Licensed to: CengageBrain User
Licensed to: CengageBrain User
This is an electronic version of the print textbook. Due to electronic rights restrictions,
some third party content may be suppressed. Editorial review has deemed that any suppressed
content does not materially affect the overall learning experience. The publisher reserves the right
to remove content from this title at any time if subsequent rights restrictions require it. For
valuable information on pricing, previous editions, changes to current editions, and alternate
formats, please visit www.cengage.com/highered to search by ISBN#, author, title, or keyword for
materials in your areas of interest.
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Licensed to: CengageBrain User
Diversity of Life
Biology: The Unity and Diversity of Life,
Thirteenth Edition
Cecie Starr, Ralph Taggart, Christine Evers,
Lisa Starr
Senior Acquisitions Editor, Life Sceinces:
Peggy Williams
Publisher: Yolanda Cossio
Assistant Editor: Shannon Holt
© 2013, 2009 Brooks/Cole, Cengage Learning
Unless otherwise indicated, all art in this text © Cengage Learning.
ALL RIGHTS RESERVED. No part of this work covered by the copyright herein
may be reproduced, transmitted, stored or used in any form or by any means
graphic, electronic, or mechanical, including but not limited to photocopying,
recording, scanning, digitizing, taping, Web distribution, information networks,
or information storage and retrieval systems, except as permitted under
Section 107 or 108 of the 1976 United States Copyright Act, without the prior
written permission of the publisher.
Editorial Assistant: Sean Cronin
Media Editor: Lauren Oliveira
Marketing Manager: Tom Ziolkowski
Marketing Coordinator: Jing Hu
Marketing Communications Manager: Linda Yip
Content Project Manager: Hal Humphrey
Design Director: Rob Hugel
Art Director: John Walker
Print Buyer: Karen Hunt
Rights Acquisitions Specialist: Dean Dauphinais
Production Service: Grace Davidson &
Associates
Text Designer: John Walker
Photo Researcher: Myrna Engler Photo
Research Inc.
For product information and technology assistance, contact us at
Cengage Learning Customer & Sales Support, 1-800-354-9706.
For permission to use material from this text or product,
submit all requests online at www.cengage.com/permissions.
Further permissions questions can be emailed to
permissionrequest@cengage.com.
Library of Congress Control Number: 2011938382
ISBN-13: 978-1-111-58067-4
ISBN-10: 1-111-58067-7
Brooks/Cole
20 Davis Drive
Belmont, CA 94002
USA
Text Researcher: Pablo D’Stair
Copy Editor: Anita Wagner
Illustrators: Gary Head, ScEYEnce Studios,
Lisa Starr
Cover Image: The diversity of body forms
among flower mantids, wolves, and sea
anemones conceals an underlying unity. All
are animals, and thus belong to the same
branch of life that you do.
Top: Bob Jensen Photography; middle, Jeff
Vanuga/Corbis; bottom: John Easley.
Compositor: Lachina Publishing Services
Cengage Learning is a leading provider of customized learning solutions with
office locations around the globe, including Singapore, the United Kingdom,
Australia, Mexico, Brazil, and Japan. Locate your local office at:
www.cengage.com/global.
Cengage Learning products are represented in Canada by Nelson ­Education, Ltd.
To learn more about Brooks/Cole visit www.cengage.com/brookscole.
Purchase any of our products at your local college store or at our preferred
online store www.CengageBrain.com.
Printed in Canada
1 2 3 4 5 6 7 15 14 13 12 11
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Licensed to: CengageBrain User
20 Viruses, Bacteria, and Archaea
Learning Roadmap
Where you have been Section 1.4 gave you an early
glimpse of the bacteria and archaea. Section 4.4 began our
discussion of their structure and Section 19.5 put these groups
into an evolutionary time frame. Section 17.5 focused on antibiotic resistance in bacteria. This chapter also discusses bacteriophage, a type of virus that will be familiar from Section 8.3.
Where you are now
Viral Structure and Function
A virus is a noncellular infectious
particle that must infect a living
cell to replicate. All organisms
are susceptible to viral infection.
Viruses and Human Health
Viruses can be pathogens, meaning
they cause disease. Most viral
diseases are mild and pass quickly,
but some persist; a few are fatal.
Two Lineages of Simple Cells
Bacteria and archaea are two distinct
lineages of asexually reproducing,
structurally simple cells that do not
have a nucleus. They are extremely
diverse and abundant.
Bacteria
Bacteria play essential ecological
roles. They put oxygen into the air,
make nitrogen available to plants,
and act as decomposers. A minority
cause disease.
Archaea
Archaea live in some astonishingly
hostile environments such as hot
springs and pools of brine. They also
live alongside bacteria in soil and in
the animal gut.
Where you are going You will learn more about the effects of viral and bacterial pathogens in
chapters that discuss physiology. For example, Section 37.11 looks at the immune effects of AIDS, and
Section 41.10 discusses sexually transmitted diseases. You will also learn about the beneficial effects of
the bacteria in your gut (39.1), and the bacteria that partner with plants (28.3). Bacteria play an integral
role in food webs (46.3) and biogeochemical cycles (46.5).
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Licensed to: CengageBrain User
20.1 Evolution of a Disease
Billions of years before there were fungi, plants, or
animals, Earth’s seas were home to two groups of
microscopic organisms: bacteria and archaea. These
single-celled organisms do not have a nucleus or other
typical eukaryotic organelles. Viruses are simpler still,
with no chromosomes or metabolic machinery. By
many definitions, viruses are not even alive. Despite
their simplicity, viruses can evolve because like living
organisms they have a genome that can mutate.
In recent years, scientists have learned quite a bit
about the origin and evolution of HIV (human immunodeficiency virus). This virus causes the emerging
disease AIDS (acquired immune deficiency syndrome).
An emerging disease is a disease that is relatively new
to humans or has newly expanded its range.
HIV was first isolated in the early 1980s. Since
then, gene sequence comparisons have revealed that
the most common strain (HIV-1) evolved from the
strain of simian immunodeficiency virus (SIV) that
infects chimpanzees in west central Africa. A recent
study investigated the health effects of SIV in a wild
chimpanzee population that has been studied by primatologist Jane Goodall and others for many years.
Researchers used DNA analysis of chimpanzee feces
to identify individual animals and determine whether
they were infected by SIV (Figure 20.1). This information was combined with observational data from the
field. The researchers found that, in this population,
SIV reduces fitness. SIV-infected chimpanzees die earlier than unaffected animals and leave fewer offspring.
How did SIV get from chimpanzees into people?
Some African populations eat nonhuman primates
and it is likely that a person became infected while
butchering an infected chimpanzee for use as food.
Butchery is a bloody process, and SIV-infected blood
could have gotten into a butcher’s body through a cut.
Presumably the virus survived and mutated inside its
unusual host. Over time it became HIV.
So far, the earliest known evidence of HIV infection
comes from two tissue samples stored at a hospital in
west central Africa. One is a blood sample taken from
a man in 1959. The other is a woman’s lymph node
that was removed in 1960. The viral gene sequences
from the two samples differ a bit, which implies that
HIV had already been around and mutating by the
time these two people became infected. Given the
known mutation rate for HIV, researchers estimate that
HIV first infected humans in the early 1900s.
emerging disease Disease that is relatively new to a species, or has
recently expanded its range.
HIV (human immunodeficiency virus) Retrovirus that causes AIDS.
Figure 20.1 Analyzing wild chimpanzee feces for SIV. Rebecca
Rudicell is part of a team that is studying the effects of this virus,
from which HIV evolved.
Gene sequence comparisons have also allowed
researchers to trace the movement of the virus out of
Africa. One recent study concluded that HIV-1 was
carried from Africa to Haiti in about 1966. The virus
diversified in Haiti and acquired distinctive mutations not seen in Africa. In about 1969, HIV-1 with
Haiti-specific mutations was introduced to the United
States. It may have arrived in an infected individual
or in infected blood. Once there, it spread quietly until
AIDS was identified as a threat in 1981.
Today, more than 20 million people worldwide
have died from AIDS. About 30 million are currently
infected with HIV. The virus infects and replicates
inside white blood cells that are essential to immune
responses. Eventually, the infected white blood cells
die, destroying the body’s ability to defend itself. As a
result, disease-causing organisms run rampant, causing symptoms of AIDS and health problems that can
be fatal. Section 37.11 discusses in detail how AIDS
affects the immune system.
Knowing about HIV’s ancestry may help us
develop new weapons against the virus. For example,
although SIV does harm chimpanzees, the effects
are not as devastating as untreated HIV in humans.
Determining how the chimpanzee immune system
fights against SIV may provide insights that we can
put to use in our own fight against AIDS.
323
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Licensed to: CengageBrain User
20.2 Viruses and Viroids
■ A virus consists of nucleic acid and protein. It is smaller than
any cell and has no metabolic machinery of its own.
■ Links to Discovery of DNA function 8.3, Endocytosis 5.10
In the late 1800s, biologists studying stunted tobacco
plants discovered a new kind of disease-causing agent,
or pathogen. It was so small that it passed through
screens that filtered out bacteria, and it could not be
seen with a light microscope. The scientists called this
unseen infectious entity a virus, a term that means
“poison” in Latin.
Today, we define a virus as a noncellular infectious
particle that can replicate only in a living cell. A virus
is an obligate intracellular parasite. It does not have
ribosomes or other metabolic machinery and it cannot make ATP. To replicate, the virus must insert its
genetic material into a cell of a specific type of organism. We call that organism its host.
The fact that viruses can only replicate in cells
suggests that they evolved from cells. They may
be derived from bits of DNA or RNA that escaped.
Alternatively, viruses may be remnants of a time
before cells. This would explain why many viral genes
have no counterpart in cells.
Viral Structure
Viruses are typically so small (about 25 to 300 nanometers) that they can only be seen with an electron microscope. A free viral particle, or virion, always includes
a viral genome enclosed within a protein shell, or
capsid. The viral genome may be RNA or DNA, and it
may be single-stranded or double-stranded.
The capsid consists of many protein subunits that
bond together in a repeating pattern, producing a
RNA
protein
subunits
of coat
Figure 20.2 Animated Tobacco mosaic
virus, a virus that infects tobacco (above)
and related plants. The helical arrangement
of the capsid subunits (right) gives the virus
a rodlike structure. The genome is singlestranded RNA.
324 UNIT IV
DNA
inside
protein
coat
sheath
tail
fiber
Figure 20.3 Animated Model (left) and electron micrograph (right) of a bacteriophage, a bacterial virus with a
complex structure.
helical or many-sided (polyhedral) shape. The capsid
protects the viral genetic material and facilitates its
delivery into a host cell. In all viruses, some components of the viral coat bind to proteins at the surface
of a host cell. The capsid may also enclose one or more
viral enzymes.
Many plant viruses have a helical structure. The
tobacco mosaic virus is an example. Its coat proteins
bond together in a tight helix around its genetic material, a single strand of RNA (Figure 20.2). Viruses
typically enter a plant through a wound made by an
insect, pruning, or another mechanical injury. They
move throughout the plant body and even enter seeds.
Bacteriophages, viruses that infect bacteria, have a
complex structure (Figure 20.3). Their headlike capsid
encloses the viral DNA. Other protein components
of the virus allow it to pierce a bacterial cell wall
and inject DNA into the cell. You learned earlier how
Hershey and Chase used a type of bacteriophage
called lambda to identify DNA as the genetic material
of all organisms (Section 8.3).
Polyhedral viruses have a many-sided protein coat.
Adenoviruses are an example. These animal viruses
have a 20-sided capsid with a distinctive protein spike
at each corner (Figure 20.4A). Adenoviruses frequently
cause common colds. They are “naked,” or nonenveloped viruses; their capsid is their outermost layer.
In most animal viruses, the capsid is enclosed
within an “envelope,” a layer of membrane derived
from the host cell in which the virus assembled. For
example, herpesvirus is an enveloped DNA virus.
EVOLUTION AND BIODIVERSITY
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Licensed to: CengageBrain User
DNA and enzymes
protein coat
beneath the
envelope
envelope composed
of lipids and proteins
(derived from host)
A Model of an adenovirus, a polyhedral
virus. Protein subunits form a 20-sided
polyhedron around double-stranded DNA.
B Model (left) and electron micrograph (right) of a herpesvirus, an enveloped virus.The
envelope is derived from the nuclear membrane of the cell in which the virus assembled.
In the micrograph, the envelope is peeled back to reveal the protein coat beneath.
Figure 20.4 Animated Two animal viruses.
It has a 20-sided capsid surrounded by an envelope
made of bits of a host cell’s nuclear membrane (Figure
20.4B). More frequently, an enveloped virus derives its
envelope from the host’s plasma membrane.
Viruses also harm us directly by impairing our health.
We discuss viral diseases of humans in Section 20.4.
Ecological Role of Viruses
Viroids are small RNAs that cause disease in many
Everywhere there is life, there are viruses. Viruses
infect and replicate in all organisms, no matter how
simple or complex. A viral infection often decreases
a host’s ability to survive and reproduce, so viruses
affect ecological interactions throughout the biosphere.
Some viruses assist humans through their effects
on other species. For example, we benefit when baculoviruses infect and kill caterpillars that feed on crop
species or when bacteriophages kill bacteria that could
cause food poisoning.
On the other hand, viruses can have devastating
economic effects when they infect livestock or agriculturally important plants. In recent years, outbreaks
of influenza among pigs and chickens have led to
the slaughter of hundreds of thousands of animals.
Viroids
commercially valuable plants, including potatoes,
tomatoes, citrus, apples, coconuts, avocados, and chrysanthemums. They were discovered in the 1970s by
the plant pathologist Theodor Diener. He named the
tiny new pathogen he had isolated a viroid, because it
seemed like a stripped-down version of a virus.
All viroids are circular, single-stranded RNAs. They
are remarkably small, with fewer than 400 nucleotides.
By comparison, even the smallest viral genome has
thousands of nucleotides. Unlike the genetic material of a virus, viroid RNA does not encode proteins.
The viroid is replicated in a plant cell nucleus by the
plant’s RNA polymerase.
Take-Home Message
What are viruses and viroids?
bacteriophage Virus that infects bacteria.
pathogen Disease-causing agent.
viroid Small, noncoding, infectious RNA.
virus Noncellular, infectious particle of protein and nucleic acid;
replicates only in a host cell.
» Viruses are noncellular infectious particles made of protein and nucleic acid.
» A virus is an obligate intracellular parasite. It has no metabolic machinery of
its own and can multiply only inside living cells.
» Viroids are infectious noncoding RNAs that cause some plant diseases.
CHAPTER 20
viruses, Bacteria, and Archaea 325
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Licensed to: CengageBrain User
20.3 Viral Replication
■ A viral infection is like a cellular hijacking; viral genes take
over a host cell’s machinery and direct it to synthesize viral
components that can self-assemble as new viral particles.
■ Links to Transcription 9.3, Translation 9.5
Overview of Viral Replication
The details of viral replication processes vary, but all
involve the steps outlined in Table 20.1. A virus cannot
propel itself toward a host, so infection begins with a
chance encounter. During attachment, viral proteins
bind to receptor proteins on the surface of a host cell.
The virus’s genetic material enters the host cell and
takes over its genetic machinery. Under the influence
of the virus, the cell puts aside its normal tasks and
Table 20.1 Steps in Most Viral Replication Cycles
1. Attachment Proteins on viral particle chemically recognize and lock
onto specific receptors at the host cell surface.
2. Penetration Either the viral particle or its genetic material crosses the
plasma membrane of a host cell and enters the cytoplasm.
3. Replication and synthesis Viral DNA or RNA directs host to make
viral nucleic acids and viral proteins.
4. Assembly Viral components self-assemble as new viral particles.
5. Release The new viral particles are released from the cell.
E Lysis of host cell lets
new virus particles escape.
turns to replicating and expressing the viral genome.
When viral proteins and nucleic acid come into contact, they self-assemble as new virions. The virus
either buds from the host cell or escapes when the host
cell lyses (breaks open).
Bacteriophage Replication
Bacteriophages replicate in bacteria by two pathways.
Both begin when a bacteriophage attaches to a bacterial cell and injects its DNA (Figure 20.5). In the lytic
pathway, viral genes are expressed immediately 1 .
The infected host first produces viral components that
self-assemble as virus particles. Then, a viral-encoded
enzyme breaks down the cell wall causing lysis of the
cell—the cell disintegrates and dies.
In the lysogenic pathway, viral DNA becomes integrated into the host cell’s genome and viral genes are
not expressed, so the cell remains healthy 2 . When
the cell reproduces, viral DNA is copied and passed
on along with the host’s genome. Like miniature time
bombs, the viral DNA inside these descendant cells
awaits a signal to enter the lytic pathway.
Some bacteriophages can only replicate by the
lytic pathway. They kill their host cell quickly and are
not passed from one bacterial generation to the next.
Others embark upon either the lytic or lysogenic pathway, depending on conditions in the host cell.
A Virus particle binds,
injects genetic material.
A1 Viral DNA is inserted
into host chromosome by
viral enzyme action.
2 Lysogenic
1 Lytic
Pathway
Pathway
B Host replicates
viral genetic material,
builds viral proteins.
D Accessory parts are
attached to viral coat.
A2 Chromosome
and integrated viral
DNA are replicated.
C Viral proteins self-assemble
into a coat around viral DNA.
A3 Cell divides;
recombinant DNA is in
each descendant cell.
A4 Viral enzyme excises
viral DNA from chromosome.
Figure 20.5 Animated Pathways in the replication cycle of a bacteriophage.
Figure It Out: What is the blue circle in A?
Answer: Bacterial chromosome
326 UNIT IV
EVOLUTION AND BIODIVERSITY
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Licensed to: CengageBrain User
viral glycoprotein
(binds to host proteins)
HIV DNA
2
3
viral coat
proteins
4
reverse
transcription
5
transcription
HIV RNA
translation
6
one of two
strands of
viral RNA
lipid envelope
with proteins
HIV
Figure 20.6 Animated Replication of HIV, a retrovirus (right).
The structure of the HIV virion is shown above.
Replication of HIV
HIV is an enveloped RNA virus that replicates inside
human white blood cells (Figure 20.6). It attaches to a
cell via a glycoprotein that extends through its envelope 1 . The glycoprotein binds two different proteins
on the host cell. After attachment, the viral envelope
fuses with the blood cell’s plasma membrane, releasing viral enzymes and RNA into the cell 2 .
One of the viral enzymes is reverse transcriptase, a
polymerase that uses viral RNA as a template to synthesize double-stranded DNA 3 . This DNA enters the
nucleus together with another viral enzyme, integrase.
Integrase inserts the DNA into one of the host’s chromosomes 4 . Once integrated, the viral DNA is replicated and transcribed along with the host genome 5 .
Some of the resulting viral RNA is translated into viral
proteins 6 and some becomes the genetic material of
new HIV virions 7 . The virions self-assemble at the
plasma membrane 8 . As the virus buds from the host
cell, some of the host’s plasma membrane becomes
the viral envelope 9 . Each new virion can then infect
another white blood cell. New HIV-infected cells are
also produced when an infected cell replicates.
lysogenic pathway Bacteriophage replication mechanism in which
viral DNA becomes integrated into the host’s chromosome and is
passed to the host’s descendants.
lytic pathway Bacteriophage replication mechanism in which a
virus replicates in its host and kills it quickly.
reverse transcriptase A viral enzyme that uses RNA as a template
to make a strand of cDNA.
7
1
9
8
1 Viral protein binds to proteins at the surface of a white
blood cell.
2 Viral RNA and enzymes enter the cell.
3 Viral reverse transcriptase uses viral RNA to make doublestranded viral DNA.
4 Viral DNA enters the nucleus and becomes integrated into
the host genome.
5 Transcription produces viral RNA.
6 Some viral RNA is translated to produce viral proteins.
7 Other viral RNA forms the new viral genome.
8 Viral proteins and viral RNA self-assemble at the host
plasma membrane.
9 New virus buds from the host cell, with an envelope of host
plasma membrane.
Drugs designed to fight HIV take aim at steps in
viral replication. Some interfere with the way HIV
binds to a host cell. Others impair the viral reverse
transcriptase. Integrase inhibitors prevent viral DNA
from integrating into a human chromosome. Protease
inhibitors prevent the processing of newly translated
polypeptides into mature viral proteins.
These antiviral drugs lower the number of HIV
particles, so a person stays healthier. Less HIV in body
fluids also means reduced risk of passing the virus to
others. However, no drug eliminates the virus, all have
unpleasant side effects, and all must be taken for life.
Take-Home Message
How do viruses replicate?
» A virus binds to a specific type of host cell, and viral genetic material
enters the cell. Viral genes direct the production of viral components that
then self-assemble as new viral particles.
CHAPTER 20
viruses, Bacteria, and Archaea 327
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Licensed to: CengageBrain User
20.4 Viruses as Human Pathogens
■ Viral diseases range from merely inconvenient to potentially
deadly.
■ Links to DNA replication and repair 8.5–8.6, Directional
selection 17.5
The Threat of Infectious Disease
An infection occurs when one organism enters another
and replicates inside it. An infectious disease arises
when the activities of the “guests” interfere with the
host’s normal functions. Viruses, bacteria, fungi, and
protists cause human disease (Table 20.2).
Most infectious diseases are spread by contact with
tiny amounts of mucus, blood, or other body fluid that
contains the pathogen. Washing your hands regularly
is the best defense against such diseases. Other infectious diseases require a vector, an animal that carries
the pathogen from host to host. Biting insects and ticks
are the most important common disease vectors.
Common Viral Diseases
Most viral diseases cause mild symptoms and trouble
us only briefly. For example, some adenoviruses infect
the membranes of our upper respiratory system and
cause common colds. Others colonize the lining of our
gut and cause a brief bout of vomiting and diarrhea.
A minority of viral diseases can be more persistent.
Various types of herpesviruses cause cold sores, genital herpes, mononucleosis, or chicken pox. Typically
Table 20.2 Major Causes of Death From
Infectious Disease
Disease
Type of
Pathogen
Deaths
per year worldwide
Acute respiratory
infections
Viruses, bacteria
4 million
AIDS
Virus (HIV)
2.7 million
Diarrheas
Viruses, bacteria,
protists
1.8 million
Tuberculosis
Bacteria
1.6 million
Malaria
Protists
1.3 million
Measles
Virus
164,000
Whooping cough
Bacteria
294,000
Tetanus
Bacteria
204,000
Meningitis
Viruses
173,000
Syphilis
Bacteria
157,000
328 UNIT IV
Figure 20.7 Sign of an active herpes simplex virus I infection.
Fluid rich in viral particles leaks from the open sore.
the initial infection causes symptoms that subside
quickly. However, the virus remains in the body in a
latent state, and can reawaken later on. Many people
have been infected by herpes simplex virus 1 (HSV-1),
which can remain latent in nerve cells for years. When
activated, the virus replicates and causes painful “cold
sores” on the edge of the lips (Figure 20.7). Similarly,
HIV can persist in a latent state inside white blood
cells that are not actively dividing.
Measles, mumps, rubella (German measles),
and chicken pox are childhood diseases that, until
recently, were common worldwide. Today, most children in developed countries are protected against
these illnesses because they have been vaccinated.
Administering a vaccine primes the body to fight off
a specific pathogen, a process explained in detail in
Section 37.12).
New Flus: Viral Mutation and Reassortment
Like living organisms, viruses have genomes that
can be altered by mutation. RNA viruses such as HIV
and influenza virus mutate especially quickly. The
viral reverse transcriptase makes frequent replication
errors. These errors remain uncorrected because the
host’s proofreading and repair mechanisms evolved to
fix errors of transcription and do not operate during
reverse transcription.
To keep up with ongoing mutations in influenza
viruses, scientists create a new flu shot every year.
The flu shot is a vaccine designed to thwart the newly
mutated influenza viruses that scientists predict are
most likely to pose a threat during the upcoming flu
season. Unfortunately, determining which flu strains
will be circulating is not an exact science. Even after a
flu shot, a person is susceptible to a virus that differs
from the virus types targeted by the vaccine.
EVOLUTION AND BIODIVERSITY
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Licensed to: CengageBrain User
Influenza subtypes are named after the structure of
two proteins at the viral surface (Figure 20.8). The glycoprotein hemagglutinin (H) allows the virus to bind
to a host cell. The enzyme neuraminidase (N) helps
new viral particles to exit from that cell.
In April of 2009, a new version of the H1N1 subtype of influenza appeared unexpectedly. Although
the media referred to this virus as “swine flu,” it
had a composite genome, with genes from a human
flu virus, bird flu virus, and two different swine flu
viruses. Such novel genomes arise as a result of viral
reassortment, the swapping of genes between viruses
that infect a host at the same time (Figure 20.9).
The 2009 H1N1 virus was discovered when it
caused an epidemic in Mexico. An epidemic is an outbreak of disease in a limited region. Within months,
there was a pandemic, an outbreak of disease that
simultaneously affects people throughout the world.
Health officials became concerned because unlike a
typical seasonal flu that kills mainly the elderly, the
new flu seemed to be causing deaths largely among
young, healthy people. Fortunately, initial fears of
a high mortality rate proved largely unfounded.
Governments quickly released reserves of antiviral
drugs to treat those infected. The drugs interfere with
neuraminidase function, and so impair the virus’s
ability to infect cells. A vaccine was created to prevent new infections. The World Health Organization
declared the pandemic over in August 2010.
Another strain of influenza, influenza H5N1, is
a bird flu that occasionally infects people who have
direct contact with birds. When the virus does infect
people, the death rate is disturbingly high. From
2003 to 2009, the World Health Organization received
reports of 417 human cases of influenza H5N1, mainly
in Asia. Of these, 257 (about 60 percent) were fatal.
Fortunately, person-to-person transmission of the
H5N1 virus is exceedingly rare.
Health officials continue to carefully monitor H5N1
and H1N1 influenza. Either virus could mutate, and
their coexistence raises the possibility of a potentially
disastrous gene exchange. If H1N1 picked up genes
from avian H5N1, the result could be a flu virus that is
both easily transmissible and deadly.
hemagglutinin
neuraminidase
Figure 20.8 Influenza virus. Subtypes such as H1N1 or H5N2
are defined by the structure of viral proteins—hemagglutinin (H)
and neuraminidase (N)­—that extend through the outer envelope.
1 Two strains of influenza
viruses (shown here as red
and blue) infect a host at
the same time.
2 Inside a host
cell, viral genes
are copied and
the copies mix
together.
3 A mix of genes
is packaged into
each new viral
particle that buds
from the host cell.
Figure 20.9 Viral reassortment. When a host cell is infected by
two viruses of the same type, copies of viral genes reassort to
form new combinations.
Take-Home Message
epidemic Disease outbreak that occurs in a limited region.
pandemic Disease outbreak with cases worldwide.
vector Of a disease, an animal that transmits a pathogen from one
How do viruses affect human health?
host to the next.
» Viruses cause many diseases, most short-lived and relatively mild, but some
that are deadly.
viral reassortment Two related viruses infect the same individual
» Viral pathogens can change by mutation or reassortment.
simultaneously and swap genes.
CHAPTER 20
viruses, Bacteria, and Archaea 329
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Licensed to: CengageBrain User
20.5 “Prokaryotes”—Enduring, Abundant, and Diverse
■ The most widespread and abundant forms of life belong to
two lineages of single-celled organisms that do not have
a nucleus.
■ Links to Prokaryotes 4.4, Aerobic respiration 7.2, Classification
systems 18.2, Early life 19.5
Two Lineages of “Prokaryotes”
Biologists have historically divided all life into two
groups. Cells without a nucleus were prokaryotes and
those with a nucleus were eukaryotes (Figure 20.10A).
More recently we learned that the “prokaryotes” actually constitute two distinct lineages, now referred to as
the domains Bacteria and Archaea. Eukaryotes are the
third domain (Figure 20.10B).
prokaryotes
A
eukaryotes
bacteria
archaea
Figure 20.11 Asexual reproduction in Escherichia coli, one of the
many species of bacteria that lives in the human gut. Meiosis and
sexual reproduction do not occur in bacteria or archaea.
eukaryotes
B
Figure 20.10 Comparison of (a) two-domain and (b) threedomain trees of life. The three-domain model is now in wide use.
Bacteria are the more well-known and widespread
group of cells that do not have a nucleus. Archaea are
less well studied, and many live in extreme habitats.
In light of the realization that bacteria and archaea
are not a monophyletic group, some microbiologists
have advocated abandoning the term “prokaryote.”
They point out that biological groups are defined by
shared traits, not the lack of a trait, such as a nucleus.
Other scientists argue that the term remains useful as a
way to refer to two lineages that share many structural
and functional similarities.
Bacteria and archaea are smaller and structurally
simpler than eukaryotes. This structural simplicity
does not imply inferiority. Bacteria and archaea existed
before eukaryotes and have coexisted with them for
more than a billion years. The number of bacterial cells
currently living on Earth has been estimated at five
million trillion trillion. From an evolutionary perspective, bacteria and archaea are highly successful.
Identifying Species and Investigating Diversity
In eukaryotes, a species is defined on the basis of the
ability of its members to mate and produce fertile offspring. This definition of a species does not apply to
organisms such as bacteria and archaea that typically
330 UNIT IV
0.25 µm
reproduce only asexually (Figure 20.11). In these groups,
a species is defined as a group of individuals that
share an ancestor and have a high degree of similarity
in many independently inherited traits.
Historically, classification of bacteria was based on
numerical taxonomy. By this process, an unidentified
cell is compared against a known group on the basis
of cell shape, cell wall properties, and metabolism.
The more traits the cell shares with the known group,
the closer is their inferred relatedness. This approach
works best for cells that can be grown in the laboratory, stained, and viewed with a microscope. However,
many bacteria and archaea do not grow in the lab.
The relatively new field of metagenomics is devoted
to assessing microbial diversity by analysis of DNA
in samples collected directly from an environment.
Metagenomic studies often reveal a remarkable degree
of species diversity. For example, air samples collected
in two cities in Texas contained about 1,800 different
kinds of bacteria.
The Human Microbiome Project is an ongoing
metagenomic study of the microorganisms that live
in or on the human body. Already, a survey of bacteria that live on the skin of the inner elbow turned up
nearly 200 species. Another study found that more
than a thousand species of bacteria and a few archaeal
species can live in the human gut.
Metabolic Diversity
Autotroph or Heterotroph? Metabolic diversity gives
prokaryotes the collective ability to live just about
anywhere there is a source of energy and carbon.
EVOLUTION AND BIODIVERSITY
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Licensed to: CengageBrain User
archaea Most recently discovered and less well-known lineage of
single-celled organisms without a nucleus.
bacteria Most diverse and well-known lineage of single-celled
organisms without a nucleus.
chemoautotroph Organism that uses carbon dioxide as its carbon
source and obtains energy by oxidizing inorganic molecules.
chemoheterotroph Organism that obtains both energy and carbon
by breaking down organic compounds.
metagenomics Study of microbial diversity that relies on analysis
of DNA samples collected directly from the environment.
photoautotroph Organism that obtains carbon from carbon dioxide and energy from light.
photoheterotroph Organism that obtains its carbon from organic
compounds and its energy from light.
prokaryote Member of one of two single-celled lineages (bacteria
and archaea) that do not have a nucleus; a bacterium or archaeon.
Energy Source
Carbon
Source
CO2
Organic
molecules
Light
Chemicals
Photoautotrophs
Chemoautotrophs
some bacteria,
photosynthetic
protists, plants
some bacteria,
most archaea
Photoheterotrophs
Chemoheterotrophs
some bacteria,
some archaea
most bacteria, some
archaea, fungi, animals,
nonphotosynthetic
protists
Figure 20.12 Modes of nutrition in bacteria and archaea.
Figure It Out: Which group of organisms can build their own
food from CO2 in the dark?
Answer: Chemoautotrophs
Organisms harvest energy and nutrients from the
environment in four different ways. All of these nutritional modes occur among bacteria, archaea, or both
(Figure 20.12). In addition, some bacteria and archaea
can switch from one metabolic mode to another.
As you learned in Section 6.1 autotrophs build their
own food using carbon dioxide (CO2) as their carbon
source. There are two subgroups of autotrophs: those
that obtain energy from light, and those that obtain
energy from chemicals.
Photoautotrophs are photosynthetic. They use the
energy of light to assemble organic compounds from
CO2 and water. Many bacteria are photoautotrophs, as
are plants and photosynthetic protists. As Section 19.6
explained, eukaryotes have chloroplasts that evolved
from cyanobacteria, a type of photosynthetic bacteria.
Chemoautotrophs obtain energy by oxidizing
(removing electrons from) inorganic molecules such
as hydrogen sulfide or methane. They use energy
released by this process to build food from CO2.
Chemoautotrophic bacteria and archaea are the main
producers in dark environments such as the sea floor.
No eukaryotes are known to be chemoautotrophs.
Heterotrophs cannot use inorganic sources of carbon. Instead, they obtain carbon by taking up organic
molecules from their environment. Photoheterotrophs
harvest energy from light, and carbon from alcohols, fatty acids, or other small organic molecules.
Heliobacteria that live in the soils of rice paddies are
an example.
Chemoheterotrophs obtain both energy and carbon
by breaking down carbohydrates, lipids, and proteins.
Most bacteria and some archaea are chemoheterotrophs, as are animals, fungi, and nonphotosynthetic
protists. All pathogenic bacteria are chemoheterotrophs that extract the organic compounds they need
to live from their host. Other bacterial chemohetero-
trophs serve as decomposers. They convert organic
molecules into inorganic ones. By their activity,
decomposers make nutrients that were tied up in
organic wastes and remains accessible to plants.
Aerobe or Anaerobe? With rare exceptions, eukaryotic
organisms are aerobes; they rely on aerobic respiration (Section 7.2) and thus require oxygen. By contrast
many bacteria and most archaea are anaerobes. Some
can tolerate an oxygen-free environment. Others are
obligate anaerobes, meaning oxygen either slows their
growth or kills them outright. Anaerobes are harmed
by oxygen because oxidation reactions damage their
biological molecules and, unlike aerobic cells, they do
have enzymes that can repair that damage. We find
obligate anaerobes in aquatic sediments and the animal gut. They also can infect deep wounds.
Take-Home Message
Why do biologists consider prokaryotes successful?
» Both bacteria and archaea have survived for billions of years and continue to
coexist beside the eukaryotes.
» Bacteria and archaea are Earth’s most abundant organisms. We are only
beginning to appreciate their enormous species diversity.
» Collectively, prokaryotes can live in a wider range of habitats than
eukaryotes because they are more diverse in terms of their metabolism.
Prokaryotes utilize all four modes of nutrition and may be aerobic or
anaerobic.
CHAPTER 20
viruses, Bacteria, and Archaea 331
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Licensed to: CengageBrain User
20.6 Structure and Function of “Prokaryotes”
■ Bacteria and archaea are small and structurally simple, but
they are well adapted to their environments.
■ Links to Discovery of DNA function 8.3, Molecular toolkit 15.2
Cell Size and Structure
Bacteria and archaea are similar in many ways (Table
20.3). The typical bacterial or archaeal cell cannot be
seen without a light microscope. Thousands can fit on
the head of a pin (Figure 20.13A). A bacteria is the size
of a eukaryotic mitochondrion and, as you know, these
organelles are likely descended from bacteria.
Three cell shapes are common: rods, spheres, and
spirals. Rod-shaped cells are described as bacilli (singular, bacillus), spherical cells as cocci (singular, coccus), and spiral cells as spirilla (singular, spirillum).
Nearly all bacteria and archaea secrete a semirigid,
porous cell wall around their plasma membrane (Figure
20.13B). Bacterial cells walls include peptidoglycan, a
molecule not found in archaea. Bacteria may also have
a slime layer or capsule around the cell wall. Slime
helps a cell stick to surfaces. A capsule is tougher and
Table 20.3 Traits Bacteria and Archaea Share
1. No nuclear envelope; chromosome in nucleoid
2. Generally a single chromosome (a circular DNA
molecule); many species also contain plasmids
3. Cell wall (in most species)
4. Ribosomes distributed in the cytoplasm
5. Asexual reproduction by binary fission.
6. Capacity for gene exchange among existing cells
via conjugation, transduction, and transformation.
1 A bacterium has one circular
chromosome that attaches to the
inside of the plasma membrane.
2 The cell duplicates its chromosome, attaches the copy beside the
original, and adds membrane and
wall material between them.
3 When the cell has almost
doubled in size, new membrane
and wall are deposited across its
midsection.
4 Two genetically identical
cells result.
Figure 20.14 Animated Binary fission, the reproductive mode
of bacteria and archaea.
helps some bacterial pathogens evade the immune
defenses of their vertebrate hosts.
Bacteria and archaea typically have a single chromosome. This circle of double-stranded DNA attaches
to the plasma membrane and resides in a cytoplasmic
region called the nucleoid. There is no nuclear envelope
as in eukaryotes, although at least one bacterial species
has a membrane around its nucleoid. Membranes of
some bacteria fold inward, but no bacteria or archaea
have an endoplasmic reticulum or Golgi apparatus.
Many bacteria and archaea have flagella. The flagella do not bend side to side as in eukaryotes, but
rather rotate like a propeller, and they do not contain
microtubules. Hairlike filaments called pili (singular,
pilus) may also extend from the cell surface. Some
cells use pili to stick to surfaces. Others glide along by
DNA
cytoplasm with
ribosomes
plasma
membrane
cell wall
capsule
a Bacteria on the head of a pin.
40 µm
Figure 20.13 Animated Typical bacterial size and structure.
332 UNIT IV
flagellum
pilus
B Rod-shaped bacterium (a bacillus).
EVOLUTION AND BIODIVERSITY
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Licensed to: CengageBrain User
using their pili as grappling hooks. A pilus extends out
to a surface, sticks to it, then shortens, drawing the cell
forward. Another type of retractable pilus draws cells
together for gene exchanges, as described below.
Reproduction and Gene Transfers
Bacteria and archaea have staggering reproductive
potential. Some can divide every twenty minutes.
Both groups reproduce by binary fission (Figure 20.14).
During this process, a cell replicates its single chromosome and this DNA replica attaches to the plasma
membrane adjacent to the parent molecule. The addition of more membrane moves the two DNA molecules apart. Eventually, the membrane and cell wall
extend across the cell’s midsection and divide the parent cell into two genetically identical descendants.
In addition to inheriting DNA “vertically” from a
parent, bacteria and archaea engage in horizontal gene
transfers, by which an individual acquires genes from
another individual. The gene donor can be a cell of the
same species or a different species.
Conjugation involves transfer of genes on a plasmid
(Figure 20.15). A plasmid is a small circle of DNA separate from the chromosome (Section 15.2). During conjugation, a special sex pilus draws two cells together.
Then, one cell puts a copy of a plasmid into the other.
Both bacteria and archaea have plasmids and can
engage in conjugation. Members of the two groups
sometimes swap genes in this way.
With transduction, bacteriophages move genes
between cells. The virus picks up a bit of DNA from
one host cell, then transfers the DNA to its next host.
Bacteria and archaea also take up DNA from the
environment, a process called transformation. For
example, Frederick Griffith changed Streptococcus
bacteria from harmless to deadly by transformation
binary fission Cell reproduction process of bacteria and archaea.
conjugation Mechanism of horizontal gene transfer in which one
bacterial or archaeal cell passes a plasmid to another.
horizontal gene transfer Transfer of genetic material among exist-
ing individuals.
1 Conjugation in E. coli
begins when a cell with a
specific type of plasmid
extends a sex pilus to
another E. coli cell that
lacks this plasmid. The
pilus attaches the cells
to one another. When it
shortens, the cells are
drawn together.
sex pilus
nicked plasmid
2 A conjugation tube
forms, connecting the
cytoplasm of the cells.
An enzyme nicks the
plasmid in the donor cell.
conjugation tube
3 As a single strand of
plasmid DNA moves into
the recipient, each cell
makes a complementary
DNA strand.
4 The cells separate and
the plasmid resumes its
circular shape.
Figure 20.15 Animated Conjugation, a mechanism of gene transfer.
For clarity, the plasmid’s size has been greatly exaggerated and the
chromosome is not shown.
(Section 8.3). Griffith mixed the harmless cells with
dead cells of a harmful strain. The heat had damaged
the membranes of the harmful cells, thus killing them
and releasing their DNA. That DNA was picked up by
the harmless bacteria and put to use.
The ability of bacteria to acquire new genes has
important implications for public health. Suppose a
gene for antibiotic resistance arises by mutation in a
bacterial cell. This gene not only can be passed on to
that cell’s descendants, but can also be transferred to
other existing cells. Gene transfers increase the rate at
which a gene spreads through a population of bacteria, thus enhancing the population’s ability to respond
to any selective pressure.
nucleoid Region of cytoplasm where the DNA is concentrated in
a bacterial or archaeal cell.
pilus Protein filament that projects from the surface of some
bacterial and archaeal cells.
plasmid Of many bacteria and archaea, a small ring of nonchromosomal DNA replicated independently of the chromosome.
transduction In bacteria and archaea, a mechanism of horizontal
gene transfer in which a bacteriophage carries DNA from one cell
to another.
transformation In bacteria and archaea, a type of horizontal gene
transfer in which DNA is taken up from the environment.
Take-Home Message
What structural and functional features do bacteria and archaea share?
» In both lineages, cells are typically walled and a single chromosome is not
enclosed in a nucleus.
» Cells reproduce by binary fission and swap genes by conjugation and other
mechanisms of horizontal gene transfer.
CHAPTER 20
viruses, Bacteria, and Archaea 333
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Licensed to: CengageBrain User
20.7 Bacterial Diversity
■ Most bacteria play important ecological roles. A small minority are human pathogens.
■ Links to Photosynthesis 6.4, PCR 15.3, Hydrothermal vents
19.3, Evolution of chloroplasts and mitochondria 19.6
Bacteria that cause human disease often get the spotlight, but most bacteria are either harmless or beneficial. Here we consider a few of the major lineages to
give you an idea of bacterial diversity.
The Heat Lovers
If life emerged in thermal pools or near hydrothermal
vents, the modern heat-loving bacteria may resemble
those early cells. Biochemical comparisons put them
near the base of the bacterial family tree. One species,
Thermus aquaticus, was discovered in a volcanic spring
in Yellowstone National Park. Biochemist Kary Mullis
isolated a heat-stable DNA polymerase from T. aquaticus and put the enzyme to work in the first PCR reactions. He won a Nobel Prize for inventing this process,
which is widely used in biotechnology (Section 15.3).
Oxygen-Producing Cyanobacteria
Photosynthesis evolved in many bacterial lineages, but
only cyanobacteria (Figure 20.16A) utilize the noncyclic
pathway and release free oxygen. If, as evidence suggests, chloroplasts evolved from ancient cyanobacteria,
we have cyanobacteria and their chloroplast descen-
dants to thank for nearly all of the oxygen in Earth’s
atmosphere (Section 19.6).
When cyanobacteria incorporate the carbon from
carbon dioxide into an organic compound, we say that
they fix carbon. Some cyanobacteria also carry out
nitrogen fixation: They incorporate nitrogen from the
air into ammonia (NH3).
Nitrogen fixation is an important ecological service
provided only by bacteria. Photosynthetic eukaryotes
need nitrogen, but they cannot use the gaseous form
(N≡N) because they do not have an enzyme that can
break the molecule’s triple bond. They can, however,
take up ammonia released by nitrogen-fixing bacteria.
Highly Diverse Proteobacteria
Proteobacteria are the most diverse bacterial group.
Some are photoautotrophs that carry out photosynthesis but do not release oxygen. Others are chemoautotrophs. One of these, Thiomargarita namibiensis, is the
largest bacterium known and can be seen without a
microscope (Figure 20.16B). It gets energy by stripping
electrons from sulfur that it stores in a giant vacuole.
Rhizobium, a chemoheterotroph, lives in roots of
legumes such as peas. It gets sugars from its host and
in return aids the plant by fixing nitrogen.
Myxobacteria, or slime bacteria, are tiny hunters
that live in soil. They glide about as a swarm and feed
on other bacteria. When food dwindles, hundreds of
thousands of cells form a multicelled fruiting body
nitrogen-fixing
cell
photosynthetic
cells
capsule
with
spores
a Anabaena, a type of aquatic
cyanobacterium. It carries out
oxygen-producing photosynthesis and fixes nitrogen.
B Thiomargarita namibiensis, the
biggest bacterium known. It lives
in sea sediments and takes up and
stores sulfates (white dots).
C Chondromyces crocatus, a myxobacterium, hunts other soil bacteria. When
food runs out, thousands of cells form
a fruiting body with spores at its tip.
D Lactobacillus ferments sugars
and produces lactate. The cells
shown here were used to turn milk
into yogurt. Other lactobacilli are
important as decomposers.
Figure 20.16 A sampling of bacterial diversity. Most bacteria do not cause disease.
334 UNIT IV
EVOLUTION AND BIODIVERSITY
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Licensed to: CengageBrain User
Table 20.4 Examples of Disease-Causing Bacteria
Group/Species
Disease
Description
Proteobacteria
A
B
Figure 20.17 Example of a bacterial pathogen. (A) Spirochete
that causes Lyme disease. (B) Bull’s-eye rash at the site of a tick
bite is often the first sign of infection.
with spores (hard-walled resting structures) at its tips
(Figure 20.16C).
Escherichia coli is a chemoheterotroph that lives in
the mammalian gut. It is part of the normal flora, a
collection of microorganisms that typically live in and
on a body. Other proteobacteria that enter the body
cause disease (Table 20.4). One pathogenic group, the
rickettsias, is thought to be the closest relatives of
mitochondria. Rickettsias live as intracellular parasites.
Thick-Walled, Gram-Positive Bacteria
Gram-positive bacteria are a lineage of thick-walled
cells that stain purple when prepared for microscopy by Gram staining. Most are chemoheterotrophs.
Lactobacillus species ferment sugars and produce lactate (Figure 20.16D). The related Streptococcus species
cause strep throat and impetigo. Actinomycetes grow
as threadlike filaments in soil. One genus, Streptomyces,
is the source of the antibiotic streptomycin. Clostridium
and Bacillus are soil bacteria that form endospores
when conditions are unfavorable. An endospore contains the cell’s genome and a bit of cytoplasm in a
protective coat. It can withstand drying, boiling, and
radiation. When endospores enter a human body and
germinate, the resulting infection by toxin-secreting
bacteria can cause fatal anthrax, tetanus, or botulism.
chlamydias Bacteria that are intracellular parasites of vertebrates.
cyanobacteria Oxygen-producing photosynthetic bacteria.
endospore Resistant resting stage of some soil bacteria.
Gram-positive bacteria Lineage of thick-walled bacteria that are
colored purple by Gram staining.
nitrogen fixation Incorporation of nitrogen gas into ammonia.
proteobacteria Most diverse bacterial lineage; includes species that
carry out photosynthesis, fix nitrogen. Some cause disease.
spirochetes Lineage of bacteria shaped like a stretched-out spring.
Bordetella Whooping cough
pertussis
Childhood respiratory
infection
Neisseria
gonorrhoeae
Gonorrhea
Sexually transmitted disease
Rickettsia
rickettsii
Rocky Mountain
spotted fever
Fever accompanied by rash,
spread by ticks
Vibrio cholerae
Cholera
Diarrheal illness
Gram-Positive Bacteria
Clostridium
species
Tetanus, botulism
Toxin released by bacteria
causes paralysis
Streptococcus
Impetigo, boils
aureus
Blisters, sores on skin
Streptococcus
Strep throat
pyogenes
Sore throat, fever, damage
to heart valves if not treated
Spirochetes
Borrelia Lyme disease
burgdorferi
Rash, flulike symptoms
spread by ticks
Treponema
Syphilis
pallidum
Sexually transmitted disease
Mycobacteria
Mycobacterium
tuberculosis
Tuberculosis
Respiratory disease
Other Groups That Include Human Pathogens
Spirochetes resemble a stretched-out spring (Figure
20.17A). One species transmitted by ticks causes Lyme
disease (Figure 20.17B). Another causes the sexually
transmitted disease syphilis.
Chlamydias are small intracellular parasites of vertebrates. One species, Chlamydia trachomatis, is known in
the United States mainly as a cause of sexually transmitted disease. In developing countries, it is a major
cause of blindness.
Mycobacteria are rod-shaped cells with a waxy coat.
One species causes tuberculosis, a respiratory disease
that kills more than a million people each year.
Take-Home Message
What ecological roles do bacteria play?
» Bacteria benefit us by releasing oxygen, fixing nitrogen, and otherwise
participating in nutrient cycles.
» A small minority of the bacterial chemoheterotrophs cause disease.
CHAPTER 20
viruses, Bacteria, and Archaea 335
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Licensed to: CengageBrain User
20.8 Archaea
■ Archaea, the more recently discovered prokaryotic lineage,
are found in some very inhospitable places.
■ Links to Pigments 6.2, Chemiosmosis 6.5
Discovery of the Third Domain
The distinctive features of archaea first came to light
in the 1970s. Carl Woese was comparing the ribosomal
RNAs among what he thought were bacterial species
to find out how they related to one another. He discovered that some species fell into a distinct group. Their
rRNA gene sequences positioned them between all
other bacteria and the eukaryotes. On the basis of this
evidence, Woese proposed the three-domain classification system (Section 20.5).
As years went by, evidence in support of Woese’s
conclusions mounted. Archaea differ from bacteria in
the composition of their cell wall and plasma membrane. Like eukaryotes, archaea organize their DNA
around histone proteins, which bacteria do not have.
The first sequencing of an archaeal genome provided
the definitive evidence that archaea and bacteria are
distinct lineages—most of this archaeon’s genes have
no counterpart in bacteria.
Woese has compared the discovery of archaea to the
discovery of a new continent, which he and others are
now exploring.
a Thermally heated waters. Pigmented archaea color the rocks in
waters of this hot spring in Nevada.
Here, There, Everywhere
Many archaea thrive in seemingly hostile habitats. The first archaeon to have
its genome sequenced, Methanococcus
jannaschii (left), was discovered near a
hydrothermal vent on the seafloor. It
is an extreme thermophile, an organism
that grows only at a very high temperature. Some archaea that live near hydrothermal vents can grow even at 110°C
(230°F). Heat-loving archaea also live
in volcanically heated geysers and hot
springs (Figure 20.18A).
Other archaea are among the extreme
halophiles, organisms that live in highly
salty water. Salt-loving archaea live
0.5 µm
in the Dead Sea, the Great Salt Lake,
and many smaller brine-filled lakes (Figure 20.18B).
One extreme halophile, Halobacterium, has gas-filled
vesicles that keep it afloat in well-lit surface waters. Its
plasma membrane contains a unique protein, a purple
pigment called bacteriorhodopsin. When excited by
light, this protein pumps protons (H+) out of the cell,
336 UNIT IV
B Highly salty waters. Pigmented extreme halophiles color the
brine in this California lake.
C The gut of many animals. Cows belch frequently to
release the methane produced by archaea in their stomach.
Figure 20.18 Examples of archaeal habitats.
EVOLUTION AND BIODIVERSITY
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Licensed to: CengageBrain User
Evolution of a Disease (revisited)
Infectious diseases that are immediately fatal are rare.
This is fortunate and not surprising. Evolutionarily
speaking, the pathogens that leave the most descendants win. Think of a person infected with HIV as a
factory that makes and distributes virus (Figure 20.19).
Killing the host would shut down this facility.
Of course, hosts also evolve in response to disease.
A disease with a high mortality rate acts as a selective
agent, favoring individuals capable of resisting infection or surviving in spite of it. For example, about 10
percent of people of European ancestry have a mutation that lessens the likelihood of infection by HIV. The
mutation is absent in American Indian, east Asian, and
African populations.
People with this protective mutation currently enjoy
a selective advantage as a result of the AIDS epidemic.
However, the protective mutation did not become
prevalent in Europeans as a result of AIDS. Studies
of ancient remains tell us it has been in the northern
European gene pool for thousands of years. Like all
mutations, it arose randomly. It probably increased to
its current frequency in Europe because it provided
protection against one of the many historical epidemics that occurred there. Its current selective advantage
is simply a matter of luck.
How would you vote? Antiviral drugs help keep people
25 µm
Figure 20.19 Micrographs of a new HIV particle budding
from an infected white blood cell.
against their gradient. The H+ flows back into the cell
through ATP synthases, thus driving the formation of
ATP. A similar process drives ATP formation during
photosynthesis (Section 6.5). However, Halobacterium
does not use energy stored in ATP energy to fix carbon
dioxide as photosynthetic organisms do. It is a photoheterotroph that obtains carbon by taking up small
organic molecules from its environment.
Many archaea, including some extreme thermophiles and extreme halophiles, are methanogens, or
methane makers. These chemoautotrophs form ATP
by pulling electrons from hydrogen gas or acetate.
Methane (CH4) gas forms as a product of these reactions. Methanogenic archaea abound in sewage, marsh
sediments, and the animal gut (Figure 20.18C). All are
strictly anaerobic, meaning they cannot live in the
presence of oxygen.
By their metabolic activity, methanogens produce
2 billion tons of methane annually. The release of this
with HIV healthy and lessen the likelihood of viral transmission. However, an estimated 25 percent of HIV-infected
Americans do not know they are infected. Annual, voluntary
HIV tests with drug treatment for those infected could help
curtail the AIDS pandemic. Do you favor an expanded, voluntary testing program?
carbon-containing gas into the air has a major impact
on the global carbon cycle.
As biologists continue to explore archaeal diversity,
they are finding that archaea live alongside bacteria
nearly everywhere. They are more abundant than bacteria in deep, dark ocean waters.
So far, scientists have not discovered any archaea
that pose a major threat to human health. However,
the presence of archaea may have some ill effects. For
example, methanogenic archaea live in the human gut,
and their abundance may affect our weight. By taking up hydrogen, methanogens make the gut more
hospitable for bacteria that break down complex carbohydrates. As a result, the more methanogens in your
gut, the more calories you can extract from food. Some
studies have found a correlation between an abundance of gut methanogens and obesity.
Take-Home Message
extreme halophile Organism adapted to life in a highly salty
environment.
extreme thermophile Organism adapted to life in a very high-
temperature environment.
methanogen Organism that produces methane gas as a metabolic
by-product.
What are archaea?
» Archaea are single cells without a nucleus that are closer to eukaryotes
than to bacteria. Many live in very hot or very salty habitats, but there are
archaea nearly everywhere. Unlike bacteria, they are not a major cause of
human disease.
CHAPTER 20
viruses, Bacteria, and Archaea 337
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Licensed to: CengageBrain User
Summary
Section 20.1 AIDS is an emerging disease
caused by HIV. The oldest evidence of this
virus comes from central Africa, where
chimpanzees are infected by a related
virus (SIV). Analysis of viral genes has
allowed researchers to trace the spread of HIV.
Section 20.2 A virus consists of protein,
nucleic acid, and, in some cases, a bit of
membrane from a host cell. A virus replicates inside a cell of a specific host type.
For example, bacteriophages infect
bacteria. Some viruses are pathogens that cause human
disease. Viruses may have evolved before cells, or they
may be descended from cells or their components.
Viroids are infectious particles consisting only of a
small circle of RNA that does not encode any proteins.
They enter plants through a wound and cause disease.
E Lysis of host cell lets
new virus particles escape.
1 Lytic
Pathway
C Viral proteins self-assemble
into a coat around viral DNA.
Section 20.3 To replicate, a virus attaches
to a host cell and its genetic material
enters the cell. Viral genes and enzymes
A1 Viral
DNAmachinery
is inserted
direct
host
to replicate viral
into host chromosome by
A2 Chromosome
A Virus particle binds,
genetic
material
makeaction.
viral proteins.
Viral particles
and integrated
viral
injects genetic
material. and
viral enzyme
DNA are replicated.
self-assemble and are released.
Bacteriophages may
multiply by a lytic pathway, in
2 Lysogenic
Pathway are made fast and released
which the new viral particles
by lysis, or by a lysogenic pathway, in which viral DNA
becomes part of the host chromosome.
B Host
replicates
A3 The
Cell divides;
The
genetic material of HIV is RNA.
viral
viral genetic material,
recombinant DNA is in
reverse transcriptase uses viral
RNA ascell.
its
enzyme
builds
viral proteins.
each descendant
Viral enzyme
excises
template to make A4
DNA
that the
host cell can read.
viral DNA from chromosome.
Section 20.4 Viral diseases may be spread
by contact with a viral particle or delivered into the body by a vector such as a
tick. Most viral diseases such as common
colds cause symptoms only briefly. Some
viruses persist in the body and reawaken after a latent
period. Viral genes mutate and viruses can swap genes
in a host individual, a process called viral reassortment.
An epidemic is an outbreak of disease in only a limited
region. A pandemic is a worldwide outbreak.
Section 20.5 The prokaryotes are now
known to include two distinct lineages:
bacteria and archaea. Unlike eukaryotes,
these lineages do not typically have a
nucleus or endomembrane system, and
they do not reproduce sexually. Metagenomics, the
study of DNA in samples drawn directly from the environment, is revealing previously unknown diversity.
Bacteria and archaea are small, abundant, and­, as a
group, metabolically diverse. Some are aerobic and others cannot tolerate oxygen. Photoautotrophs carry out
photosynthesis. Photoheterotrophs capture light, but
they get carbon from organic molecules rather than CO2.
Chemoautotrophs build food from CO2 using energy
from inorganic substances. Chemoheterotrophs extract
both energy and carbon from organic molecules.
338 UNIT IV
Section 20.6 Most bacteria and archaea
have cell walls. Many have surface projections such as flagella and pili. The
chromosome is a circular molecule of
DNA that resides in a region of cytoplasm
called the nucleoid. There may also be one or more plasmids, circles of DNA that are separate from the chromosome and carry a few genes. Reproduction occurs by an
asexual process called binary fission.
Three types of horizontal gene transfer move genes
between existing cells. Conjugation transfers a plasmid
from one cell to another. Virus-assisted transfer of genes
is transduction. With transformation, cells take up DNA
from the environment.
Section 20.7 Bacteria are the most wellknown and diverse cells without a
nucleus. Many are ecologically important. Cyanobacteria produce oxygen as
a by-product of photosynthesis. They
and other bacteria carry out nitrogen fixation, the incorporation of nitrogen from nitrogen gas into ammonia.
Proteobacteria and Gram-positive bacteria are the most
highly diverse bacterial lineages. Both include some
pathogens. Some Gram-positive bacteria such as those
that cause anthrax survive unfavorable conditions as
endospores. Coiled cells called spirochetes and intracellular parasites called chlamydias can also cause disease.
Section 20.8 Archaea are the more recently
discovered lineage of cells without a
nucleus. Comparisons of structure, function, and genetic sequences position them
in a separate domain, between eukaryotes
and bacteria. Many archaea live in extreme environments. Halophiles live in salty waters and extreme thermophiles live at very high temperatures. Some archaea
are photoheterotrophs with a unique purple protein that
captures light energy. Most, including the methanogens,
are chemoautotrophs. Some archaea live in our bodies,
but none are considered pathogens.
Self-Quiz
Answers in Appendix III
1. DNA or RNA may be the genetic material of
.
a.a bacterium b.a viroid c. a virus d.an archaeon
2. A viroid consists entirely of
a.DNA
b.RNA
.
c. protein d.lipids
3. Bacteriophages can kill their host quickly by
.
a.binary fission
c. a lysogenic pathway
b.a lytic pathway
d. both b and c
4. The genetic material of HIV is
5. The
a.archaea
b.bacteria
.
do not reproduce sexually.
c. eukaryotes
d.both a and b
6. One cell transfers a plasmid to another by
a.binary fission
c. conjugation
b. transformation
d.the lytic pathway
.
EVOLUTION AND BIODIVERSITY
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Licensed to: CengageBrain User
3. Do these results support the hypothesis that the virus is
adapting to host defenses?
4. Japan has a high frequency of HLA-B*51; about half the
population has it. How might this explain the high frequency
of the I135X mutation in Japanese with other HLAs?
8. E. coli cells that live in your gut are
.
a.photoautotrophs
c. chemoautotrophs
b.photoheterotrophs
d. chemoheterotrophs
are intracellular parasites.
a.Spirochetes
c. Cyanobacteria
b.Chlamydias
d. Proteobacteria
10.Some Gram-positive bacteria (e.g., Bacillus anthracis)
survive harsh conditions by forming a(n)
.
a.pilus
c. endospore
b.heterocyst
d. plasmid
11.
reproduce by binary fission.
c. Bacteria
a.Viruses
b.Archaea
d. both b and c
12.A plasmid is a circle of
a.RNA
b.DNA
.
c. either RNA or DNA
13.Which of the following infectious diseases is caused by
bacteria?
a.flu
b.AIDS
c. measles
d.syphilis
14.A worldwide outbreak of a disease is a(n)
100
88
9 358
100
40 441
60 217
98
98
20
29
18
Kumamoto,
Japan
Perth,
Australia
Durban,
South Africa
16
Gaborone,
Botswana
42
28
0
Cohort
100
66
50
25
3 294
Figure 20.20 Regional variation in the frequency of the I135X escape mutation among HIVpositive people. For each region, pink bars represent people whose blood cells have HLAB*51, and thus cannot detect I135X mutants. Blue bars represent people with other versions
of the HLA protein. These people have blood cells that can detect and fight HIV even if it has
the I135X mutation.
Critical Thinking
7. All
are oxygen-releasing photoautotrophs.
a.spirochetes
c. cyanobacteria
d. proteobacteria
b.chlamydias
9.
13 91
London, UK
2. Overall, are people with HLA-B*51 more or less likely
than those with other HLAs to have virus with the mutation?
25 54
Oxford, UK
1. What percentage of people with HLA-B*51 in Vancouver
had HIV with the escape mutation for this protein?
n = 60 455
100
95
75
Vancouver,
Canada
Adapting to Host Defenses Surface proteins called HLAs
allow white blood cells to detect HIV particles and fight
an infection. In a recent study, scientists tested whether
HIV is adapting to this host defense. They did so by looking at the frequency of a specific mutation (I135X) in HIV.
This “escape mutation” helps the virus avoid detection by
a version of the HLA protein (HLA-B*51) that is common
in some regions of the world, but not in others.
Figure 20.20 shows the percentage of HIV-positive people who had HIV with the I135X mutation. Data was collected at medical centers from several parts of the world.
I135X variant (%)
data analysis activities
.
15.Match the terms with their most suitable description.
methanogen
a.infectious RNA
nucleoid
b.nonliving infectious particle;
virus nucleic acid core, protein coat
plasmid
c. draws cells together
extreme
d.releases methane
halophile e.region with DNA
viroid
f. circle of nonchromosomal DNA
sex pilus
g.rod-shaped cell
bacillus
h.salt lover
1. If a cut or scrape becomes infected, Staphylococcus
aureus is probably the culprit (right). These bacteria often
live on the skin and they can cause a problem if they get
into a wound. Most “staph” infections can be cured with
the antibiotic methicillin. Unfortunately, methicillinresistant S. aureus (MSRA) is on the rise. Antibioticresistant staph infections previously occurred mainly in
hospitals and nursing homes. Now they are breaking out
in schools and health clubs. The bacteria are transmitted
by contact with an infected person or something that
person has touched, as by sharing towels and razors.
The gene conferring methicillin resistance is on a plasmid. Explain why a gene on a plasmid can spread more
quickly than a gene that is on the bacterial chromosome.
2. The adenoviruses that cause colds do not have a lipid
envelope and they tend to remain infectious outside the
body for longer than enveloped viruses. “Naked” viruses
are also less likely to be rendered harmless by soap and
water. Why might possession of an envelope make a
virus less hardy?
3. A farmer growing peas can ensure the plants get
enough nitrogen by adding a nitrogen fertilizer that
contains ammonia, or by applying Rhizobium inoculum
to seeds or young plants. What are the ecological advantages of encouraging the growth of nitrogen-fixing bacteria rather than using a chemical fertilizer?
4. Scientists on a drilling project in Virginia discovered
a new species of bacteria living 3 kilometers (a little less
than 2 miles) beneath the soil surface. The temperature
here is 75°C (167°F) and the pressure is tremendous.
They named the new species Bacillus infernus, which
means “bacterium from hell.” Which of the four possible
modes of nutrition could these cells be using?
CHAPTER 20
viruses, Bacteria, and Archaea 339
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
S. aureus
Wound infected
by MRSA
Appendix III. Answers to Self-Quizzes
Italicized numbers refer to relevant section numbers
Chapter 20
1. c
2. b
3. b
4. RNA
5. d
6. c
7. c
8. d
9. b
10.c
11.d
12.b
13.d
14.pandemic
15.d
e
b
f
h
a
c
g
20.2
20.4
20.3
20.3
20.5
20.6
20.7
20.5, 20.7
20.7
20.7
20.6
20.6
20.4, 20.7
20.4
20.8
20.6
20.2
20.6
20.8
20.4
20.6
20.6
This page contains answers for this chapter only
Appendix III
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.