1. Biology Sylvia S. Mader Michael Windelspecht
Chapter 20
Viruses, Bacteria, and Archaea
Lecture Outline
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2. 20.1 Viruses, Viroids, and Prions
Biology, 9th ed,Sylvia Mader
20.1 Viruses, Viroids, and Prions
Chapter 21
Viruses, Bacteria & Archaea
Virus
Associated with a number of plant, animal, and human diseases
noncellular
May have a DNA or RNA genome.
Invention of the electron microscope allowed these infectious agents to be first seen
French chemist Louis Pasteur suggested that something smaller than a bacterium was the cause of rabies

3. Viruses, Viroids, and Prions
Biology, 9th ed,Sylvia Mader
Chapter 21
Viruses, Viroids, and Prions
Viruses, Bacteria & Archaea
Viral Structure and Classification:
Viruses are characterized by
Size and shape
10–400 nm in diameter
Type of nucleic acid core
Single stranded or double stranded? DNA or RNA?
Capsid or no capsid (outer layer composed of protein subunits):
Some are enveloped by membrane
Others “naked”

4. Viruses, Viroids, and Prions
Biology, 9th ed,Sylvia Mader
Chapter 21
Viruses, Viroids, and Prions
Viruses, Bacteria & Archaea
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Capsid (protein)
Covering
Envelope (not found in all viruses)
Virus particle
Nucleic acid molecule (DNA or RNA)
Inner core
Various proteins (enzymes)

5. Viruses, Viroids, and Prions
Biology, 9th ed,Sylvia Mader
Viruses, Viroids, and Prions
Chapter 21
Viruses, Bacteria & Archaea
Viruses:
Are obligate intracellular parasites
Are either active or inactive, instead of living or non-living
Can be cultured only inside living cells
Chicken egg
Tissue culture
Three hypotheses about viral origin and evolution
Proteins and nucleic acids, organic molecules in viruses, evolved and
viruses may have arisen from these two basic polymers when cells did.
Viruses may have evolved backwards from living cells.
Viruses degenerated from living cells.

6. Viruses, Viroids, and Prions
Biology, 9th ed,Sylvia Mader
Viruses, Viroids, and Prions
Chapter 21
Viruses, Bacteria & Archaea
Reproduction of Viruses:
1. Attachment: Virus gains entry into specific host cell based on host-specific match between virus surface molecules and host cell receptors.
2. Penetration: Host cell engulfs virus or virus injects its genome into the cytoplasm.
3. Biosynthesis: New viral components are synthesized using host cell’s machinery and energy.
4. Maturation:
5. Release: New viruses exit host cell through lysis or budding in order to infect new host cells.

7. Viruses, Viroids, and Prions
Biology, 9th ed,Sylvia Mader
Viruses, Viroids, and Prions
Chapter 21
Viruses, Bacteria & Archaea
Bacteriophages – Viruses that infect bacterial cells
There are two types of bacteriophage life cycles.
The lytic cycle
Viral reproduction occurs
Hundreds of virus particles are released
The lysogenic cycle
Viral reproduction does not occur immediately but may occur in the future.
Virus becomes integrated into the host genome and may reenter lytic cycle.
Diphtheria is caused by a prophage-carrying bacterium, which produces a toxin that damages the lining of the upper respiratory tract, restricting breathing.

8. Viruses, Viroids, and Prions
Biology, 9th ed,Sylvia Mader
Chapter 21
Viruses, Viroids, and Prions
Viruses, Bacteria & Archaea
Reproduction of Animal Viruses:
Animal virus enters the host cell
Uncoating releases viral DNA or RNA
Budding:
Acquires a membranous envelope
Retroviruses (HIV, the virus that causes AIDS)
Contain reverse transcriptase
Carries out RNA  cDNA reverse transcription
cDNA becomes integrated into host DNA
HIV may remain latent for years
Viral DNA is transcribed; new viruses are produced

9. Biology, 9th ed,Sylvia Mader
Flu Virus
Chapter 21
Viruses, Bacteria & Archaea
A flu virus has an H (hemagglutinin) spike and an N (neuraminidase) spike
H spike allows the virus to bind to the receptor
16 different types
N spike attacks host plasma membranes
Allows mature viruses to exit the cell
9 different types
Our immune system only recognizes H spikes and N spikes it has been exposed to.
Currently the H7N9 and H5N1 subtypes of flu virus are of great concern because they can potentially become pandemics (global outbreaks).

10. Biology, 9th ed,Sylvia Mader
Chapter 21
Emerging Viruses
Viruses, Bacteria & Archaea
Emerging viruses are new or previously uncommon illnesses.
Examples are AIDS, West Nile encephalitis, hantavirus pulmonary syndrome (HPS), severe acute respiratory syndrome (SARS), Ebola, hemorrhagic fever, and avian influenza
Several types of events can cause emergence of viruses
A virus may extend its range.
A genetic mutation may occur.
Example: Influenza strains H5N1, H1N1, and H7N9 were created through mutation of flu viruses which only infected animals
It is necessary to obtain flu vaccine each year due to the rapidly mutating flu virus

11. Viruses, Viroids, and Prions
Biology, 9th ed,Sylvia Mader
Viruses, Viroids, and Prions
Chapter 21
Viruses, Bacteria & Archaea
Viruses are best known for causing infectious diseases in plants and animals.
Herpes, HIV, cancer
Viruses lack metabolism; thus, antibiotics have no effect
Viroids
Naked strands of RNA
Prions
Protein molecules with contagious tertiary structure
TSEs are neurodegenerative diseases which destroy nerve tissue in the brain
They are untreatable and fatal
Some practices, including eating brains of cattle, transmitted the cattle disease (BSE or bovine spongiform encephalopathy) to humans.