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Viruses
Warm Up


What is a virus?
How do you think (or do you remember from
the video) a virus replicates?
Homework:
 Virus Replication WS due Friday

What is a virus?

A virus is a particle (not a cell) made of
nucleic acid (DNA or RNA) and a protein
coat
 Viruses don’t require food or water
 Viruses can only replicate in a host cell by
using the host cell’s “machinery”
How does its size compare to that of
various cells?

Viruses are smaller than prokaryotic cells
(like bacteria) and significantly smaller than
eukaryotic cells (like human cells)
 http://www.cellsalive.com/howbig.htm
Is a virus a living organism?

Most scientists describe viruses as non-living
 Don’t consist of cells
 Use “machinery” of other cells to carry
out necessary cell functions
Characteristics of Life
Virus
Cell
Growth
Homeostasis
Metabolism
Mutation
Nucleic acid
Reproduction
No
No
No
Yes
DNA/RNA
Only in host
Yes
Yes
Yes
Yes
DNA
Independently
Structure of a Virus

A virus has three main parts:
Nucleic acid
 Capsid (protein coat)
 Glycoproteins (proteins on surface used for
attachment)
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
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Some viruses also have an envelope (protein
layer outside of the capsid)
Bacteriophages also have tail fibers
Come in a variety of shapes
Virus Structure
Examples of Virus Shapes
Making more copies

The purpose of a virus is to infect cells so
that it can force cells to make more copies of
itself. How does the virus make this happen?
1.
2.
3.
Virus glycoproteins recognize and
attach to receptor sites on host cell
(lock and key)
Virus enters cell
• Injection or
• “enveloped” by cell
Virus takes control of host cell
machinery
Virus “Life” Cycles


Up to this point, all viruses would have
followed the same steps as they entered the
cell.
Once inside a cell, a virus may follow one of
two “life” cycles:
 Lytic
 Lysogenic
Lytic Cycle

Virus has entered host cell:
3. Replicates itself immediately
• Takes over the ribosomes to make the
necessary proteins
4. Different proteins are assembled into
whole viruses
5. When enough copies are made, an
enzyme is produced that “lyses” the host
cell, releasing copies to infect more cells
Lysogenic Cycle


Virus enters cell (same as before)
Virus inserts its DNA into the host cell’s
chromosome (latent phase)
Virus may remain part of host cell chromosome for
a short while or for many years (considered
“temperate”
 HIV is a lysogenic virus. Why is this a problem?


Eventually, trigger causes virus to remove
itself from the host cell chromosome and enter
lytic cycle (considered “virulent”)
Lysogenic Cycle
http://highered.mcgrawhill.com/sites/0072556781/student_view0/chapter17/animation_quiz_2.html
Retroviruses

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Most viruses contain DNA. This makes it easy
for them to instruct their host cell in what to do
because the host cell recognizes DNA
instructions.
Retroviruses contain RNA instead of DNA.
 Host cell won’t recognize RNA instructions,
so virus must make a DNA copy of its RNA
first
 Reverse transcriptase (an enzyme) copies
RNA instructions into DNA
HIV is an example of a retrovirus
http://www.whfreeman.com/kuby/content/anm/kb03an01.htm
Reverse Transcriptase
RNA virus
DNA (single strand)
A
T
U
A
G
Reverse
transcriptase
C
C
G
C
G
A
T
DNA (double strand)
Single DNA
strand used as
template for
complementary
DNA strand
T
A
A
T
C
G
G
C
G
C
T
A
Particles Similar to Viruses

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Viroids
 Contain RNA
 No capsid
 Infect plants
Prions
 Clumped proteins
 Mad cow disease,
Creutzfeldt-Jakob
disease
Preventing Viruses

Vaccines exist for certain viruses
 Vaccines contain a pathogen (like a virus) in
a form that will trigger the immune system,
but not make the person sick
 Immune system will “remember” the
pathogen if the person is ever exposed again
and will be able to fight it quickly
Treating Viral Infections
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Very few treatments are available for viral
infections. Why?
 Viruses mutate very frequently- “old”
vaccines won’t work on “new” mutation
Some antiviral drugs exist that interfere
with protein production