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Viruses and Prokaryotes
Chapter 21 Part 1
Impacts, Issues
The Effects of AIDS
 Some viruses and bacteria help us; others, such
as the HIV virus that causes AIDS, can kill
21.1 Viral Characteristics and Diversity
 A virus consists of nucleic acid and protein
 A virus is smaller than any cell and has no
metabolic machinery of its own
Viruses
 Viruses
• Noncellular infectious particles that multiply only
inside living cells
• Consist of genetic material and a protein coat;
some also have a lipid envelope
• Some viruses cause disease (pathogens); others
control disease-causing organisms
Characteristics of a Virus
Examples of Viruses
 Viruses that infect plants (tobacco mosaic virus)
 Viruses that infect bacteria or archaeans
(bacteriophages)
 Naked viruses (adenoviruses)
 Enveloped viruses (herpesviruses)
Examples of Viruses
RNA
protein
subunits
of coat
Fig. 21-2a, p. 334
DNA
inside
protein
coat
sheath
tail
fiber
Fig. 21-2b, p. 334
Fig. 21-2c, p. 334
Fig. 21-2d, p. 334
viral DNA and enzymes
lipid envelope with
protein components
protein coat
inside envelope
Fig. 21-2e (1), p. 334
lipid envelope with
protein components
protein coat
inside envelope
Fig. 21-2e (2), p. 334
Effects of Plant Viruses
Viral Origins and Evolution
 Viruses may have descended from cells that
were parasites of other cells
 Viruses may be genetic elements that escaped
from cells
 Viruses may represent a separate evolutionary
branch
21.2 Viral Replication
 All viruses replicate only inside host cells, but
the details of the process vary among viral
groups
Steps in Viral Replication
Stepped Art
Table 21-2, p. 336
Bacteriophage Replication
 Lytic pathway
• Under direction of viral genes, the host makes an
enzyme that lyses and kills the cell
 Lysogenic pathway
• Virus enters a latent state
• Host replicates viral genes and passes them on
to descendents before entering lytic pathway
Bacteriophage Replication
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
D Accessory parts are
attached to viral coat.
C Viral proteins selfassemble into a coat
around viral DNA.
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
daughter cell.
Fig. 21-4, p. 337
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
daughter cell.
Stepped Art
Fig. 21-4, p. 337
Animation: Bacteriophage multiplication
cycles
Animation: Lytic pathway
Animation: Lysogenic pathway
Replication of an Enveloped DNA Virus
 Example: Herpes
• Viral envelope fuses with host membrane; viral
DNA enters host cytoplasm
• Viral DNA enters nucleus, directs synthesis of
new viral DNA and proteins
• New viral particles are assembled and enveloped
in host nuclear membrane
• New viral particles exit cell by exocytosis
Replication of a Retrovirus
 Example: HIV
• Virus binds to receptors on certain white blood
cells; viral envelope fuses with host membrane;
viral RNA enters host cytoplasm
• Enzyme (reverse transcriptase) converts viral
RNA to DNA, which integrates with host DNA
• Host cell produces viral RNA and proteins which
assemble into new viral particles
• New viruses are enveloped in host plasma
membrane and exit by exocytosis
HIV Replication
viral enzyme
(reverse transcriptase)
C Viral DNA gets
integrated into the
host’s chromosome.
A Viral RNA
and protein
enter the
host cell.
viral coat
proteins
one of two
strands of
viral RNA
lipid envelope
with proteins
B Viral reverse
transcriptase
uses viral RNA
to make doublestranded viral
DNA.
nucleus
viral DNA
D Viral DNA gets
transcribed along
with the host’s genes.
E Some RNA
transcripts are
viral proteins new viral RNA.
Others are
translated into
viral proteins.
RNA and
viral RNA
proteins
assemble as
new virus
particles.
F Viral particles
bud from the
infected cell.
Fig. 21-5, p. 337
Animation: HIV replication cycle
21.3 Viroids and Prions
 Viroids and prions are infectious particles that
are even simpler than viruses
 Viroid
• Infectious RNA, not surrounded by a protective
protein coat
 Prion
• Proteins in the nervous system that can misfold,
and cause other prions to misfold
Viroid Disease in Plants
Prion Diseases
 Scrapie: A prion disease that affects sheep
 Bovine spongiform encephalopathy (BSE or mad
cow disease): Affects cattle that have eaten feed
made with infected sheep
 Variant Creutzfeldt-Jacob disease (vCJD):
Affects humans who have eaten infected beef
Prion Diseases
21.1-21.3 Key Concepts: Viruses and
Other Noncellular Infectious Particles
 Viruses are noncellular particles made of protein
and nucleic acid; they replicate by taking over
the metabolic machinery of a host cell
 Viroids are short sequences of infectious RNA
 Prions are infectious misfolded versions of
normal proteins
21.4 Prokaryotes—Enduring,
Abundant, and Diverse
 Prokaryotes
• Structurally simple cells that lack a nucleus
• Evolved before eukaryotes
 Prokaryotes still persist in enormous numbers
and show great metabolic diversity
Evolutionary History and Classification
 Automated gene sequencing and comparative
biochemistry helps classify species and
subgroups (strains) of prokaryotes
to ancestors of eukaryotic cells
DOMAIN BACTERIA
DOMAIN ARCHAEA
biochemical and molecular origin of life
p. 339
Abundance and Metabolic Diversity
 Prokaryotes are Earth’s most abundant organisms
 Metabolic diversity contributes to their success
• Example: Saprobes that break down wastes or
remains are important decomposers
 All four forms of nutrition are used by prokaryotes
Prokaryotic Nutritional Modes
21.5 Prokaryotic Structure and Function
 Prokaryotic cells have many structural features
that adapt them to their environment
 The typical prokaryote is a walled cell with
ribosomes but no nucleus
Prokaryotic Cell Characteristics
 Prokaryotic structure
• Nucleoid region contains a single, circular
chromosome
• Cell wall surrounds the plasma membrane, with
a slime layer (capsule) outside the cell wall
• Flagella rotate like propellers
• Pili extend from the cell surface for adhesion or
motion
Prokaryotic Cell Characteristics
Prokaryotic Body Plan
Prokaryotic Cell Size and Shape
 Prokaryotic cells are
much smaller than
eukaryotic cells (about
the size of
mitochondria)
 Prokaryotes have
three typical shapes:
coccus
bacillus
spirillum
Fig. 21-8a, p. 340
DNA, in
cytoplasm, with ribosomes nucleoid region
pilus
bacterial
flagellum
outer capsule
cell wall
plasma
membrane
Fig. 21-8b, p. 340
Prokaryotic Reproduction
 Prokaryotic chromosome
• A circular, double-stranded DNA molecule
 Prokaryotic fission
• DNA replicates; parent cell divides in two
Prokaryotic Fission
A The bacterial
chromosome is
attached to the
plasma membrane
prior to DNA
replication.
C The DNA copy
becomes attached at
a membrane site
near the attachment
site of the parent
DNA molecule.
B Replication starts
and proceeds in two
directions from a
certain site in the
bacterial chromosome.
E Lipids, proteins, and
carbohydrates are built
for new membrane and
new wall material. Both
get inserted across the
cell’s midsection.
D Then the two DNA
molecules are moved
apart by membrane
growth between the
two attachment sites.
F The ongoing,
orderly deposition
of membrane and
wall material at the
midsection cuts
the cell in two.
Fig. 21-10, p. 341
A The bacterial
chromosome is
attached to the
plasma membrane
prior to DNA
replication.
C The DNA copy
becomes attached at
a membrane site
near the attachment
site of the parent
DNA molecule.
B Replication starts
and proceeds in two
directions from a
certain site in the
bacterial chromosome.
E Lipids, proteins, and
carbohydrates are built
for new membrane and
new wall material. Both
get inserted across the
cell’s midsection.
D Then the two DNA
molecules are moved
apart by membrane
growth between the
two attachment sites.
F The ongoing,
orderly deposition
of membrane and
wall material at the
midsection cuts
the cell in two.
Stepped Art
Fig. 21-10, p. 341
Animation: Prokaryotic fission
Horizontal Gene Transfers
 Conjugation
• Transfer of a plasmid (non-chromosomal DNA)
between prokaryotic cells via a sex pilus
 Transduction
• Transfer of prokaryotic genes via bacteriophages
 Transformation
• Prokaryotic genes acquired from the environment
Conjugation
sex pilus
A 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.
Fig. 21-11a, p. 341
nicked plasmid conjugation tube
B A conjugation tube forms,
connecting the cytoplasm of
the cells. An enzyme nicks
the plasmid in the donor cell.
C As a single strand of
plasmid DNA moves into the
recipient, each cell makes a
complimentary DNA strand.
D The cells separate and the
plasmid resumes its circular
shape.
Fig. 21-11 (b-d), p. 341
sex pilus
A 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.
nicked plasmid conjugation tube
B A conjugation tube forms,
connecting the cytoplasm of
the cells. An enzyme nicks the
plasmid in the donor cell.
C As a single strand of plasmid DNA
moves into the recipient, each cell
makes a complimentary DNA strand.
D The cells separate and the
plasmid resumes its circular
shape.
Stepped Art
Fig. 21-11, p. 341
Animation: Prokaryotic conjugation
21.4-21.5 Key Concepts
Features of Prokaryotic Cells
 Prokaryotes are single-celled organisms that do
not have a nucleus or the diverse cytoplasmic
organelles found in most eukaryotic cells
 Collectively, prokaryotes show great metabolic
diversity; they divide rapidly and exchange DNA
by a variety of mechanisms