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Virus, or “Poison”
 Is
it alive? Not really…
 Depend on cells to reproduce  obligate
intracellular parasites
 Inert organic particles when outside cells
 Viruses for all - infect bacteria, protists,
and all higher plants and animals
 Infect specific cell types in host
Virus size comparison at Cellsalive.com
Edward Jenner – 1700’s, Smallpox vaccine
Vaca = Cow,
noticed milkmaids immune to smallpox
Louis Pasteur – rabies vaccine, 1800’s
Wendell Stanley – crystallized TMV
(Tobacco Mosaic Virus), 1900’s,
showed that viruses are NOT small bacteria
Viruses - Structure
acid (RNA or DNA) – instructions
for making proteins
 protein capsid
 may have plasma membrane, or envelope
 Surface proteins for attachment to host
cell
 Little or no metabolism:
 nucleic
Who will read the DNA/RNA and make viral proteins that
assemble into a viral capsid?
The host cell
If a virus has an envelope (membrane), where does it come
from?
The host cell’s plasma membrane
Influenza Virus
(membrane
from host
cell)
(nucleic acid,
instructions for
making proteins)
(protein)
Surface proteins
Criteria used to Classify Viruses
1. Type of nucleic acid
2. Presence or absence of an envelope
3. Shape/morphology of the capsid
4. Specific surface proteins  different
strains
Viral Replication by Host Cell
1. Adsorption
2. Entry
Lytic Cycle:
Cells lyses and releases
new viral particles
3. Replication
4. Assembly
5. Lysis / Cytolysis
Viral
Replication
by Host Cell
Lysogenic Cycle:
Period of dormancy
until stimulus causes
lytic cycle
Integration of viral
DNA into host’s
DNA  Provirus
(in this case a
prophage)
Viral Replication by Host Cell
(compare & contrast based on lecture diagrams)
Lytic Cycle
1. Viral DNA/RNA
transcribed and translated
by host cell
2. Cells lyse and releases
new viral particles
Lysogenic Cycle
1.
2.
Period of dormancy
until stimulus causes cell
to transcribe and translate
viral DNA (viral DNA is
incorporated into cell’s
DNA)
Cells lyse & release new
viral particles
Retroviruses
 RNA
core
 Replication – cells make viral
DNA from RNA (reverse
order)
 Enzyme: reverse
transcriptase – virus contains
it
 Ex.: HIV and tumor producing
viruses
HIV inside human
T4 lymphocyte
Retrovirus injects RNA and reverse transcriptase enzyme into
host cell;
viral DNA is integrated into cell’s DNA (provirus forms) and may
remain dormant for long various periods of time (lysogenic).
Reverse
transcriptase
New retroviruses
Provirus (viral DNA in cell’s
chromosome)
Viruses can be used to insert genetic material into cells.
Ex.: Gene therapy to cure genetic diseases
(Cystic Fibrosis)
Gene Therapy for CF
Possible Evidence of Virus Evolution
“Chunks” of DNA that can move around and be independent:
Plasmids and Transposons
Today = Plasmid in Bacteria
Today = jumping genes in Corn
Viral Disease Examples
 Common
cold
 Influenza
 Rhinovirus
 Smallpox
(eradicated)
 Chicken pox
 Rabies
 Polio (still out
there…)
 Mumps
 Rubella
 Ebola
(90%
lethality)
 Lassa fever
 Dengue fever
 Many other
hemorrhagic
fevers
 The list goes on
and on and on….
Are Viruses Considered to be Living or Non-Living?
Characteristic
Nucleic Acid
Growth
Mutate
Production of ATP
Cell Organelles
Homeostasis
Metabolism
Independent
Reproduction
No, Viruses do not display this trait
Yes, Viruses do display this trait
Other Infectious Particles
– single strand of RNA, no capsids
Ex.: plant diseases
 Viroids
– infectious protein molecules
Ex.: CJD, scrapie, mad cow disease (BSE)
 Prions
Prion causes proper proteins to fold incorrectly
 cannot do their ‘job’ in the cell,
collect inside cell  cell death
Prion Disease – ‘holes’ in brain tissue