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Intimate Strangers
The Infectious Agents of Disease
Infectious Agents
•Viruses
•Bacteria
•Protozoa
Viruses
• Sub microscopic “primitive life
form”
• Can not self replicate
• Obligate intracellular parasite
(totally dependent on host cells for
replication)
• Unaffected by antibiotics
• Made of a core of DNA or RNA
surrounded by a protein coat
Virus-From the Latin for ‘poison’
Viruses straddle the definition of life. They lie
somewhere between molecular complexes and
very simple biological entities. Viruses contain
some of the structures and exhibit some of the
activities that are common to organic life, but they
are missing many of the others. In general, viruses
are entirely composed of a single strand of genetic
information encased within a protein capsule.
Viruses lack most of the internal structure and
machinery which characterize “life”. In order for a
virus to replicate it must infect a suitable host cell.
Basic Structure of a Virus
• Capsid- protein coat
containing DNA or
RNA
• Envelope- similar to a
membrane surrounding
the capsid in some
viruses; proteins, lipids,
and glycoproteins make
up the envelope
Nucleic Acid-DNA or RNA?
• Viruses have either
single-strand RNA
or double-strand
DNA, but never
both.
• Those that contain
RNA are termed
retroviruses
• Ex: HIV
Virus Structure
Polyhedral
(Capsid has 20 triangular
faces and 12 corners)
Ex: Influenza Virus
Helical
(Capsid is a coiled spring)
Ex: Tobacco Mosaic Virus
Bacteriophage
• Virus that infects bacteria
• Structure is unique: polyhedral head
on a helical tail with DNA inside the
head
• Has “legs” that land on the surface of
the bacteria to infect it
Viral Roles
• Viruses can be
harmful.
 Virulent disease
causing.
 Temperate not
immediately
disease
causing.
 Viruses play a
role in causing
some cancers.
• Viruses may be
helpful.
 A temperate
virus can be used
to cause genetic
variation within a
host population.
(tulips)
 Can be used to
introduce genes
into defective
DNA “biologic
vectors” (genetic
engineering)
Viral Infection
• Lytic Infection
• A. Virus injects DNA into host cell and
forms a circle
• B. Host begins making viral proteins (can’t
tell the difference b/t viral and its own
DNA)
• C. Assembles new viruses
• D. Lyses (bursts) the cell releasing new
viruses
• Ex: T4 Bacteriophage
Viral Infection
• Lysogenic Infection
• A. Virus injects DNA into host cell
• B. DNA forms a prophage (literally
embeds itself into the hosts’ DNA)
• C. Remains dormant for a period of time
until environmental factors activate it into
the lytic cycle (chemicals,radiation,illness,
stress, etc)
• D. Goes directly into the lytic cycle
• Ex: Herpes Simplex Virus
Virus Life Cycle Animations
1.
2.
3.
4.
http://www.hopkinsaids.edu/hiv_lifecycle/hivcycle_txt.html
http://www.sumanasinc.com/webcontent/anisamples/maj
orsbiology/lifecyclehiv.html
http://faculty.ccbcmd.edu/courses/bio141/lecguide/unit3/
viruses/adlyt.html
http://student.ccbcmd.edu/courses/bio141/lecguide/unit3
/viruses/lysosum01.html
1= HIV animation w/drug info
2= HIV narrated
3= Lytic Cycle
4= Lysogenic Cycle
Just how big is a virus?
• http://www.cellsalive.com/howbig.ht
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Some Diseases Caused by
Viruses
Plants
Tobacco mosaic
Tomato bushy
Animals
Foot and Mouth
Rabies
Humans
Common cold
German measles
Alfalfa mosaic
Distemper influenza
Cold Sores
Sugar beet curly top Cowpox
Polio
Dwarfism in rice
Herpes, AIDS,
Hepatitis
Smallpox