viruses and prokaryotes - del mar...
TRANSCRIPT
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Viruses and Prokaryotes
Chapter 21 Part 1
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Impacts, Issues
The Effects of AIDS
Some viruses and bacteria help us; others, such
as the HIV virus that causes AIDS, can kill
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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
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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
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Characteristics of a Virus
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Examples of Viruses
Viruses that infect plants (tobacco mosaic virus)
Viruses that infect bacteria or archaeans
(bacteriophages)
Naked viruses (adenoviruses)
Enveloped viruses (herpesviruses)
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Examples of Viruses
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Fig. 21-2a, p. 334
RNA
protein
subunits
of coat
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Fig. 21-2b, p. 334
DNA
inside
protein
coat
sheath
tail
fiber
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Fig. 21-2c, p. 334
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Fig. 21-2d, p. 334
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Fig. 21-2e (1), p. 334
viral DNA and enzymes
lipid envelope with
protein components
protein coat
inside envelope
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Fig. 21-2e (2), p. 334
lipid envelope with
protein components
protein coat
inside envelope
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Effects of Plant Viruses
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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
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21.2 Viral Replication
All viruses replicate only inside host cells, but
the details of the process vary among viral
groups
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Steps in Viral Replication
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Table 21-2, p. 336
Stepped Art
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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
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Bacteriophage Replication
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Fig. 21-4, p. 337
A1 Viral DNA is
inserted into host
chromosome by
viral enzyme
action.
A Virus particle binds,
injects genetic material.
A2 Chromosome and integrated viral DNA are replicated.E Lysis of host cell
lets new virus particles
escape.
Lytic
Pathway
Lysogenic
Pathway
D Accessory parts are
attached to viral coat.B Host replicates
viral genetic material,
builds viral proteins.
A3 Cell
divides;
recombinant
DNA in each
daughter cell.
C Viral proteins self-
assemble into a coat
around viral DNA.
A4 Viral
enzyme excises
viral DNA from
chromosome.
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Fig. 21-4, p. 337
Stepped Art
A2 Chromosome and integrated viral DNA are replicated.E Lysis of host cell
lets new virus particles
escape.
A Virus particle binds,
injects genetic material.
Lytic
Pathway
A1 Viral DNA is
inserted into host
chromosome by
viral enzyme
action.
Lysogenic
Pathway
D Accessory parts are
attached to viral coat.B Host replicates
viral genetic material,
builds viral proteins.
A3 Cell
divides;
recombinant
DNA in each
daughter cell.
C Viral proteins self-
assemble into a coat
around viral DNA.
A4 Viral
enzyme excises
viral DNA from
chromosome.
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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
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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
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HIV Replication
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Fig. 21-5, p. 337
viral enzyme
(reverse transcriptase)
C Viral DNA gets
integrated into the
host’s chromosome.
D Viral DNA gets
transcribed along
with the host’s genes.
A Viral RNA
and protein
enter the
host cell.
E Some RNA
transcripts are
new viral RNA.
Others are
translated into
viral proteins.
RNA and
proteins
assemble as
new virus
particles.
viral coat
proteins
nucleus
B Viral reverse
transcriptase
uses viral RNA
to make double-
stranded viral
DNA.
viral proteins
viral RNA
viral DNAone of two
strands of
viral RNA
lipid envelope
with proteinsF Viral particles
bud from the
infected cell.
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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
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Viroid Disease in Plants
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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
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Prion Diseases
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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
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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
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Evolutionary History and Classification
Automated gene sequencing and comparative
biochemistry helps classify species and
subgroups (strains) of prokaryotes
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p. 339
to ancestors of eukaryotic cells
DOMAIN BACTERIA DOMAIN ARCHAEA
biochemical and molecular origin of life
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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
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Prokaryotic Nutritional Modes
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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
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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
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Prokaryotic Cell Characteristics
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Prokaryotic Body Plan
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Prokaryotic Cell Size and Shape
Prokaryotic cells are
much smaller than
eukaryotic cells (about
the size of
mitochondria)
Prokaryotes have
three typical shapes:
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Fig. 21-8a, p. 340
coccus
bacillus
spirillum
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Fig. 21-8b, p. 340
cytoplasm, with ribosomes
DNA, in
nucleoid region
pilus
bacterial
flagellum
outer capsule
cell wall
plasma
membrane
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Prokaryotic Reproduction
Prokaryotic chromosome
• A circular, double-stranded DNA molecule
Prokaryotic fission
• DNA replicates; parent cell divides in two
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Prokaryotic Fission
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Fig. 21-10, p. 341
A The bacterial
chromosome is
attached to the
plasma membrane
prior to DNA
replication.
B Replication starts
and proceeds in two
directions from a
certain site in the
bacterial chromosome.
C The DNA copy
becomes attached at
a membrane site
near the attachment
site of the parent
DNA molecule.
D Then the two DNA
molecules are moved
apart by membrane
growth between the
two attachment sites.
E Lipids, proteins, and
carbohydrates are built
for new membrane and
new wall material. Both
get inserted across the
cell’s midsection.
F The ongoing,
orderly deposition
of membrane and
wall material at the
midsection cuts
the cell in two.
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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.
B Replication starts
and proceeds in two
directions from a
certain site in the
bacterial chromosome.
C The DNA copy
becomes attached at
a membrane site
near the attachment
site of the parent
DNA molecule.
E Lipids, proteins, and
carbohydrates are built
for new membrane and
new wall material. Both
get inserted across the
cell’s midsection.
Stepped Art
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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
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Conjugation
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Fig. 21-11a, p. 341
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.
sex pilus
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Fig. 21-11 (b-d), 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.
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Fig. 21-11, p. 341
Stepped Art
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.sex pilus
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.
nicked plasmid conjugation tube
B A conjugation tube forms,
connecting the cytoplasm of
the cells. An enzyme nicks the
plasmid in the donor cell.
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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