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3.a.1 – DNA, and in some cases RNA, is the
primary source of heritable information (19.2).
3.c.3 – Viral replication results in genetic
variation, and viral infection can introduce
genetic variation into the hosts (19.1 & 19.2).
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An infectious particle consisting of nucleic acid
enclosed in a protein coat
Characteristics:
1. Small size
 Large - barely visible with the light microscope
 Small - down to 20nm
 Much smaller than cells (including bacteria)
2. Infectious
3. Reproduction
4. Alcohol resistance
 Not immediately killed by alcohol (like your cells)
 Infection:
◦ Viruses cause many diseases
◦ They can be spread from one organism to
another
 Reproduction:
◦ Can not reproduce without a host
◦ Viruses are obligate parasites – must have
host to survive
1. Genome - the genetic information
2. Capsids and Envelopes - the outer
covering
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Protein shell that encloses the viral
genome
Made from a large number of protein
subunits
Number of kinds of subunits usually small
Often geometric in shape
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Membranes cloaking the capsids of some viruses
Made from host membrane, but may have virusoriginated glycoproteins added
1.
2.
3.
4.
5.
Cell Entry
Genome Replication
Capsid Protein formation
Self-assembly of offspring
Exit from Host
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Locate host by "Lock-and-Key” fit between virus
proteins and cell receptor molecules
Infection - Begins when viral genome enters the
cell
Some viruses can enter many hosts
◦ Ex: West Nile – mosquitoes, birds and humans
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This step begins the actual viral infection
Host cell is reprogrammed to copy virus
genome
Use host DNA polymerase
Use virus enzymes to copy RNA to DNA
Use host's resources to make copies of viral
proteins
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Host's protein synthesis machinery used to
make virus proteins
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genome + capsid  virus
Particles usually assemble spontaneously
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Host cell may burst or lyse, releasing the virus
May "bud-off" host membrane, forming envelopes
around the capsids
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Best understood of all viruses
Called phages
May show two types of virus life cycles:
◦ Lytic Cycle
◦ Lysogenic Cycle
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Virus reproductive cycle that kills the host cell
Note - the previously described virus life cycle
was a Lytic Cycle
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Uses Lytic Cycle
Example of an Virulent Virus (a virus that only
uses the lytic cycle)
Has about 100 genes
Completes cycle in 20-30 minutes
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Virus reproduction that doesn't immediately kill
the host cell
◦ Usually doesn’t kill the host AT ALL
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Viral DNA is inserted into the host DNA, but not
expressed
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Belong to several viral classes
Specific Interests:
1. Viruses with envelopes
2. RNA viruses
Ex. of Infections caused by animal viruses: Cold
sores/canker sores, influenza, common cold
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Use nuclear membrane
Causes: cold sores, shingles, chicken pox,
mononucleosis
Viral DNA integrated into Host DNA as a
provirus
Shows both lytic and lysogenic life cycles
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Classes III – VI
Ex: yellow fever, rubella, common cold, SARS,
ebola
Class VI - Retrovirus - use Reverse Transcriptase
to make DNA from an RNA template.
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HIV - causes AIDS
Retrovirus from chimps
Destroys the body’s immune system, allowing
other diseases to kill
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Measles
Polio
Smallpox
Influenza
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Vaccines (preventative)
Some Drugs
◦ Ex: Ara-A Acyclovin (Herpes treatment)
◦ Ex: AZT – HIVirus
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Comment - some treatments are working on the
reverse transcriptase
Problems:
◦ Since viruses work within a host cell, they are very
difficult to treat
◦ Best Treatment – prevention
◦ Can also cause cancer
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Infectious particles of naked RNA
Affect plants
Similar to viruses, but lack a capsid
Ex:
◦ Coconut Blight
◦ Chrysanthemum Wilt
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Infectious protein particles
Affect animals
Cause “mad-cow” and Creutzfeldt-Jakob disease
Transmitted in food
Slow-acting
Virtually indestructible
Protein folding problem?
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DNA circle (Chromosome)
Plasmids - Small circle of DNA that is
independent of the chromosome
Carries a small number of traits
1.
2.
3.
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Transformation
Conjugation
Plasmids
This is how bacteria mix genetic material
into new combos
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Alteration of a cell's DNA by the uptake of foreign
DNA
Ex: Griffith's experiment
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Direct transfer of genetic material between two
bacterial cells
Bacterial "sex”
Used to map genetic sequences in bacteria
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Small circular piece of DNA
Carry many important traits
Ex: Fertility Factor
Antibiotic Resistance
(R Plasmids)
Virus movie
http://www.youtube.com/watch?v=Rpj0emEGShQ