Viruses
Viruses
What are they?
How do they work?
Where do they come from?
And… What good are they?
What is a virus?
An infectious agent made up of:
a core of nucleic acid – DNA or RNA
(ONLY ABOUT 5 GENES)
a protein coat (capsid)
How Big are viruses?
Polio virus = 20 nanometers
20nm
1 nm – 0ne billionth of a meter!
3000 polio viruses fit across the diameter of a
period at the end of a sentence in your book.
They
havehave
no envelope
Some viruses
no envelope –
they’re naked!
Some viruses have an
envelope to cover them!
Envelope – came from host’s
cell membrane when virus
budded out of host
Viruses don’t reproduce…..
They Replicate.
Once inside a cell, the cell does
all the work and produces
new viruses using the inserted
Dna or Rna as a
Guide.
Attachment
The lytic cycle
Lytic Cycle
a
injection
Lysis
replication
assembly
Sometimes the virus doesn’t kill the
cell right away and it becomes part
of the cell’s genes.
If this happens, the virus DNA
becomes a prophage and can become
activated at any time (like a time
bomb).
In the meantime, the prophage is
passed on to all the offspring of
that cell …. Maybe for many
generations.
Assembly
lysis
Replication
Lytic
cycle
Viral dna becomes
activated
Attachment
Integration – virus
DNA joins cell’s DNA
Lysogenic
cycle
Each daughter cell
contains both
bacterial and viral dna
Cell’s DNA (along with
viral dna is replicated
Are viruses living or nonliving?
What are some characteristics of life?
Do viruses exhibit these characteristics?
When were viruses discovered?
Viruses have apparently always been around.
However, it wasn’t until 1897 that a Dutch
scientist named Beijerinck called an invisible
agent that was smaller than bacteria a virus
(Latin for poison). He was studying tobacco
leaves that had been infected with what we
now know as tobacco mosaic virus.
Classification of Viruses
By Shape
Host type
Animal viruses
function
Retroviruses
attack a certain
way.
DNA viruses attack
another way.
Plant viruses
Bacteria viruses
Retroviruses
Contain RNA
When infecting a cell, these viruses
have to transcribe the RNA to DNA
before the viral code can be read.
This requires an enzyme, reverse
transcriptase, to Change RNA to DNA
then the viral code can be added to the
cell’s DNA.
Once part of the cell’s Dna, the viral
code can cause the cell to make more
viruses.
HIV IS A RETROVIRUS
rna
CAPSID
REVERSE
TRANSCRIPTASE
Nonviral infectious agents
• Prions
– PIECE OF PROTEIN
– CAUSE OF MAD-COW
DISEASE
– CAN INFECT ANIMALS –
INCLUDING HUMANS
• vIROIDS
– Single strand of
RNA
– Causes plant
diseases
Human diseases caused by viruses
•
•
•
•
•
•
•
•
Common cold
Influenza (flu)
Chickenpox
Polio
HIV
Some pneumonia
Some meningitis
herpes
Viruses in the biosphere
• Animal diseases
– Distemper
– Rabies
– pneumonia
• Plant Diseases
– Discolor leaves
– Stunt growth
– Kill plants
Uses for viruses
• Vaccines – dead or weakened form that
stimulates the immune system to fight the
virus when exposed to it.
• Genetic engineering – use a virus carrier to
insert genes into diseased cells.
• Agriculture
– Pest control
– Colorful variations in some plants - flowers
interesting viruses
• Ebola virus – kills quickly; requires close
contact, such as when preparing a body for
burial or using dirty needles; since it kills so
fast, it generally dies out quickly.
• Human sarcoma virus –
causes tumors
• Adenovirus – causes
The Common cold
Bacteriophages – infect bacteria; very
well studied.
Smallpox – once wiped out entire nations, now a
memory –
Most successful vaccination program in world.
Smallpox virus
Aren’t you glad
there’s no smallpox
anymore?
Vaccines – dead or
weakened form of virus
injected to provide
immunity.
First vaccine
• Chinese had been using
the idea for centuries,
but didn’t market it.
• Dr. Edward Jenner noticed that
milkmaids who had had cowpox
rarely got smallpox.
– Injected a boy with cowpox pus – he got
cowpox
– When injected with smallpox pus, he did
not get it!
The end