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Chapter 20
Viruses and Bacteria
Table of Contents
Section 1 Viruses
Section 2 Bacteria
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Chapter 20
Section 1 Viruses
Objectives
• Describe why a virus is not considered a living
organism.
• Describe the basic structure of a virus.
• Summarize the steps of viral replication.
• Explain how HIV infects immune system cells.
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Chapter 20
Section 1 Viruses
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 are segments of nucleic acids contained in a
protein coat. Viruses are not cells.
• Viruses are pathogens—agents that cause disease.
• Viruses do not grow, do not have homeostasis, and
do not metabolize.
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Chapter 20
Section 1 Viruses
Virus
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Chapter 20
Section 1 Viruses
Pathogen
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Chapter 20
Section 1 Viruses
Is a Virus Alive?, continued
Discovery of Viruses
• Near the end of the nineteenth century, scientists
were trying to find the cause of tobacco mosaic
disease, which stunts the growth of tobacco plants.
• In 1935, biologist Wendell Stanley of the Rockefeller
Institute purified tobacco mosaic virus (TMV) and
determined that the purified virus is a crystal.
• Stanley concluded that TMV is a chemical rather
than an organism.
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Chapter 20
Section 1 Viruses
Viral Structure
• The virus protein coat, or capsid, may contain either
RNA or DNA, but not both.
• Many viruses have a membrane, or envelope,
surrounding the capsid.
• The envelope helps the virus enter cells. It consists of
proteins, lipids, and glycoproteins, which are
proteins with attached carbohydrate molecules that
are derived from the host cell.
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Chapter 20
Section 1 Viruses
Parts of a Virus
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Chapter 20
Section 1 Viruses
Viral Structure, continued
• Some viruses are long rods that form filaments.
• Spherical viruses are typically studded with
receptors.
• A helical virus is rodlike in appearance, with capsid
proteins winding around the core in a spiral.
• Viruses that infect bacteria, called bacteriophages,
have a complicated structure. A T4 bacteriophage,
for example, has a polyhedron capsid attached to a
helical tail.
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Chapter 20
Section 1 Viruses
Structures of
TMV and
Influenza Virus
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Chapter 20
Section 1 Viruses
Structures of
Adenovirus and
Bacteriophage
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Chapter 20
Section 1 Viruses
Bacteriophage
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Chapter 20
Section 1 Viruses
Viral Reproduction
• Viruses must rely on living cells (host cells) for
replication.
• Before a virus can replicate, it must first infect a living
cell.
• An animal virus enters its host cell by endocytosis.
• A bacterial virus, or bacteriophage, punches a hole in
the bacterial cell wall and injects its DNA into the cell.
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Chapter 20
Section 1 Viruses
Viral Reproduction, continued
Lytic Cycle
• In bacterial viruses, the cycle of viral infection,
replication, and cell destruction is called the lytic
cycle.
• After the viral genes have entered the cell, they 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 is
broken open and releases newly made viruses.
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Chapter 20
Section 1 Viruses
Lytic Cycle
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Chapter 20
Section 1 Viruses
Viral Reproduction, continued
Lysogenic Cycle
• During an infection, some viruses stay inside the cells
but instead of producing virus particles, the viral gene
is inserted into the host chromosome and is called a
provirus.
• Whenever the cell divides, the provirus also divides,
resulting in two infected host cells.
• In this cycle, called the lysogenic cycle, the viral
genome replicates without destroying the host cell.
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Chapter 20
Section 1 Viruses
Prophages and Proviruses
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Chapter 20
Section 1 Viruses
Lysogenic Cycle
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Chapter 20
Section 1 Viruses
Viral Replication in Bacteria
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Chapter 20
Section 1 Viruses
Relationships between the Lytic and
Lysogenic Cycles
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Chapter 20
Section 1 Viruses
Viral Reproduction, continued
Host Cell Specificity
• Viruses are often restricted to certain kinds of cells.
• Viruses may have originated when fragments of host
genes escaped or were expelled from cells.
• The hypothesis that viruses originated from a variety
of host cells may explain why there are so many
different kinds of viruses. Biologists think there are at
least as many kinds of viruses as there are kinds of
organisms.
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Chapter 20
Section 1 Viruses
Viral Reproduction, continued
Structure of HIV—an Enveloped Virus
• The human immunodeficiency virus (HIV) causes
acquired immune deficiency syndrome (AIDS).
• Within HIV’s envelope lies the capsid, which in turn
encloses the virus’s genetic material.
• In the case of HIV, the genetic material is composed
of two molecules of single-stranded RNA.
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Chapter 20
Section 1 Viruses
How HIV Infects Cells
Attachment
• Studding the surface of each HIV are spikes
composed of a glycoprotein.
• This particular glycoprotein precisely fits a human cell
surface receptor called CD4.
• Thus the HIV glycoprotein can bind to any cell that
possesses CD4 receptors.
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Chapter 20
Section 1 Viruses
How HIV Infects Cells, continued
Entry into Macrophages
• HIV cannot enter a cell merely by docking onto a
CD4 receptor. Rather, the glycoprotein must also
activate a second co-receptor, called CCR5.
• It is this event at CCR5 that starts endocytosis.
• Because human macrophages possess both CD4
and CCR5 receptors, HIV can enter macrophages.
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Chapter 20
Section 1 Viruses
How HIV Infects Cells, continued
Replication
• Once inside a cell, the HIV particle sheds its capsid.
The particle then releases an enzyme called reverse
transcriptase.
• Reverse transcriptase copies the naked viral RNA
into a complementary DNA version.
• Translation of the viral DNA by the host cell’s
machinery directs the production of many copies of
the virus.
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Chapter 20
Section 1 Viruses
Infection of
Macrophage
by HIV
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Chapter 20
Section 1 Viruses
How HIV Infects Cells, continued
AIDS
• For years after the initial infection, HIV continues to replicate
(and mutate). Eventually and by chance, HIV’s surface
glycoproteins change to the point that they now recognize a new
cell surface receptor. This receptor is found on the subset of
lymphocytes called T cells.
• Unlike its activity in macrophages, HIV reproduces in T cells and
then destroys them.
• It is this destruction of the body’s T cells that blocks the body’s
immune response and signals the onset of AIDS.
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Chapter 20
Section 1 Viruses
AIDS (Acquired Immune Deficiency
Syndrome)
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Chapter 20
Section 1 Viruses
Viral Diseases
• Perhaps the most lethal virus in human history is the
influenza virus.
• Certain viruses can also cause some types of cancer.
• Viruses associated with human cancers include
hepatitis B (liver cancer), Epstein-Barr virus (Burkitt’s
lymphoma), and human papilloma virus (cervical
cancer).
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Chapter 20
Section 1 Viruses
Important Viral Diseases
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Chapter 20
Section 1 Viruses
Viral Diseases, continued
Emerging Viruses
• Viruses that evolve in geographically isolated areas
and are pathogenic to humans are called emerging
viruses.
• These new pathogens are dangerous to public
health. People become infected when they have
contact with the normal hosts of these viruses.
• Examples of emerging viruses include West Nile
virus and hantavirus.
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Chapter 20
Section 1 Viruses
Viral Diseases, continued
Prions and Viroids
• Prions are composed of proteins but have no nucleic
acid. A disease-causing prion is folded into a shape
that does not allow the prion to function.
• Contact with a misfolded prion will cause a normal
prion to misfold, too. In this way the misfolding
spreads.
• A viroid is a single strand of RNA that has no capsid.
Viroids are important infectious disease agents in
plants.
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Chapter 20
Section 2 Bacteria
Objectives
• List seven differences between bacteria and
eukaryotic cells.
• Describe three different ways bacteria can obtain
energy.
• Describe the external and internal structure of
Escherichia coli.
• Distinguish two ways that bacteria cause disease.
• Identify three ways that bacteria benefit humans.
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Chapter 20
Section 2 Bacteria
Bacteria
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Chapter 20
Section 2 Bacteria
Characteristics of Bacteria
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Chapter 20
Section 2 Bacteria
Bacterial Structure
• Bacteria differ from eukaryotes in at least seven
ways.
• Bacteria are prokaryotes. Unlike eukaryotes,
prokaryotes lack a cell nucleus.
• Most bacterial cells are about 1 µm in diameter; most
eukaryotic cells are more than 10 times that size.
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Chapter 20
Section 2 Bacteria
Bacterial Structure, continued
• All bacteria are single cells.
• Bacterial chromosomes consist of a single circular
piece of DNA. Eukaryotic chromosomes are linear
pieces of DNA that are associated with proteins.
• Bacteria reproduce by binary fission, a process in
which one cell pinches into two cells.
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Chapter 20
Section 2 Bacteria
Bacterial Structure, continued
• Bacterial flagella are simple structures composed of a
single fiber of protein that spins like a corkscrew to
move the cell.
• Some bacteria also have shorter, thicker outgrowths
called pili.
• Bacteria have many metabolic abilities that
eukaryotes lack. For example, bacteria perform
several different kinds of anaerobic and aerobic
processes, while eukaryotes are mostly aerobic
organisms.
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Chapter 20
Section 2 Bacteria
Structure of Cilia and Flagella
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Chapter 20
Section 2 Bacteria
Pilus
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Chapter 20
Section 2 Bacteria
Structural Characteristics of a Bacterial Cell
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Chapter 20
Section 2 Bacteria
Parts of a Prokaryotic Cell
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Chapter 20
Section 2 Bacteria
Comparing Organisms That Are Unicellular
and Multicellular
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Chapter 20
Section 2 Bacteria
Bacterial Cell Shapes
• A bacterial cell is usually one of three basic shapes:
bacillus, a rod-shaped cell; coccus, a round-shaped
cell; or spirillum, a spiral cell.
• Members of the kingdom Eubacteria have a cell wall
made of peptidoglycan, a network of polysaccharide
molecules linked together with chains of amino acids.
• Outside the cell wall and membrane, many bacteria
have a gel-like layer called a capsule.
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Chapter 20
Section 2 Bacteria
Three Bacterial Cell Shapes
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Chapter 20
Section 2 Bacteria
Bacterial Capsule
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Chapter 20
Section 2 Bacteria
Bacterial Cell Shapes, continued
• Eubacteria can have two types of cell walls,
distinguished by a dye staining technique called the
Gram stain.
• Gram staining is important in medicine because the
two groups of eubacteria differ in their susceptibility
to different antibiotics.
• Antibiotics are chemicals that interfere with life
processes in bacteria.
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Chapter 20
Section 2 Bacteria
Gram Staining
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Chapter 20
Section 2 Bacteria
Gram Stain
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Chapter 20
Section 2 Bacteria
Bacterial Cell Shapes, continued
• Some bacteria form thick-walled endospores around
their chromosomes and a small bit of cytoplasm
when they are exposed to harsh conditions.
• Pili enable bacteria to adhere to the surface of
sources of nutrition, such as your skin. Some kinds of
pili enable bacteria to exchange genetic material
through a process called conjugation.
• Conjugation is a process in which two organisms
exchange genetic material.
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Chapter 20
Section 2 Bacteria
Conjugation
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Chapter 20
Section 2 Bacteria
Escherichia coli
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Chapter 20
Section 2 Bacteria
Obtaining Energy
Photosynthesis
• Photosynthetic bacteria can be classified into four
major groups based on the photosynthetic pigments
they contain: purple nonsulfur bacteria, green sulfur
bacteria, purple sulfur bacteria, and cyanobacteria.
• Green sulfur bacteria and purple sulfur bacteria grow
in anaerobic environments.
• Cyanobacteria are thought to have made the Earth’s
oxygen atmosphere.
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Chapter 20
Section 2 Bacteria
Obtaining Energy, continued
Chemoautotrophs
• Bacteria called chemoautotrophs obtain energy by
removing electrons from inorganic molecules such as
ammonia and hydrogen sulfide or from organic
molecules such as methane.
• In the presence of one of these hydrogen-rich
chemicals, chemoautotrophic bacteria can
manufacture all their own amino acids and proteins.
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Chapter 20
Section 2 Bacteria
Chemoautotroph
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Chapter 20
Section 2 Bacteria
Obtaining Energy, continued
Heterotrophs
• Most bacteria are heterotrophs.
• Many are aerobic, that is, they live in the presence of
oxygen. Some other bacteria can live without oxygen.
• Together with fungi, heterotrophic bacteria are the
principal decomposers of the living world; they break
down the bodies of dead organisms and make the
nutrients available to other organisms.
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Chapter 20
Section 2 Bacteria
Pathogenic Bacteria
Bacteria Can Metabolize Their Host
• Heterotrophic bacteria obtain nutrients by secreting
enzymes that break down complex organic structures
in their environment and then absorbing them. If that
environment is your throat or lungs, this can cause
serious problems.
• Several common bacterial diseases include dental
cavities, strep throat, tuberculosis, and acne.
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Chapter 20
Section 2 Bacteria
Important Bacterial Diseases
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Chapter 20
Section 2 Bacteria
Pathogenic Bacteria, continued
Bacterial Toxins
• The second way bacteria cause disease is by
secreting chemical compounds into their
environment. These chemicals, called toxins, are
poisonous to eukaryotic cells.
• When bacteria grow in food and produce toxins,
the toxins can cause illness in humans who eat
those contaminated foods.
• Most bacteria can be killed by boiling water or
various chemicals.
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Chapter 20
Section 2 Bacteria
Pathogenic Bacteria, continued
Biowarfare
• Biowarfare is the deliberate exposure of people to
biological toxins or pathogens such as bacteria or
viruses.
• Biologists are working on new approaches to
recognize the onset of an attack with a bioweapon, to
treat infected people, and to slow the spread of any
outbreak of disease.
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Chapter 20
Section 2 Bacteria
Antibiotics
• In 1928, the British bacteriologist Alexander Fleming
discovered the antibiotic penicillin.
• Today different antibiotics are used to interfere with
different cellular processes.
• Because these processes do not occur in viruses,
antibiotics are not effective against them.
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Chapter 20
Section 2 Bacteria
Antibiotics, continued
Antibiotic-Resistant Bacteria
• Some bacteria have become resistant to antibiotics.
• Susceptible bacteria are eliminated from the population, and
resistant bacteria survive and reproduce, thus passing on their
resistance traits.
• Usually, if the full course of the antibiotic is administered, all the
targeted bacteria are killed and there is no chance for a resistant
strain to develop. If antibiotic treatment ends prematurely, some
of the more-resistant bacteria may survive and reproduce.
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Chapter 20
Section 2 Bacteria
Importance of Bacteria
Food and Chemical Production
• Many of the foods that we eat, such as pickles,
cheese, sauerkraut, olives, vinegar, and sourdough
bread, are processed by specific kinds of bacteria.
• Humans are able to use different bacteria to produce
different kinds of chemicals for industrial uses.
• Genetic engineering companies use genetically
engineered bacteria to produce their many products,
such as drugs for medicine and complex chemicals
for research.
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Chapter 20
Section 2 Bacteria
Bacteria and Food
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Chapter 20
Section 2 Bacteria
Importance of Bacteria, continued
Mining and Environmental Uses of Bacteria
• Mining companies can use bacteria to concentrate
desired elements from low-grade ore.
• Bacteria metabolize different organic chemicals and
are therefore used to help clean up environmental
disasters such as petroleum and chemical spills.
• Powders containing petroleum-metabolizing bacteria
are used to help clean oil spills.
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Chapter 20
Standardized Test Prep
Multiple Choice
The diagram below illustrates viral replication in
bacteria. Use the diagram to answer questions 1–3.
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Chapter 20
Standardized Test Prep
Multiple Choice, continued
1. Which type of cycle is represented in the diagram?
A.
B.
C.
D.
aerobic
anaerobic
lysogenic
lytic
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Chapter 20
Standardized Test Prep
Multiple Choice, continued
1. Which type of cycle is represented in the diagram?
A.
B.
C.
D.
aerobic
anaerobic
lysogenic
lytic
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Chapter 20
Standardized Test Prep
Multiple Choice, continued
2. What is the virus doing to the bacterium in step 1?
F.
G.
H.
J.
injecting its capsid
injecting its DNA
withdrawing proteins
withdrawing DNA
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Chapter 20
Standardized Test Prep
Multiple Choice, continued
2. What is the virus doing to the bacterium in step 1?
F.
G.
H.
J.
injecting its capsid
injecting its DNA
withdrawing proteins
withdrawing DNA
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Chapter 20
Standardized Test Prep
Multiple Choice, continued
3. What is happening in step 4?
A. Viruses are entering the bacterium through a
hole in the cell membrane.
B. Viruses are repairing the cell membrane using
viral proteins.
C. Newly formed viruses are being released from
the bacterium.
D. The bacterium is ejecting its own chromosome
inside a capsid.
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Chapter 20
Standardized Test Prep
Multiple Choice, continued
3. What is happening in step 4?
A. Viruses are entering the bacterium through a
hole in the cell membrane.
B. Viruses are repairing the cell membrane using
viral proteins.
C. Newly formed viruses are being released from
the bacterium.
D. The bacterium is ejecting its own chromosome
inside a capsid.
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