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Viruses and
Infectious
Disease
Viral Structure
and
Reproduction
What is a Virus?
• Viruses are very
tiny, nonliving,
particles made of
either DNA or
RNA which is
enclosed in a
protein coat
called a capsid.
What makes them NONliving?
• Viruses do not exhibit ALL of the characteristics
of living things.
Characteristic
Viruses
Living Cells
Structure
DNA or RNA core,
protein capsid
Cell membrane, cytoplasm;
eukaryotes also contain
nucleus and organelles
Reproduction
Only with the help
of a host cell
Independent cell division
either asexually or sexually
Genetic Code
DNA or RNA
DNA
Growth and Development
No
Yes
Obtain and use energy
No
Yes
Change over time
Yes
Yes
Naming Viruses
• Most viruses are named after the disease
they cause.
• Some are named for the organ or tissue
that they infect.
Naming Viruses
• Viruses that infect bacteria are called
bacteriophages, or phage for short.
Virus Structure
• Viruses have two
basic parts:
– Inner core made of
nucleic acid (DNA or
RNA)
– Outer protein coat
called a “capsid”.
Virus Structure
• Some large viruses
have an envelope
surrounding their
capsid.
– Made of lipids, just like
the plasma membrane
of cells.
– Makes viruses more
infectious because
they can more easily
infect living cells.
Virus Structure
• Viral DNA or RNA contains only the
instructions for making more copies of the
virus.
Virus Structure
• The arrangement of
proteins in the capsid
determines the virus’
shape.
• Tobacco Mosaic virus –
long; helical
• Polyhedral – resemble
crystals
• Shape helps determine
what cell the virus infects
and how.
Viral
Reproduction
Viral Reproduction
• The cell in which a virus replicates
is called a host cell.
•Before a virus
can replicate, it
must attach to the
host cell and
inject its nucleic
acid (DNA or
RNA) into the cell.
Viral Reproduction
• Once initial infection has taken place, a
virus will enter one of two reproductive
cycles.
– Lytic
- Lysogenic
Lysogenic Infection
• Viral DNA integrates itself into host DNA. As the
host cell’s DNA replicates itself, it replicates the
viral DNA as well.
• Imbedded DNA is called a prophage.
• Viral DNA remains inactive for a period of time,
slowly incorporating itself into more and more
cells as the host cell divides.
• The host organism may not feel sick or show
any signs of infection in a lysogenic infection.
Lysogenic Infection
Lytic Infection
• The viral DNA enters the host cell and
begins to break down the host’s DNA.
• It then uses the host cells nucleotides to
produce more viral DNA copies.
• Soon, the cell is so full of new copies of
the virus that the cell bursts and
thousands of viruses leave the cell to
infect other cells.
Lytic Infection
Retroviruses
• Contain RNA as their
genetic material.
• Viral RNA enters the
host cell and
produces a DNA copy
of itself which
becomes a prophage.
– Ex. HIV (AIDS)
• Retroviruses may
remain dormant
(lysogenic) for a very
Infectious
Disease
What are pathogens?
• Bacteria and viruses are found
everywhere in nature and most are
completely harmless.
• A FEW cause disease.
• Disease-causing bacteria and viruses are
called pathogens.
Bacterial Disease
• There are 2 ways that bacteria produce
disease.
– Some damage cells and tissues by breaking
down the cells for food
• Ex. Tuberculosis is inhaled where it destroys lung
tissue.
Mycobacterium tuberculosis
Bacterial Disease
– Some release toxins (poisons) that travel
through the body, and interferes with the
normal activity of the host.
• Streptococcus –
– Causes Strep Throat
– Can release toxins into
the bloodstream, causing
Scarlet Fever.
• Tetanus –
– Toxins cause severe
muscle spasms, paralysis and death.
Preventing Bacterial Disease
• Vaccine – a preparation of weakened or
killed pathogens.
– Prompts the body to develop immunity to the
disease.
– Immunity is the body’s ability to destroy new
pathogens.
Preventing Bacterial Disease
• Antibiotics
– Compounds that block the growth and
reproduction of bacteria.
• Ex. Penicillin and tetracyclins
Controlling Bacterial Growth
• Sterilization by heat
– Most bacteria cannot survive high
temperatures for very long.
Controlling Bacterial Growth
• Disinfectants
– Chemical solutions that
kill bacteria
– Overuse of antibacterial
compounds increases
the likelihood that
bacteria will evolve to
become resistant to
them which makes them
more difficult to kill.
Controlling Bacterial Growth
• Food Storage and Processing
– Refrigeration
• Food stored at low temperatures lasts
longer because it takes bacteria longer
to multiply.
– Canning
• Preserves food almost indefinitely
– Food is sterilized by heat and then
immediately placed in sterile jars or cans.
Viral Disease
• Viruses produce disease by disrupting the
body’s normal equilibrium.
– Viruses attack and kill living tissue.
• Viral Diseases cannot be treated with antibiotics.
• The best treatment is prevention, usually
through the use of vaccines.
Common Infectious Diseases
Bacterial
Viral
Lyme Disease
Tetanus
Diptheria
Bacterial Meningitis
Strep throat
Tooth decay
Common cold
Influenza
Smallpox
Warts
AIDS (HIV)
Chickenpox
Measles
Polio
Hepatitis A, B, and C
West Nile