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Viruses
Objectives
 Identify different types of viruses
 Compare replication cycles
 Discuss how viruses cause disease
The Viruses
• Viruses are noncellular and thus cannot be classified
with cellular organisms
• Generally smaller than 200 nm in diameter
• Usually named after the diseases they cause
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Viral Structure
• Each type has at least two parts
– Capsid: Outer layer composed of protein subunits
•
•
Some enveloped by membrane
Others “naked”
– Nucleic acid core: DNA or RNA
• Vary in shape from thread-like to polyhedral
Virus Structure
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Parasitic Nature
 Viruses are:
 Obligate intracellular parasites

Cannot reproduce outside a living cell
 Can be cultured only inside living cells


Chicken egg
Tissue culture
 Most are host specific
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Viral Categorization
 Classification is based on:
 Type of nucleic acid
 Size and shape
 Presence / absence of outer envelope
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The Bacteriophages:
Reproduction
 Bacteriophages – Viruses that infect bacterial cells
 Portions of capsid adhere to specific receptor on the
host cell
 Viral nucleic acid enters the cell
 Once inside, the virus takes over metabolic machinery
of the host cell
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Bacteriophages:
The Lytic Cycle
 Lytic cycle may be divided into five stages:
 Attachment
 Penetration
 Biosynthesis
 Maturation
 Release
 Kills the host cell
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The Bacteriophages:
The Lysogenic Cycle
 Phage becomes a prophage (provirus)
 Becomes integrated into the host genome
 Becomes latent

May later reenter the lytic cycle
 Doesn’t kill the host (at first anyway)
 Disease causing viruses
 Herpes simplex I (cold sores) and II (genital herpes),
and hepatitis B, chicken pox (shingles)
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Reproduction of Animal Viruses
 Animal virus enters the host cell
 Uncoating releases viral DNA or RNA
 Budding:
Viral particles released in a bud
 Acquires a membranous envelope
Examples:

 Retroviruses (HIV >>>>AIDS)
 Contain reverse transcriptase
 Carries out RNA  cDNA reverse transcription
 cDNA becomes integrated into host DNA
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Retrovirus HIV-1
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Viral Infections
• Viruses are best known for causing infectious
diseases in plants and animals
• Examples:
– Herpes, HIV, cancer
•
Viruses lack metabolism; thus, antibiotics have no effect
– Viroids
• Naked strands of RNA
• Many crop diseases
– Prions
• Protein molecules with contagious tertiary structure
• Some human and other animal diseases - Mad cow disease
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Diseases caused by viruses:
In Humans
 Cold/flu
 Measles
 Herpes
 polio
 Smallpox
Warts
HIV
mononucleosis
In Animals
Distemper
Foot and Mouth disease
Feline Leukemia
In Plants
Tobacco Mosaic Disease
Control of Viruses
White Blood Cells in humans
Antibodies
Interferon (chemical
produced in the body)
Vaccines
Examples of Viruses
18-2 Archaebacteria and
Eubacteria
Objectives
 Compare types of Prokaryotes
 Explain the characteristics and adaptations of bacteria
 Evaluation the economic importance of bacteria
The Prokaryotes
 Include eubacteria and archaeabacteria, which are
fully functioning cells
 A single spoonful of earth can contain >1000 prokaryotes
 Range in size from 1-10 µm in length and 0.7-1.5 µm in
width
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Prokaryote Structure
 Characteristics:
• Lack a membrane-bounded nucleus (DNA in
•
•
•
•
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nucleoid region)
Outer cell wall containing peptidoglycan
Some move by means of flagella
Lack membranous organelles
May have accessory ring of DNA (plasmid)
a
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Reproduction in Prokaryotes
 Asexual
 Prokaryotes reproduce asexually by means of binary
fission
 Methods of genetic recombination
 Conjugation


Sex pilus forms between two cells
Donor cell passes DNA to recipient cell through pilus
 Transformation – taking up pieces of DNA secreted
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by live bacteria or released by dead bacteria
 Transduction-transfer of portions of bacterial DNA
from one cell to another
Pilus
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Fission
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Reproduction in Prokaryotes
 Some bacteria form resistant endospores under
unfavorable conditions
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Clostridium tetani
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Prokaryotic Nutrition
 Oxygen requirements:
 Obligate aerobes – unable to grow in the absence of free
oxygen
 Obligate anaerobes – unable to grow in the presence of
free oxygen
 Facultative anaerobes – able to grow in either the
presence or absence of free oxygen
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Legume
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The Bacteria (eubacteria)
 Bacteria are commonly diagnosed using the Gram
stain procedure
 When washed after staining:
 Gram-positive bacteria retain dye and appear purple
 Gram-negative bacteria do not retain dye and appear
pink
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The Bacteria
 Structure of cell wall also of diagnostic use
 Bacteria can be further classified in terms of their
three basic shapes
 Spiral (spirilli),
 Rod (bacilli), and
 Round (cocci)
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Bacteria
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Cyanobacteria
 Formerly called the Blue-Green algae (Cyanophyta)
 Cyanobacteria are Gram-negative bacteria that
photosynthesize
 *Believed to be responsible for introducing oxygen into
the primitive atmosphere
 Lack visible means of locomotion
 Can live in extreme environments
 When commensals with fungi, form lichens
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Cyanobacteria
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The Archaea
 Archaea were earlier considered bacteria
 their rRNA differs from Bacteria
 Other differences:
 cell walls
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Archaea Metabolism
• Most are chemoautotrophs
• Some mutualistic
• Some commensalistic
• None known to be parasitic
• None are photosynthetic
• Many live in harsh conditions
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Types of Archaea
 Many live in harsh conditions:
 Anaerobic marshes


Methanogens
Produce methane from hydrogen gas and carbon
dioxide
 Salty lakes


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Halophiles
Require high salt concentrations for growth
 Hot sulfur springs



Thermoacidophiles
Reduce sulfides and survive best at temperatures
above 80ºC
Plasma membranes contain unusual lipids convey
tolerance of high temperatures
Other Facts about bacteria
can cause disease
(Pneumonia, Strep. throat,
TB, food poisoning,
syphilis, gonorrhea…)
Structure
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Control of harmful bacteria
some can be destroyed or
growth slowed with high
temps, chemicals, steam,
cold, antiseptics
sterilization, sanitation,
pasteurization
Bacteria can be beneficial
 Decomposition or recycling of
nutrients
 Digestion (vitamins)
 make breads, beers, and wine
 antibiotics
 food texture (yogurt, cheeses)
 genetic engineering – insulin
 Nitrogen fixation
 Borrelia burgdorferi
Staphylococcus