VIRUSES
Campbell and Reece Chapter 19
Virus

an infectious particle incapable of
replicating outside of a cell, consisting
of RNA or DNA genome surrounded by a
protein coat (capsid) & for some viruses
a membranous envelope
SEM of Phage Attack
on E. coli
Discovery of Viruses

1883: German scientist, Mayer,
discovered that he could transmit
tobacco mosaic disease (stunts growth
of tobacco plant) by rubbing sap of
affected plant on healthy plant
Discovery of Viruses


Mayer then hypothesized there was an
unusually small bacteria that could not
be seen with microscope
10 yrs later Ivanowsky put sap of
infected plant thru filter designed to
remove all bactreria ….filtrate still
caused healthy plants to get disease
Discovery of Viruses



next hypothesis: bacteria produce a
toxin that causes disease so it would
have been in filtered sap ….Beijerinck
proved the filtered sap contained
infectious agents that reproduced but
only inside host it infected
Beijerinck credited with being 1st to
describe concept of virus
1935: Stanley crystallized the virus
Tobacco Mosaic Virus
Viral Genomes




DNA:
DNA:
RNA:
RNA:
dbl stranded
single stranded
single stranded
dbl stranded
Viral Genomes



smallest have 4 genes
largest several 100
named as DNA virus or RNA virus
Capsid


protein shell enclosing genome
shapes:
 rod
 polyhedral
 icosohedral
Viral Envelopes



membranous accessory structure
derived from host cell membranes
+ proteins & glycoproteins of viral
origin
Bacteriophages


virus that infects
bacteria
1st 7 identified
infected E. coli
 named
Type 1 –
Type 7
 Types 2, 4, & 6
very similar:
Structure of Viruses


smallest virus 20 nm in diameter
(smaller than a ribosome)
largest virus several 100 nm barely
visible with light microscope
Host Cells



each particular virus can only infect
cells of limited # of host species called
the host range of the virus
specificity result of viral recognition
system
most: protein on viral surface fits into
receptor of host cell
Simplified Viral
Replicative Cycle




1. virus enters cell & is uncoated
releasing viral DNA & capsid proteins
2. host enzymes replicate the viral
genome
3. different host enzymes transcribe
the viral genome into viral mRNA 
translated by host ribosomes to make
viral proteins
4. viral genomes & capsid proteins selfassemble into new virus particles
which exit cell
Replication of RNA Viruses

use virally encoded RNAP that can use
RNA as a template
Lytic Cycle
Lytic Cycle


culminates in death of host cell
phages that replicate only by lytic
cycle called virulent phage
Bacterial Defenses
1.
2.
natural selection will favor mutants
having receptors that no longer allow
phage to attach
viral DNA may be recognized as
foreign  cut up by bacterial enzymes
called restriction enzymes (restrict
ability of phage to infect bacterium)
Phage Natural Selection


allows phage mutants that can bind to
altered receptors or are resistant to
particular restriction enzyme
so this parasite-host relationship in
constant evolutionary flux
The Lysogenic Cycle
Lysogenic Cycles



does not destroy the host
phages capable of using both modes of
replication called temperate phages
Λ phage (lambda) used widely in
biological research
Prophage



viral DNA that has been integrated into
bacterial loop of DNA
when that bacterium replicates the
viral DNA is passed on to all daughter
cells & so on & so on
when λ genome induced to leave the
loop of bacterial DNA  lytic cycle &
cell (bacterium) dies
Phage Genes in Bacteria


diptheria, botulism, & scarlet fever
would not be so harmful to humans
w/out certain prophage genes that
cause the host bacteria to make toxins
difference between E.coli that lives in
out GI tract (no problem) & the one
that’s found in food poisoning:
presence of prophages
Animal Viruses

nature of viral genome basis for classification of viruses that
infect animals:
DNA: dbl stranded
1.

dsDNA
DNA: single stranded
2.

ssDNA
RNA: dbl stranded
3.

dsRNA
RNA: single stranded
4.

ssRNA
RNA: template for mRNA synthesis
5.

ssRNA template for mRNA synthesis
RNA: template for DNA synthesis
6.

ssRNA template for DNA synthesis
Classes of Animal Viruses
Animal Viruses vs. Phages
Animal Viruses


many have both
envelope & RNA
some with DNA
also have envelope
Phages

few have envelope
or RNA
Viral Envelopes



outer membrane around capsid
used to enter host cell
viral glycoproteins protrude that will
bind to specific receptors on surface of
host cell
Replicative Cycle of
an Enveloped RNA Virus
RNA as Viral Genetic Material


includes most plant viruses & some
RNA viruses
broadest variety infect animals
RNA Viruses

Class IV:
 genomes
can serve directly as mRNA 
immediately after infection can translate
viral proteins

Class V:
 genome
serves as template for mRNA
synthesis
 C’ strands of RNA made which serve as
templates for both mRNA & new RNA
strands
RNA Viruses

Class VI:
 retroviruses
 have
enzyme: reverse transcriptase 
transcribes RNA template  DNA
(opposite normal direction of information
flow)
 HIV a retrovirus: enveloped with 2
identical molecules ssRNA & 2 reverse
transcriptase
Replication Cycle of HIV
Replication of HIV
1. envelope glycoproteins allow virus to
bind to specific receptors on certain
WBCs
2. virus fuses with cell’s plasma
membrane & capsid proteins removed
 viral RNA & proteins
3. reverse transcriptase catalyzes
synthesis of a dsDNA strand c’ to the
virus’s RNA
Replication of HIV
4. reverse transcriptase catalyzes
synthesis of 2nd DNA strand c’ to the
1st
5. dsDNA incorporated as a provirus into
host cell’s genome
6. proviral genes transcribed into ssRNA
which are genomes for next generation
& as mRNAs for translation into viral
protein
Replication of HIV
7. viral proteins include capsid proteins
& reverse transcriptase (made in
cytosol) & envelope glycoproteins
(made in ER)
8. vesicles transport the glycoproteins to
host cell plasma membrane
9. capsids assemble around viral genome
+ reverse transcriptase molecules
10. new viruses bud off from host cell
Evolution of Viruses



there are viruses that infect every
known form of life
use same universal genetic code as all
living things
probable that viruses evolved after the
1st cells appeared
Evolution of Viruses

most accepted hypothesis: evolved
from naked bits of nucleic acids that
moved from 1 cell  another
 possibly
plasmids or tranposons  1st
virus
 transposon: a transposable element that
moves w/in a genome by means of a DNA
intermediate
Plasmids



small, circular DNA molecules found in
bacteria & in unicellular yeast
(eukaryotic)
replicate independently of genome
occasionally transferred between
bacteria


Viruses, transposons, & plasmids are
all mobile genetic elements
virus may have more in common
genetically with its host cell than with
other viruses that infect same species
Mimivirus


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
largest virus to date (size of small
bacterium)
dsDNA
mimi: mimicking microbe
~1,000 genes: some code for proteins
used in translation, DNA repair, protein
folding, & polysaccharide synthesis
Viruses, Viriods, & Prions



viruses cause disease in all life forms
viriods cause disease in plants
prions cause disease in animals
Viral Disease in Animals

How viruses cause disease:
 damage
or kill cells by release of
hydrolytic enzymes from lysosomes
 cause host cell to produce toxins
 have molecular components (in envelope)
that are toxic to host
 many of symptoms ass’c with viral
infection result of immune system
reacting to infection (fever, chills, aches)
Vaccines

harmless variant or derivative of a
pathogen that stimulates a host’s
immune system to mount defenses
against the pathogen
“New” Viral Diseases in Humans


usually caused by existing viruses that
expand their host territory
example: H1N1 (2009) was a new
combination of pig, human, & avian
viral genes
Viral Infection in Plants

enter host cells thru damaged cell walls
(horizontal transmission) or are
inherited from a parent (vertical
transmission)
Viriods


naked RNA molecules that infect
plants & disrupt their growth
can cause misshaped potatoes
Prions

slow-acting, virtually indestructible
infectious proteins that causes brain
diseases in mammals
Model for How Prions Propagate