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Viruses
Viruses
• Virus – non-cellular particle made up of genetic
material and protein. Reproduce only by
infecting living cells.
• TMV – tobacco mosaic virus – first heavily
studied virus
• Dmitri Ivanovski – 1892, cause of TMV – juice
extracted from infected plants
• Martinus Beijerinck – 1897, named TMV particle
viruses
• Wendell Stanley – 1935, purified TMV into a
crystal – viruses are not living things.
Virus Anatomy
• Many shapes possible
• Typical virus composed of a core of either DNA or RNA (not both),
surrounded by an outer protein coat, or capsid. Capsid includes
proteins that enable the virus to enter a host cell.
Some Virus Structures
Tobacco Mosaic
Virus
T4 Bacteriophage
DNA
Head
Influenza
Virus
RNA
Capsid
proteins
RNA
Capsid
Tail
sheath
Tail
fiber
Surface
proteins
Magnification: 82,000X
Magnification: 200,000X
Membrane
envelope
Magnification: 1,000,000X
Viral Infection
• *Viruses attach with tail fibers and inject
genetic material into host cell. Most
viruses are highly specific to the cells they
infect. Generally, they are species
specific, but they may infect larger groups
of organisms (rabies and mammals).
• - Bacteriophages – viruses which invade
bacteria.
Lytic Infection
• Host cell lysed and destroyed – kills cells
quickly!
– Ex. Cold, flu viruses
• Virus takes over cell and uses cell to make
copies of itself
The Lytic Cycle
Bacteriophage
protein coat
Bacteriophage DNA
Bacterial
chromosome
Bacteriophage attaches to
bacterium’s cell wall
Bacteriophage enzyme lyses the
bacterium’s cell wall, releasing
new bacteriophage particles that
can attack other cells.
Lytic Cycle
Bacteriophage injects DNA
into bacterium
Bacteriophage proteins and
nucleic acids assemble into
complete bacteriophage
particles
Bacteriophage takes over
bacterium’s metabolism, causing
synthesis of new bacteriophage
proteins and nucleic acids
Bacteriophage
Bacteriophage DNA
Bacteriophage protein
Lysogenic Infection
• Does not lyse host cell right away – lies in wait and
doesn’t kill cell quickly
• Ex. Herpes (cold sores), HIV viruses
• Virus embeds its DNA into DNA of host cell and is
replicated along with the host cell’s DNA. Can lie
dormant for years!
The Lysogenic Cycle
Bacterial
chromosome
Bacteriophage
DNA
Bacteriophage injects
DNA into bacterium
Bacteriophage DNA
(prophage) can exit the
bacterial chromosome
Lytic
Cycle
Bacteriophage enzyme lyses the
bacterium’s cell wall, releasing
new bacteriophage particles that
can attack other cells
Lysogenic
Cycle
Bacteriophage DNA
(prophage) may replicate
with bacterium for many
generations
Bacteriophage DNA
forms a circle
Bacteriophage proteins and
nucleic acids assemble into
complete bacteriophage particles
Prophage
Bacteriophage DNA
inserts itself into
bacterial chromosome
Lytic vs. Lysogenic
Retroviruses
• Important group of viruses
• AIDS, contain RNA, once inside the cell,
the RNA produces DNA which acts as a
prophage and binds with the cell’s DNA.
• Virus may remain inactive for many years
• “Retro” means backward, RNA to DNA,
may cause some forms of cancer and
AIDS
Are Viruses Alive?
• Viruses are known as parasites
• Viruses are not cells and cannot
reproduce independently. But, when infect
living cells, can make copies of
themselves, regulate gene expression,
and even evolve.
Diseases Caused by Viruses
• Pathogens – disease producing agents
• Viruses – smallpox, polio, measles, AIDS,
mumps, influenza, Yellow Fever, rabies,
common cold.
– No cure, the only way to be healed is for your own
immune system to produce antibodies to resist the
virus. Antibiotics don’t work for viruses!
– Interferons – release by cells when they are attacked
by a virus, might be a possible treatment.
– Cancer – oncogenic viruses cause cancer. Not all
viruses cause cancer, not all cancers are caused by
virus.
– Prevention is best protection – vaccinations – weak or
killed virus
Common Diseases Caused by
Viruses
Type of Virus
Nucleic Acid
Disease
Oncogenic viruses
DNA
cancer
Retroviruses
RNA
cancer, AIDS
Adenoviruses
DNA
respiratory infections
Herpesviruses
DNA
chickenpox
Poxviruses
DNA
smallpox
Diseases Caused by Bacteria
• Bacteria – diphtheria, tuberculosis, typhoid
fever, tetanus, Hansen Disease, syphilis,
cholera, bubonic plague
– Bacterial illnesses are caused by toxins
produced as bacteria grow
– Bacterial disease can be treated with
antibiotics and prevented with vaccines
– Bacteria are becoming resistant to antibiotics.
Similarities Between Bacterial
and Viral Infections
• Modes of transmission similar
– Through the air, in water, in food, in body
fluids, animal bites/scratches
• Both cause diseases and can be deadly
• Both have ways of being prevented
– Vaccines for both and good
hygiene/sanitation prevents disease
Differences Between Bacterial
and Viral Infections
• Antibiotics only treat BACTERIAL infections!
They do NOT work on viral infections.
• Viral infections are treated by the body’s immune
system
• Bacteria are living cells who poison or “eat” our
own cells.
• Viral cells inject genetic material into our own
cells and use our own cells against our body