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Chapter 19 Viruses (19.2 Viruses replicate only in host cells (Replicative…
Chapter 19 Viruses
19.2 Viruses replicate only in host cells
Introduction
host range
a virus can infect cells of a limited number of host species
viruses can have either broad or narrow host ranges
example: HIV binding to certain types of immune cells vs West Nile affecting mosquitoes, humans and more
obligate intracellular parasites
can only replicate in it's host cell
General Features of Viral Replicative Cylces
3 basic steps
bind itself to host cell
make copies of its DNA and make new viruses inside of the host cell
exit the host cell and repeat
Replicative Cycles of Phages
The Lytic Cycle
phage replicative cycle that ends with death of host cell
virulent phage
replicates only by the lytic cycle
The Lysogenic Cycle
replicates the phage genome without killing host cell
temperate phage
phages capable of both lytic and lysogenic cycles
prophage
phage/ viral DNA becomes part of the bacteria's chromosome
Bacterial Defenses against Phages
natural selection
bacterial mutants with surface proteins unrecognized as receptors by certain types of phages
restriction enzymes
cellular enzymes that cut up the phage's DNA (or restricts the phage's ability to replicate
CRISPR-Cas system
basically an immune system that cuts and destroys phage DNA
present in both bacteria and archaea
Figure 19.7
Replicative Cycles of Animal Viruses
Viral Envelopes
the replicative cycle with a viral envelope doesn't necessarily kill the host cell
animal virus uses its viral envelope to enter the cell
Viral Genetic Material
an animal virus can have either DNA or RNA
retrovirus
RNA animal viruses with the most complicated replicative cycles
retroviruses have an enzyme called reverse transcriptase
transcribes an RNA template into DNA
example: HIV, a retrovirus that causes AIDS
HIV is a provirus: basically once its viral DNA integrates with the cell, it never leaves the host's genome
Evolution of Viruses
plasmids and transposons share an important feature with viruses
all three are
mobile genetic elements
candidates for the original source of viral genomes
plasmids
small circular DNA molecules found in bacteria and yeast
occasionally
transferred between cells
exist apart from and can replicate independently of the bacterial chromosome
transposons
DNA segments that
move from one location to another
in a cell's genome
19.3 Viruses and prions are formidable pathogens in animals and plants
Emerging Viruses
epidemic
a widespread outbreak of a virus
limited to a few countries
pandemic
a global epidemic
virus has spread to several countries/worldwide
three processes
1 mutation of existing viruses
example: new strains of the flu
2 dissemination of a viral disease from a small isolated human population
example: AIDS went from unnoticed to everywhere with international travel, blood transfusions, etc
3 spread of existing viruses from other animals
example: avian flu or mad cow disease
Viral Diseases in Plants
common signs of viral infection in a plant
bleached or brown spots, stunted growth, and damaged flowers or roots
plant viruses have same basic structure and mode of replication as animal viruses
RNA genomes and helical or icosahedral capsids
spread by two ways
horizontal transmission
from a external source, like insects or gardeners
vertical transmission
plant inherits the infection from its parent
Viral Diseases in Animals
vaccine
mostly harmless derivative of a pathogen that stimulates the immune system to build up an immunity to a virus
antibiotics are useless against viruses
antiviral drugs
prevents virus from binding to cell
or stops virus from replicating inside the cell
Prions: Proteins as Infectious Agents
prions
infectious proteins that cause a number of degenerative brain diseases in animals
examples: scrapie in sheep or mad cow disease in beef
can be transmitted in food
two major characteristics
act very slowly with incubation periods of up to 10 years
they are virtually indestructible (turn normal protein into aggregate shape and clumps together
19.1 A virus consists of a nucleic acid surrounded by a protein coat
The Discovery of Viruses: Scientific Inquiry
tobacco mosaic disease experiment
Martinus Beijerinck
credited with being the first to voice the concept of a virus
Adolf Mayer
discovered disease could be transferred by rubbing infected sap on healthy plant
Dmitri Iwanosky
filtered infected sap through a filter designed to catch bacteria but still believed it was small bacteria
Structure of Viruses
basic structure
nucleic acid enclosed in a protein coat (some also have a membranous envelope)
Viral Genomes
can be single-strand or double-strand of both DNA and RNA
virus is called DNA or RNA virus based on the kind of nucleic acid in it's genome
usually genome is single linear or circular molecule of nucleic acid (but some have multiple molecules)
Capsids and Envelopes
capsid
protein shell enclosing the viral genome
capsids can be rod-shaped, polyhedral or more complex
built from capsomeres
the individual protein subunits making up capsids
viral envelope
a membranous envelope that contains parts of the host cell
derived from host cell's membrane
has host cell phospholipids and membrane proteins as well as proteins and glycoproteins from virus
bacteriophages (phages)
the viruses that affect bacteria
phages have some of the most complex capsids
Introduction
A Borrowed Life
virus
an infectious particle consisting of genes packaged in a protein coat
example: HIV attacking immune cells
viruses are smaller and simpler in structure than eukaryotic or prokaryotic cells
a parasite that has "borrowed life"
