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Chapter 20
Viruses and Bacteria
I. Viruses
A. Is a Virus Alive?
• All living things are made of cells, are able to grow
and reproduce, and are guided by information
stored in their DNA
• Viruses do not grow, do not have homeostasis,
and do not metabolize, therefore they are not
considered to be living
• Viruses are pathogens, or agents that cause
disease, and replicate by infecting cells and using
the cell to make more viruses
B. Viral Structure
• Viruses are segments of
nucleic acids contained in a
protein coat, or capsid
• The nucleic acid may either
be RNA or DNA, but not
both
- DNA viruses include those
that cause warts,
chickenpox, and
mononucleosis
• Many viruses have a membrane, or envelope,
surrounding the capsid
- helps virus enter cells
- consists of proteins, lipids, and glycoproteins
• Viruses exist in a variety of shapes
1. filaments
2. spherical
3. helical
4. polyhedral
• Bacteriophages specifically infect bacteria
- most bacteriophages consist of a polyhedron capsid
attached to a helical tail
C. Viral Reproduction
• Viruses must rely on living
cells for replication because
they have no structures to
make protein
1. Lytic Cycle
• The cycle of viral
infection, replication,
and cell destruction in
bacterial viruses
• Viral genes use the host cell
to replicate viral genes and to
make viral proteins, such as
capsids
• The proteins are then
assembled with the replicated
viral genes to form complete
viruses
• The host cell breaks open and
releases newly made viruses
2. Lysogenic Cycle
• Viral genes are inserted into
host chromosomes
- the resulting virus is called a
provirus
• Whenever the host cell divides,
the provirus also divides,
infecting all resulting host cells
without destroying them
• Changes in the environment
can cause the provirus to enter
the lytic cycle
3. Host Cell Specificity
• Viruses are often restricted to certain kinds of
cells
4. Enveloped Virus
• Many viruses that infect only animals have an
external viral envelope
ex. Human immunodeficiency virus (HIV) which
causes acquired immune deficiency
syndrome (AIDS)
• The viral envelope is composed of a lipid bilayer
derived from the membrane of the host cell
• Glycoproteins are embedded on the envelope
• The envelope holds the capsid, which in turn holds
the viral genes
D. How animal viruses infect cells
1. Attachment
• Glycoproteins on the envelope of HIV precisely
fits human cell surface receptors called CD4
receptors on macrophage (immune system)
cells
2. Entry into Macrophage
• The glycoprotein must also activate a second coreceptor, called CCR5 to start endocytosis into a
macrophage
3. Replication
• The HIV viral envelope is left
outside of the cell
• The HIV particle sheds its capsid
• The particle then releases an
enzyme called reverse
transcriptase
• Reverse transcriptase copies the
viral RNA into a complementary
DNA
• Translation of the viral DNA directs the production of
many copies of the virus by the host cell’s machinery
• New virus capsids are released from the cell by
budding and are thus covered with an envelope
derived from the host’s cell membrane
4. AIDS
• Years after the initial infection, HIV’s glycoproteins
change and start to recognize and bind to cell
surface receptors on lymphocytes called T cells
• Unlike its activity in macrophages, HIV reproduces in
T cells and then destroys them
• Upon destroying T cells, virus particles increase in
number in the blood and infect more lymphocytes
• It is this destruction of the body’s lymphocytes that
blocks the body’s immune response and signals the
onset of AIDS
E. Viral Diseases
• Emerging viruses are viruses that evolve in
geographically isolated areas and are pathogenic
to humans
• Prions are composed of proteins but have no
nucleic acid (cause mad cow disease)
• Viroids are composed of a single strand of RNA
that has no capsid
II. Bacteria
A. Bacterial Structure
• Prokaryotes that include organisms that
compose the kingdoms Eubacteria and
Archaebacteria
• Differ from eukaryotes in seven ways
1. Internal compartmentalization
• Lack a nucleus
2. Cell size
• One-tenth the size of eukaryotic cells
3. Multicellularity
• Single cells that are not specialized
4. Chromosomes
• Single circular piece of DNA
5. Reproduction
• Binary fission
- one cell pinches into two cells
6. Flagella
• eukaryotic cell flagella are more complex
7. Metabolic diversity
• Many metabolic abilities including anaerobic and
aerobic processes
B. Bacterial Cell Shapes
1. One of three basic shapes
a. coccuss – a round-shaped cell
b. bacillus – a rod-shaped cell
c. spirillum – a spiral cell
2. Cell walls
a. Eubacteria have cell walls made of
peptidoglycan surrounded by a gellike layer called a capsule
- two types distinguished by a Gram
stain; Gram- negative and Gram
positive
- important medically in
determining their susceptibility
to different antibiotics
b. Archaebacteria often lack cell walls
Gram –
Gram +
3. Endospores
• Thick-walled surroundings that form during harsh
conditions and enclose the chromosomes of some
bacteria
4. Pili
a. enable bacteria to adhere to surfaces
b. enable bacteria to exchange genetic material
through conjugation
C. Obtaining Energy
1. Photosynthesizers
a. green sulfur bacteria and purple sulfur bacteria
• Grow in anaerobic (oxygen-free)
environments
- cannot use water as a source of electrons
for photosynthesis
b. Purple nonsulfur bacteria
• Use organic compounds as a source of electrons
for photosynthesis
c. Cyanobacteria
• Often clump together in large mats of filaments
• Thought to have made the Earth’s oxygen
atmosphere
2. Chemoautotrophs
• Obtain energy by removing electrons from
inorganic molecules or organic molecules
• Manufacture their own amino acids and proteins
using hydrogen-rich chemicals
3. Heterotrophs
• Many grow in aerobic (oxygen-rich) environments
• Break down the bodies of dead organisms and
make nutrients available to other organisms
• Principal decomposers of the living world
D. Pathogenic Bacteria
1. Bacteria can metabolize their host
• Heterotrophic bacteria obtain nutrients by
secreting enzymes that break down organic
structures and then absorbing them
• The bacteria that causes tuberculosis uses
human tissue as their source of nutrients
2. Bacteria Toxins
• Bacteria cause disease by secreting chemical
toxins into their environment
• E.coli causes food poisoning by releasing a toxin
III. Antibiotics
• Different antibiotics interfere with different
cellular processes
• These processes do not occur in viruses so
antibiotics are not effective against them
A. Antibiotic- resistant bacteria
• Some bacteria have become resistant to
antibiotics due to mutations arising
spontaneously
• Susceptible bacteria are eliminated from the
population
• Resistant bacteria survive and pass on their
resistance traits
1. Antibiotic misuse
• If antibiotic treatment ends too soon, some of the
bacteria may survive
• Usually the most resistant bacteria survive and
develop antibiotic resistance
2. Multiple-antibiotic resistance
• Bacteria acquire several
antibiotic-resistance genes
• Bacteria gain resistance to
antibacterial agents in
antibacterial soaps
IV. Importance of Bacteria
A. Food and Chemical Production
• Many foods we eat are processed by specific
kinds of bacteria
• Bacteria are used to produce different kinds of
chemicals for industrial uses (acetone or
butanol)
• Genetically engineered bacteria used for
medicine
B. Mining and Environmental Uses of Bacteria
• Bacteria can convert elements into soluble
compounds so they can be washed away from
desired minerals in low-grade ore
• Bacteria metabolize organic chemicals and are
used to help clean up environmental disasters