What …
• have genetic material and can mutate and
evolve,
• can not metabolize food,
• can not respond to stimuli,
• are not made up of cells,
• can only reproduce inside of a host cell,
• act as nonliving chemicals when outside a
host cell,
• form parasitic relationships with living
organisms?
Pseudo-Life Forms…Viruses
Pseudo-Life Forms…Viruses
Structure…
Viruses contain an inner nucleic acid core
(genetic material) and an outer protein coat
(capsid).
Enveloped Virus
Lipid envelope
Protein Receptors
Enzymes
Nucleic Acid (RNA)
Capsid: core
protein coat
25 nm
Diagram of a Retrovirus or RNA carrying virus
Naked Virus
Nucleic Acid
(DNA)
Capsid: core
protein coat
Protein Receptors
on tail fibers
Diagram of a Bacteriophage
or T4 DNA carrying virus
Viruses …Structure
• The virus may also contain some
enzymes.
• An outer envelope (membrane) that is
derived from the cell membrane of the host
cell may surround the capsid.
• The envelope contains viral protein
spikes.
• They are usually smaller than 200 nm.
What is a Nanometer?
Virus (20-50 nm)
Tobacco Smoke
(100 -1,000 nm)
Bacteria ( 3,000-5,000 nm)
Fungus Spores (5,00030,000 nm)
Plant Pollen (10,000- 80,000 nm)
Rain Droplet ( 600,000 – 10,000,000 nm)
What is a Nanometer?
Object
Characteristic Size
person
1 m = 1000 mm
grain of sand
1 mm
human hair (thickness)
.1 mm = 100 µm
red blood cell
10 µm
wavelength of light
1 µm = 1000 nm
virus
100 nm
atom
.1 nm
1 m  103 mm 106 µm  109nm
~ Viron size ranges from 20-200 nm
10 x smaller
100 x
smaller
1000 x
smaller
Viruses… Structure
•The genetic material in some viruses is
DNA; in others it is RNA (as in
retroviruses).
•Both nucleic acids (NA) strands can be
either a double (ds) or single (ss) stranded.
Viruses…Reproduction
…requires viruses to be in intimate
contact with a specific host organisms
and a specific type of cell within that
host.
This is called virus-host Specificity
Viruses…Reproduction
Examples:
Influenza infects cells
lining the respiratory tracts.
Poliomyelitis virus
infects nerve cells.
Tobacco mosaic virus
infects tobacco leaves.
Viruses…Cycles
Lytic
Life Cycle…
1 - Adsorption
2 – Entry
3 – Replication
4 – Viron Assembly
5 – Lysis / Exit
Viruses…Cycles
Adsorption
…requires the virus to enter a cell
when either a part of the
capsid or the spikes in
the envelope match
receptors in the host
cell.
Viruses…Cycles
Lytic Cycle: Enveloped Virus
1 –Attachment or Adsorption
Viruses…Cycles
Lytic Cycle: Enveloped Virus
2 – Entry by Fusion and
Capsid Uncoating
Viruses…Cycles
Lytic Cycle: Enveloped Virus
2 – Entry by Endocytosis and
Capsid Uncoating
Viruses…Cycles
Lytic Cycle: Enveloped Virus
3 & 4 –Enveloped Virus
Replication & Assembly
Viruses…Cycles
Lytic Cycle: Enveloped Virus
5 – Lysis / Exit by Budding
Viruses…Cycles
Lytic
Life Cycle of an Enveloped Virus…
Viruses…Cycles
Reinfection
Some viruses, capable of causing
cell fusion, may be transported
from one cell to adjacent cells
without being released, that is, they
are transmitted by cell-to-cell
contact whereby an infected cell
fuses with an uninfected cell.
Viruses…Cycles
Lytic Cycle: Naked Virus
1 &2–Adsorption and Penetration
Viruses…Cycles
Lytic Cycle: Naked Virus
1 &2 –Adsorption and Injection
Viruses…Cycles
Adsorption and Injection…
Viruses…Cycles
Lytic Cycle: Naked Virus
3 & 4 – Replication & Assembly
Viruses…Cycles
Lytic Cycle: Naked Virus
5– Release via Cell Rupture
Viruses…Cycles
Lytic
Life Cycle of Naked Virus…
Viruses…Cycles
Reinfection
As many as 10,000 to 50,000 animal
viruses may be produced by a
single infected host cell.
Viruses…Cycles
Lysogenic Cycle ~ Replication without
Immediate Host Death
Viruses…Interrupting The Cycle
How does a Vaccination Work?
1 – Injection of broken virus particles
2 – Body responds by making
particles that recognize the virus and
kiling to it - antibodies
3 – White blood cells collect and
digest foreign particles covered in
antibodies.
Read pages 1063-1076
Viruses…Interrupting The Cycle
Neutralization of Viruses by Antibodies