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Cecie Starr
Christine Evers
Lisa Starr
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Chapter 19
Viruses, Bacteria, and Archaeans
(Sections 19.1 - 19.5)
Albia Dugger • Miami Dade College
19.1 Evolution of a Disease
• Scientists use knowledge of evolution to investigate how a
new disease such as AIDS can arise and spread in the
human population
• By sequencing the HIV-1 genome and comparing it to
genomes of primate viruses, researchers found that the
human virus evolved from simian immunodeficiency virus
(SIV), which infects wild chimpanzees
HIV Infection
• New HIV particle budding from an infected white blood cell
19.2 Viral Structure and Function
• A virus is a noncellular infectious agent that consists of a
protein coat around a core of DNA or RNA
• In some viruses, the coat is enveloped in a bit of plasma
membrane derived from a previous host
• virus
• Noncellular, infectious particle of protein and nucleic acid;
replicates only in a host cell
Viral Traits and Diversity
• A virus is far smaller than any cell and has no ribosomes or
other metabolic machinery
• Each type of virus has structural adaptations that allow it to
infect and replicate in hosts of a particular type
• A bacteriophage is a virus that infects bacteria
Examples of Viral Structure
• T4 bacteriophage has a
complex coat
• DNA is encased in a protein
“head”
• Fibers on rodlike “tail” attach
virus to host
Examples of
Viral
Structure
DNA
inside
protein
coat
sheath
tail
fiber
A T4 bacteriophage
Fig. 19.2a, p. 298
Examples of Viral Structure
• Tobacco mosaic virus has a
helical structure, with coat
proteins arranged around
RNA to form a rod
Examples of
Viral
Structure
RNA
protein
subunits
of coat
B Tobacco mosaic virus
Fig. 19.2b, p. 298
Examples of Viral Structure
• Many animal viruses have a
20-sided protein coat
• Adenoviruses are “naked,”
but the coat has a protein
spike at each corner
Examples of
Viral
Structure
20-sided
protein coat
that encloses
DNA
C Adenovirus
Fig. 19.2c, p. 298
Examples of Viral Structure
• Most animal viruses are
enveloped; membrane from
a prior host encloses the
virus
• Herpesviruses are
enveloped DNA viruses
• HIV is an enveloped RNA
virus
Examples of
Viral
Structure
viral DNA and enzymes
lipid envelope
with protein
components
D Herpesvirus
20-sided
protein coat
beneath the
envelope
Fig. 19.2d, p. 298
Viral Replication
• Because a virus lacks ribosomes and other metabolic
machinery, it must replicate inside a host cell
• Viruses attach to a host cell, then enter it or insert viral
genetic material into it
• Viral genes and enzymes direct the host to replicate viral
genetic material and make viral proteins
• New viral particles self-assemble and are released
Bacteriophage Replication
• Bacteriophages have two types of replication pathways:
• In a lytic pathway, multiplication is rapid, and new viral
particles are released by lysis
• In a lysogenic pathway, the virus enters a latent state that
extends the cycle
Key Terms
• lysogenic pathway
• Bacteriophage replication path in which viral DNA
becomes integrated into the host’s chromosome and is
passed to the host’s descendants
• lytic pathway
• Bacteriophage replication pathway in which a virus
immediately replicates in its host and kills it
Bacteriophage Replication Pathways
Bacteriophage Replication Pathways
E Lysis of host cell lets
new viral particles escape.
A Viral particle binds,
injects genetic material.
Lytic
Pathway
D Accessory parts are
attached to viral coat.
C Viral proteins selfassemble into a coat
around viral DNA.
A1 Viral DNA is
Chromosome
inserted into host A2
and integrated
chromosome by
viral DNA are
viral enzyme action. replicated.
Lysogenic
Pathway
B Host replicates
viral genetic
material, builds
viral proteins.
A4 Viral enzyme
excises viral DNA
from chromosome.
A3 Cell divides;
recombinant
DNA in each
descendant cell.
Fig. 19.3, p. 299
Bacteriophage Replication Pathways
A1 Viral DNA is
A Virus particle binds, inserted into host A2 Chromosome
injects genetic material. chromosome by and integrated viral
E Lysis of host cell
DNA are replicated.
viral enzyme
lets new virus particles
action.
escape.
Lytic
Pathway
Lysogenic
Pathway
D Accessory parts are
attached to viral coat.
B Host replicates
viral genetic material,
C Viral proteins self- builds viral proteins.
assemble into a coat
around viral DNA.
A4 Viral
enzyme excises
viral DNA from
chromosome.
A3 Cell
divides;
recombinant
DNA in each
descendant
cell.
Stepped Art
Fig. 19.3, p. 299
ANIMATION: Bacteriophage multiplication
cycles
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HIV Replication
• HIV is an enveloped RNA virus that replicates in human white
blood cells
• Viral RNA in the cell must be reverse transcribed to DNA to
begin replication
• The virus acquires its envelope as it buds from the cell
membrane
9 Steps in HIV Replication
1. Virus binds to a host cell
2. Viral RNA and enzymes enter cell
3. Viral reverse transcriptase uses viral RNA to make doublestranded viral DNA
4. Viral DNA integrates into 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 RNA self-assemble at the host membrane
9. New virus buds with an envelope of host membrane
Replication Cycle of HIV
Replication Cycle of HIV
1 Virus binds to a host cell.
2
HIV DNA
2 Viral RNA and enzymes enter cell.
4
3
3 Viral reverse transcriptase uses viral
reverse
transcription
RNA to make double-stranded viral DNA.
4Viral DNA integrates into host genome.
5 transcription
HIV RNA
5 Transcription produces viral RNA.
6 Some viral RNA is translated
6
to produce viral proteins.
7 Other viral RNA forms the new
HIV
viral genome.
translation
7
1
8
9
8 Viral proteins and viral RNA self-
assemble at the host membrane.
9 New virus buds from the host cell, with
an envelope of host plasma membrane.
Fig. 19.4, p. 299
Replication Cycle of HIV
HIV DNA
2
4
3
reverse
transcription
5
transcription
HIV RNA
6
HIV
translation
7
1
9
8
Stepped Art
Fig. 19.4, p. 299
ANIMATION: HIV replication cycle
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ANIMATION: Body plans of viruses
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ANIMATION: Lytic Pathway
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ANIMATION: Lysogenic Pathway
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19.3 Viral Effects on Human Health
• Some viruses have a positive effect on human health, as
when bacteriophages kill bacteria that could cause food
poisoning – however, other viruses are pathogens
• pathogen
• Disease-causing agent
Common Viral Diseases
• Nonenveloped viruses:
• Adenoviruses (colds)
• Viral gastroenteritis (stomach flu)
• Human papillomavirus (genital warts, cervical cancer)
• Enveloped viruses:
• Herpesviruses (cold sores, genital herpes, infectious
mononucleosis, chicken pox)
• Influenza (flu)
• Mumps, measles, and German measles
Emerging Viral Diseases
• Changes to viral genomes as a result of mutation or gene
exchanges can alter the properties of a viral disease
• emerging disease
• A disease that was previously unknown, is new to
humans, or has recently begun spreading to a new region
West Nile Fever
• West Nile virus is an enveloped RNA virus that replicates in birds;
mosquitoes carry the virus from host to host (the vector for this virus)
• In about 1 percent of cases, West Nile fever attacks the nervous system
and can be fatal
• West Nile fever is an endemic disease throughout the continental United
States (present, but at a low level)
Key Terms
• vector
• Animal that carries a pathogen from one host to the next
• endemic disease
• Disease that persists at a low level in a region or
population
SARS
• Sudden acute respiratory syndrome (SARS) first appeared in
late 2002 in China – it quickly became an epidemic, then a
pandemic, killing 774 people in 37 countries in 9 months
• epidemic
• Disease outbreak limited to one region
• pandemic
• Outbreak of disease that affects many separate regions
and poses a serious threat to human health
SARS Coronavirus
• A previously unknown type
of coronavirus (right) causes
SARS
• A similar virus in Chinese
horseshoe bats probably
evolved in other animals
before infecting humans
The SARS Epidemic
• A health care worker
puts on protective gear
during the SARS
epidemic
Influenza H5N1 and H1N1
• Influenzaviruses commonly cause seasonal flu outbreaks
• Avian influenza H5N1 (bird flu) occasionally infects people
who are in contact with birds, and has a high mortality rate
• Influenza H1N1 (swine flu) first appeared in 2009, can cause
severe respiratory symptoms, and is easily transmitted
19.4 Viroids: Tiny Plant Pathogens
• A new type of pathogen, a small RNA without a protein coat,
was discovered in 1971
• Viroids do not encode proteins, but are ribozymes (RNAs
with enzymatic activity)
• All known viroids are circular, single-stranded RNAs
• Remarkably small, viroids have fewer than 400
nucleotides
• viroid
• Small noncoding RNA that can infect plants
Key Concepts
• Viruses and Viroids
• Viruses are noncellular, with a protein coat and a genome
of nucleic acid, but no metabolic machinery
• Viruses must infect cells to replicate
• Some infect humans and cause disease
• Viroids are RNA bits that do not encode proteins
• Even so, they can infect plant cells and replicate inside
them
19.5 Bacterial Structure
and Function
• Bacteria are small,
structurally simple cells
• Biologists describe
bacteria by their shapes
• Coccus (spherical)
• Bacillus (rod-shaped)
• Spirillum (spiral)
19.5 Bacterial
Structure
and Function
coccus
bacillus
A
spirillum
Fig. 19.6a, p. 302
Cell Size, Structure, and Motility
• Bacteria do not have a nucleus or cytoplasmic organelles
typical of eukaryotes
• A single bacterial chromosome, a circle of double-stranded
DNA, resides in a cytoplasmic region called the nucleoid
• Surface structures include a cell wall, a protective capsule or
slime layer, one or more flagella, and hairlike extensions (pili)
Key Terms
• bacterial chromosome
• Circle of double-stranded DNA that resides in the bacterial
cytoplasm
• nucleoid
• Cytoplasmic region where prokaryotic chromosome lies
• pilus
• Hairlike extension from the cell wall of some bacteria
Bacterial Body Plan
Bacterial Body Plan
cytoplasm, with ribosomes
DNA, in nucleoid
pilus
bacterial
flagellum
outer capsule
cell wall
B
plasma
membrane
Fig. 19.6b, p. 302
ANIMATION: Prokaryotic fission
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Abundance and Metabolic Diversity
• 5 million trillion trillion bacterial cells live on Earth
• As a group, bacteria are metabolically diverse, including all
four known modes of nutrition:
• Photoautotrophs
• Chemoautotrophs
• Photoheterotrophs
• Chemoheterotrophs
4 Nutritional Modes
Mode of Nutrition
Energy Source
Carbon Source
Photoautotrophic
Chemoautotrophic
Photoheterotrophic
Chemoheterotrophic
Sunlight
Inorganic substances
Sunlight
Organic compounds
Carbon dioxide
Carbon dioxide
Organic compounds
Organic compounds
Key Concepts
• Structure and Function of Bacteria
• Bacteria are small cells with DNA and ribosomes, but no
nucleus or typical eukaryotic organelles
• They are the most abundant and metabolically diverse
organisms, with autotrophs (self-feeders) and heterotrophs
(feeders on others) among them