Viruses and Bacteria
Ch 19 & 27
Campbells
Structure of Viruses
• Viruses are not cells
• Viruses are very small infectious particles consisting
of nucleic acid enclosed in a protein coat and, in
some cases, a membranous envelope
Capsids and Envelopes
• A capsid is the protein shell that encloses the viral
genome
• Capsids are built from protein subunits called
capsomeres
• A capsid can have various structures
Fig. 19-3
RNA
DNA
Capsomere
Membranous
envelope
RNA
Head
DNA
Capsid
Capsomere
of capsid
Glycoproteins
Glycoprotein
18 250 nm
70–90 nm (diameter)
80–200 nm (diameter)
20 nm
(a) Tobacco mosaic
virus
50 nm
(b) Adenoviruses
50 nm
(c) Influenza viruses
Tail
sheath
Tail
fiber
80 225 nm
50 nm
(d) Bacteriophage T4
• Some viruses have membranous envelopes that
help them infect hosts
• These viral envelopes surround the capsids of
influenza viruses and many other viruses found in
animals
• Viral envelopes, which are derived from the host
cell’s membrane, contain a combination of viral and
host cell molecules
• Bacteriophages, also called phages, are viruses that
infect bacteria
• They have the most complex capsids found among
viruses
• Phages have an elongated capsid head that
encloses their DNA
• A protein tail piece attaches the phage to the host
and injects the phage DNA inside
General Features of Viral Reproductive
Cycles
• Once a viral genome has entered a cell, the cell
begins to manufacture viral proteins
• The virus makes use of host enzymes, ribosomes,
tRNAs, amino acids, ATP, and other molecules
• Viral nucleic acid molecules and capsomeres
spontaneously self-assemble into new viruses
Animation: Simplified Viral Reproductive Cycle
Fig. 19-4
VIRUS
1 Entry and
DNA
uncoating
Capsid
3 Transcription
and manufacture
of capsid proteins
2 Replication
HOST CELL
Viral DNA
mRNA
Viral DNA
Capsid
proteins
4 Self-assembly of
new virus particles
and their exit from
the cell
The Lytic Cycle
• The lytic cycle is a phage reproductive cycle that
culminates in the death of the host cell
• The lytic cycle produces new phages and digests
the host’s cell wall, releasing the progeny viruses
• A phage that reproduces only by the lytic cycle is
called a virulent phage
• Bacteria have defenses against phages, including
restriction enzymes that recognize and cut up
certain phage DNA
Animation: Phage T4 Lytic Cycle
Fig. 19-5-5
1 Attachment
2 Entry of phage
5 Release
DNA and
degradation of
host DNA
Phage assembly
4 Assembly
3 Synthesis of viral
genomes and
proteins
Head
Tail
Tail fibers
The Lysogenic Cycle
• The lysogenic cycle replicates the phage genome
without destroying the host
• The viral DNA molecule is incorporated into the
host cell’s chromosome
• This integrated viral DNA is known as a prophage
• Every time the host divides, it copies the phage
DNA and passes the copies to daughter cells
Animation: Phage Lambda Lysogenic and Lytic Cycles
Fig. 19-6
Phage
DNA
Daughter cell
with prophage
The phage injects its DNA.
Cell divisions
produce
population of
bacteria infected
with the prophage.
Phage DNA
circularizes.
Phage
Bacterial
chromosome
Occasionally, a prophage
exits the bacterial
chromosome,
initiating a lytic cycle.
Lytic cycle
Lysogenic cycle
The bacterium reproduces,
copying the prophage and
transmitting it to daughter cells.
The cell lyses, releasing phages.
Lytic cycle
is induced
New phage DNA and proteins
are synthesized and
assembled into phages.
or
Lysogenic cycle
is entered
Prophage
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
Table 19-1a
Table 19-1b
RNA as Viral Genetic Material
• The broadest variety of RNA genomes is found in
viruses that infect animals
• Retroviruses use reverse transcriptase to copy
their RNA genome into DNA
• HIV (human immunodeficiency virus) is the
retrovirus that causes AIDS (acquired
immunodeficiency syndrome)
Fig. 19-8a
Glycoprotein
Viral envelope
Capsid
Reverse
transcriptase
RNA (two
identical
strands)
HIV
HOST CELL
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
DNA
NUCLEUS
Provirus
Chromosomal
DNA
RNA genome
for the
next viral
generation
New virus
mRNA
• Vaccines are harmless derivatives of pathogenic
microbes that stimulate the immune system to
mount defenses against the actual pathogen
• Vaccines can prevent certain viral illnesses
• Viral infections cannot be treated by antibiotics
• Antiviral drugs can help to treat, though not cure,
viral infections
Fig. 19-9c
(c) Vaccinating ducks
Viral Diseases in Plants
• More than 2,000 types of viral diseases of plants
are known and cause spots on leaves and fruits,
stunted growth, and damaged flowers or roots
• Most plant viruses have an RNA genome
Viroids and Prions: The Simplest
Infectious Agents
• Viroids are circular RNA molecules that infect
plants and disrupt their growth
• Prions are slow-acting, virtually indestructible
infectious proteins that cause brain diseases in
mammals
• Prions propagate by converting normal proteins
into the prion version
• Scrapie in sheep, mad cow disease, and CreutzfeldtJakob disease in humans are all caused by prions
Fig. 27-2
1 µm
(a) Spherical
(cocci)
2 µm
(b) Rod-shaped
(bacilli)
5 µm
(c) Spiral
Cell-Surface Structures
• An important feature of nearly all prokaryotic
cells is their cell wall, which maintains cell
shape, provides physical protection, and
prevents the cell from bursting in a hypotonic
environment
• Eukaryote cell walls are made of cellulose or
chitin
• Bacterial cell walls contain peptidoglycan, a
network of sugar polymers cross-linked by
polypeptides
• Archaea contain polysaccharides and proteins
but lack peptidoglycan
• Using the Gram stain, scientists classify many
bacterial species into Gram-positive and Gramnegative groups based on cell wall composition
• Gram-negative bacteria have less peptidoglycan
and an outer membrane that can be toxic, and
they are more likely to be antibiotic resistant
Fig. 27-3
Carbohydrate portion
of lipopolysaccharide
Cell
wall
Peptidoglycan
Cell
wall
layer
Outer
membrane
Peptidoglycan
layer
Plasma membrane
Plasma membrane
Protein
Protein
Grampositive
bacteria
(a) Gram-positive: peptidoglycan traps
crystal violet.
Gramnegative
bacteria
20 µm
(b) Gram-negative: crystal violet is easily rinsed away,
revealing red dye.
Fig. 27-3c
Grampositive
bacteria
Gramnegative
bacteria
20 µm
Fig. 27-7
1 µm
0.2 µm
Respiratory
membrane
Thylakoid
membranes
(a) Aerobic prokaryote
(b) Photosynthetic prokaryote
Fig. 27-8
Chromosome
Plasmids
1 µm
• The typical prokaryotic genome is a ring of DNA
that is not surrounded by a membrane and that
is located in a nucleoid region
Reproduction and Adaptation
• Prokaryotes reproduce quickly by binary fission
and can divide every 1–3 hours
• Many prokaryotes form metabolically inactive
endospores, which can remain viable in harsh
conditions for centuries
Fig. 27-9
Endospore
0.3 µm
Transformation and Transduction
• A prokaryotic cell can take up and incorporate
foreign DNA from the surrounding environment
in a process called transformation
• Transduction is the movement of genes
between bacteria by bacteriophages (viruses
that infect bacteria)
Fig. 27-11-4
Phage DNA
A+ B+
A+
B+
Donor
cell
A+
Recombination
A+
A– B–
Recipient
cell
A+ B–
Recombinant cell
Conjugation and Plasmids
• Conjugation is the process where genetic
material is transferred between bacterial cells
• Sex pili allow cells to connect and pull together
for DNA transfer
• A piece of DNA called the F factor is required for
the production of sex pili
• The F factor can exist as a separate plasmid or
as DNA within the bacterial chromosome
Fig. 27-12
Sex pilus
1 µm
•
•
•
•
Concept 27.3: Diverse nutritional and
metabolic adaptations have evolved in
prokaryotes
Phototrophs obtain energy from light
Chemotrophs obtain energy from chemicals
Autotrophs require CO2 as a carbon source
Heterotrophs require an organic nutrient to
make organic compounds
• These factors can be combined to give the four
major modes of nutrition: photoautotrophy,
chemoautotrophy, photoheterotrophy, and
chemoheterotrophy
Table 27-1
The Role of Oxygen in Metabolism
• Prokaryotic metabolism varies with respect to
O2:
– Obligate aerobes require O2 for cellular respiration
– Obligate anaerobes are poisoned by O2 and use
fermentation or anaerobic respiration
– Facultative anaerobes can survive with or without
O2
Nitrogen Metabolism
• Prokaryotes can metabolize nitrogen in a variety
of ways
• In nitrogen fixation, some prokaryotes convert
atmospheric nitrogen (N2) to ammonia (NH3)
Fig. 27-16
Euryarchaeotes
Crenarchaeotes
UNIVERSAL
ANCESTOR
Domain Archaea
Korarcheotes
Domain
Eukarya
Eukaryotes
Nanoarchaeotes
Proteobacteria
Spirochetes
Cyanobacteria
Gram-positive
bacteria
Domain Bacteria
Chlamydias
Table 27-2
• Some archaea live in extreme environments and
are called extremophiles
• Extreme halophiles live in highly saline
environments
• Extreme thermophiles thrive in very hot
environments
Fig. 27-17
• Methanogens live in swamps and marshes and
produce methane as a waste product
• Methanogens are strict anaerobes and are
poisoned by O2
• In recent years, genetic prospecting has
revealed many new groups of archaea
• Some of these may offer clues to the early
evolution of life on Earth
• Example: Rhizobium, which forms root nodules
in legumes and fixes atmospheric N2
• Example: Agrobacterium, which produces
tumors in plants and is used in genetic
engineering
2.5 µm
Fig. 27-18c
Rhizobium (arrows) inside a root
cell of a legume (TEM)
Chlamydias
• These bacteria are parasites that live within
animal cells
• Chlamydia trachomatis causes blindness and
nongonococcal urethritis by sexual transmission
Chemical Cycling
• Prokaryotes play a major role in the recycling of
chemical elements between the living and
nonliving components of ecosystems
• Chemoheterotrophic prokaryotes function as
decomposers, breaking down corpses, dead
vegetation, and waste products
• Nitrogen-fixing prokaryotes add usable nitrogen
to the environment
• In mutualism, both symbiotic organisms benefit
• In commensalism, one organism benefits while
neither harming nor helping the other in any
significant way
• In parasitism, an organism called a parasite
harms but does not kill its host
• Parasites that cause disease are called
pathogens
• Pathogenic prokaryotes typically cause disease
by releasing exotoxins or endotoxins
• Exotoxins cause disease even if the prokaryotes
that produce them are not present
• Endotoxins are released only when bacteria die
and their cell walls break down
• Many pathogenic bacteria are potential
weapons of bioterrorism