VIRUS
CHAPTER 17
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Antibody
Antigen
DNA
Host cell
Lymphocyte
Lysogenic cycle
Lytic cycle
Membranous envelope
Mucous membrane
VOCABULARY
• Nucleic acid core
• Phagocytic white blood
cell
• Primary line of defense
• Protein capsid
• RNA
• Secondary line of
defense
• Tertiary line of defense
• Viral specificity
• White blood cell
• Describe the basic structure of a virus – p. 356-357
• Evaluate the evidence used to classify viruses as living or nonliving - p. 357-360
• Compare and contrast the lytic cycle and lysogenic cycles – p.
357-359
• Describe the body’s basic lines of defense against a viral attack
– p. 968-976
• Give examples of ways to reduce the chance of contracting a
viral disease – p.973-975, p.980-981
• Define and give examples of viral specificity CHAPTER 45 –
p.973-975, p.979-981
• Evaluate the effects of virulence on human health CHAPTER
45 – p. 973-975, p.979-981
READINGS
• Not living organisms
• Non-cellular
• Consist of a nucleic acid core (DNA or RNA) and a
protein coat called the capsid
• Capsid made of protein subunits called capsomeres
• Cannot grow or replicate on their own (inactive
particles)
Viral characterisitics
• Can only reproduce inside of a living host cell using its
raw materials & enzymes
• Lack ribosomes & enzymes needed for protein
synthesis or metabolism
• Are extremely small particles ranging from 20 - 400
nanometers on average
• Largest virus is 1000 nanometers in dimension
• Some can cause disease (smallpox, measles,
mononucleosis, influenza, colds, AIDS, Ebola
Viral characteristics cont.
EBOLA VIRUS
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Some may also cause cancers such as leukemias
Virus free cells are rare
Highly host specific (only infect certain cells)
Referred to as phages
Viruses are classified into 2 main groups by their
nucleic acid --- DNA or RNA Viruses
• DNA & RNA viruses are subdivided by capsid shape
& whether they do or don't have an envelope
Virus characteristics cont.
• DNA or RNA core surrounded by protein sheath
called capsid
• Nucleocapsid includes the viral nucleic acid & its
capsid
• Some form lipid rich covering around capsid called
the envelope
• Envelope usually formed from host cell membrane
• Envelope may have spikes to help chemically
recognize & attach to the host cell
• Shaped determined by the arrangement of proteins
making up the capsid
• TMV is rod shaped
Viral Structure
• Adenovirus & polio viruses are icosohedral (20 sided)
• T -phages have a head &
tail
• Among the most complex viruses
• Attack bacterial cells
• Composed of a icosohedral head,
tail, base plate, & tail fibers
• Long DNA molecule is inside the
head
• Tail helps inject the viral DNA into
host cell
• Tail fibers used to attach to host
Bacteriophages or T-Phages
• Contain RNA
• Have an enzyme called reverse
transcriptase which helps use the
RNA to make DNA
• Use the host cell's ribosomes &
raw materials to make viral
proteins
• Cause some cancers & AIDS
RETROVIRUSES
HIV VIRUS
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Viroids
Smallest particle able to replicate
Made of a short, single strand of RNA with no capsid
Cause disease in plants
Viroid attack on
potato
VIROID
• No nucleic acid or capsids
• Made of protein particles that have folded
incorrectly
• Attacks the central nervous system
• Cause animal diseases in cows (Mad Cow
disease), sheep, & humans
PRIONS
• Viral replication that rapidly kills the host cell causing
it to lyse or burst
• Involves 5 steps ----- Adsorption, Injection,
Replication, Assembly, & Lysis
LYTIC CYCLE
• Adsorption --- phage attaches to cell membrane of
host
• Injection --- nucleic acid (DNA) of virus injected into
host cell
• Replication --- viral DNA inactivates host cell's DNA
& uses host's raw materials & ribosomes to make
viral DNA, capsids, tails, etc.
• Assembly --- new viral parts are combined to make
new phages
• Lysis --- enzymes weaken & destroy the cell
membrane causing it to lyse releasing new viruses that
infect other cells
LYTIC CYCLE
LYTIC CYCLE
• Phases of the Lytic Cycle of a Virulent
Virus:
• Absorption: Virus attaches itself to the
cell. Entry: Enzymes weaken the cell
wall and nucleic acid is injected into the
cell, leaving the empty caspid outside the
cell. Many viruses actually enter the host
cell intact. Replication: Viral DNA takes
control of cell activity. Assembly: All
metabolic activity of the cell is directed
to assemble new viruses. Release:
Enzymes disintegrate the cell in a
process called lysis, releasing the new
• Replication in which the virus stays inactive inside of
the host cell & doesn't immediately kill it
• Viruses are called temperate phages
• Lysogenic steps include adsorption, injection,
recombination, cell reproduction, activation,
replication, assembly, & lysis
LYSOGENIC CYCLE
• Recombination ---Viral DNA joins with host cell DNA
forming an inactive prophage
• Host cell reproduces normally until activated by an
external stimuli
• External stimuli unknown, but could be ultraviolet
radiation, carcinogens, etc.
• Once activated, prophage forms new viruses &
destroys host cell
LYSOGENIC CYCLE
• The Lysogenic Cycle of a Temperate Virus:
• The virus attaches itself and injects its DNA
into the cell.
• The viral DNA attaches itself to the host DNA,
becoming a new set of cell genes called a
prophage.
• When the host cell divides, this new gene is
replicated and passed to new cells. This causes
no harm to the cell, but may alter its traits.
• Now there are two possibilities:
• The prophage survives as a permanent part
of the DNA of the host organism.
• Some external stimuli can cause the
prophage to become active, using the cell to
produce new viruses
• Primary line of defense – e.g., skin, mucous membrane,
tears
• Secondary line of defense – e.g., phagocytic white blood
cells engulf viruses
• Tertiary line of defense – e.g., white blood cells called
lymphocytes produce antibodies
DEFENSE
• How Do Vaccines Work?
• When a person is sick, the immune system makes
antibodies that have the ability to remember the pathogen.
• With subsequent exposure to the pathogen, the immune
system quickly responds by producing white blood cells
to fight the infection, which results in no or only minor
symptoms of disease.
• This adaptive immune system response occurs because
the immune system is capable of immunologic memory.
• Vaccines work the same way except they cause immunity
to a pathogen without causing the symptoms and
complications of disease.
VACCINE
• How Are Vaccines Made?
• Vaccines use altered versions of viruses or bacteria to
trigger an immune response without causing any
symptoms of disease.
• There are four basic ways vaccines are made.
HOW ARE VACCINES MADE
• 1. Weaken the virus: Viruses require cells to reproduce
themselves. If they are grown repeatedly in cells other
than those they are used to, they change and are no longer
able to grow well in the original cells. This is known as
cell culture adaptation. Examples of vaccines that are
made this way include measles, mumps, rubella,
chickenpox, shingles and rotavirus.
• 2. Inactivate the virus: The virus is killed so that it cannot
replicate. Examples include polio shot, hepatitis A,
influenza shot and rabies.
• 3. Use part of the virus: A piece of the virus is used to
make the vaccine. Examples are HPV and hepatitis B.
• 4. Use part of the bacteria:
• a. Toxoid vaccines: Some bacteria produce poisons, called
toxins, which cause illness. When the toxins are treated with a
chemical, the toxins are no longer able to cause disease.
Inactivated toxins are called toxoids. Examples include
diphtheria, tetanus and pertussis.
• b. Polysaccharide vaccines: Some bacteria are covered with a
sugar coating called a polysaccharide. In some cases, the
polysaccharides used in vaccines are attached to a helper
protein to make the vaccine. Examples include Hib,
pneumococcal and meningococcal.
• http://www.youtube.com/watch?v=EqK1CYYQIug&list=
PLFCE4D99C4124A27A&index=44&feature=plpp_vide
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• http://www.youtube.com/watch?v=L8oHs7G_syI&list=P
L7A750281106CD067&index=22&feature=plpp_video