,500 nm
barely visible under the light microscope
some have a membranous envelope
Viral Genomes
Genomes
Double Stranded DNA
single stranded DNA
double stranded RNA
single stranded RNA
depends on the type of virus
called a DNA or RNA virus based on the nucleic acid that makes up its genome
single linear or circular
smallest viruses= 3 genes, large viruses= several hundred to 2,000
bacterial genomes contain 200 to a few thousand genes
Capsids and Envelopes
the protein shell enclosing the viral genome is a capsid
rod-shaped, polyhedral, or complex depending on virus
built from protein subunits called capsomeres
rod-shaped viruses are called helical viruses
Adenoviruses
infect respiratory tracts in animals
252 protein molecules arranged in a polyhedral capsid w/ 20 triangular facets- icosahedron
icosahedral viruses
viral envelopes are from the membranes of the host cell
contain host cell phospholipids and membrane proteins
bacteriophages infect bacteria
first phages studied were sevens that infected E.coli
capsids have elongated icosahedral
protein tail is attached w/ fibers
Concept 19.3: Viruses and prions are formidable pathogens in animals and plants
Viral Disease in Plants
Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes
How much damage a virus causes depends partly on the ability of the infected tissue to regenerate by cell division
Ex: poliovirus in nerve cells
immune system is a critical part of the bodies natural defenses
vaccine: harmless derivative of a pathogen that stimulates the immune system to mount defenses against the harmful pathogen
Emerging Viruses
Viruses that suddenly become apparent
HIV
AIDS
West Nile Virus
Ebola
Zika Virus
Chikungunya Virus
Swine Flu
Epidemic: widespread outbreak
Pandemic: Global epidemic
Three processes contribute to the emergence of viral diseases
1) mutation of existing viruses
Ex:RNA viruses
2) Dissemination of viral disease from a small, isolated human population
Ex: AIDS
3) Spread of existing viruses from other animals
Ex: Swine FLu
Vector-borne illness: an organism flying or walking that can transfer the virus to you
Viral Disease in plants
most plant viruses have an RNA genome
Horizontal transmission, entering through damaged cell walls
Vertical transmission, inheriting the virus from a parent
Prions: not made of cells just like viruses
infectious proteins that appear to cause degenerative brain diseases in animals
POLYPEPTIDE
"Touch" one of our proteins, stick to it and causes an aggregate (can convert more proteins into prions)