Transcript Virus and Bacteria Notes

The Genetics of Viruses
and Bacteria
Chapter 18
Microbial Model Systems
•Recall that bacteria are prokaryotes
–With cells much smaller and more simply
organized than those of eukaryotes
•Viruses
Virus
–Are smaller and simpler still
Bacterium
Animal
cell
Animal cell nucleus
0.25 m
Figure 18.2
Viruses
•Viruses called bacteriophages
–Can infect and set in motion a genetic takeover of
bacteria, such as Escherichia coli
Figure 18.1
0.5 m
Obligate Intracellular Parasites
•A virus has a genome but can
reproduce only within a host cell
The Discovery of Viruses:
Scientific Inquiry
•Tobacco mosaic disease
–Stunts the growth of tobacco plants and
gives their leaves a mosaic coloration
Figure 18.3
TMV
•In the late 1800s
–Researchers hypothesized that a particle
smaller than bacteria caused tobacco
mosaic disease
•In 1935, Wendell Stanley
–Confirmed this hypothesis when he
crystallized the infectious particle, now
known as tobacco mosaic virus (TMV)
Viruses
–Are very small infectious particles consisting of
nucleic acid enclosed in a protein coat and, in some
cases, a membranous envelope
•Viral genomes may consist of
–Double- or single-stranded DNA
–Double- or single-stranded RNA
Capsids and Envelopes
•A capsid
–Is the protein shell that encloses the viral genome
RNA
Capsomere
DNA
Glycoprotein
70–90 nm (diameter)
18  250 mm
CAPSID
20 nm
TMV &
50 nm
(b) Adenoviruses
Adenovirus
Capsids and Envelopes
Envelopes
–Membranous coverings derived from the
membrane of the host cell
Viral Envelopes
•Many animal viruses
–Have a membranous envelope
•Viral glycoproteins on the envelope
–Bind to specific receptor molecules on the
surface of a host cell
Bacteriophages
A.K.A. phages
–Have the most complex capsids found
among viruses
General Features of Viral
Reproductive Cycles
•Viruses are obligate intracellular parasites
–They can reproduce only within a host cell
•Each virus has a host range
–A limited number of host cells that it can
infect
General Features of Viral
Reproductive Cycles
•Viruses use enzymes,
ribosomes, and small
molecules of host cells
to synthesize progeny
viruses
DNA
Capsid
VIRUS
HOST CELL
Viral DNA
mRNA
Viral DNA
Capsid
proteins
Viral Reproductive Mechanisms
–Lytic cycle
Is a phage reproductive cycle that
culminates in the death (lysis) of the host
–Lysogenic cycle
Replicates the phage genome without destroying
the host
Lytic Cycle
(Viral Reproduction)
DOCKING with the host receptor protein
PENETRATION of the viral nucleic acid into the host
cytoplasm (Restriction Endonucleases, A.K.A.
restriction enzymes break up host DNA)
BIOSYNTHESIS of the viral components
Assembly (MATURATION) of the viral components
into complete viral units
RELEASE of the completed virus from the host cell
Phage assembly
Head
Tails
Tail fibers
Lysogenic Cycle
Lambda
•Temperate phages
Are capable of using both the lytic & lysogenic
cycles of reproduction
Prophage
When viral DNA is integrated into the bacterial
chromosome (Plasmid)
Capsid
RNA
Envelope (with
glycoproteins)
HOST CELL
Viral genome (RNA)
Template
mRNA
Capsid
proteins
ER
Glycoproteins
Copy of
genome (RNA)
•Retroviruses, such as HIV, use the
enzyme reverse transcriptase
–To copy their RNA genome into DNA, which can
then be integrated into the host genome as a
provirus (Integrates into host DNA)
Glycoprotein
2 of
each
Capsid
Reverse
transcriptase
HIV
Viral envelope
RNA
(two identical
strands)
Evolution of Viruses
•Viruses do not really fit our definition of
living organisms since viruses can reproduce
only within cells
–They probably evolved after the first cells
appeared, perhaps packaged as fragments
of cellular nucleic acid
Viral Diseases in Animals
•Viruses may damage or kill cells
–By causing the release of hydrolytic
enzymes from lysosomes
•Some viruses cause infected cells
–To produce toxins that lead to disease
symptoms
Viral Diseases in Animals
•Viruses may damage or kill cells
(Amount of damage depends on the ability of infected
tissue to regenerate by mitosis)
-Respiratory tract epithelium repairs quickly from
adenovirus infection
- Nerve tracts affected by polio virus is permanent
•Find host cells using “lock & key” fit with
proteins on virus & host cell receptors
Prions
• Protein infectious particles
• Contain no RNA or DNA
• Long incubation period (~10 years)
Prions
–Are slow-acting, virtually indestructible infectious
proteins that cause brain diseases in mammals
–Propagate by converting normal proteins into the
prion version
Prion
Original
prion
Many prions
Normal
protein
Prions
New
prion
Emerging Viruses
Are those that appear suddenly or suddenly
come to the attention of medical
scientists
3 processes contribute to emerging viruses
1.
Mutation of existing viruses as RNA is not corrected by
proofreading e.g. SARS
2. Spread from one host species to another e.g. Hanta Virus
3. Dissemination from a small isolated population e.g. HIV
SARS – Severe Acute Respiratory Syndrome
(b) The SARS-causing agent is a coronavirus
(a) Young ballet students in Hong Kong
like this one (colorized TEM), so named for the
wear face masks to protect themselves
“corona” of glycoprotein spikes protruding from
from the virus causing SARS.
the envelope.
Figure 18.11 A, B
Emerging Viruses are NOT new
They are existing viruses that
•Mutate
•Spread to new host species
•Disseminate more widely in the host species
Small Pox
Polio
Polio
Herpes Simplex
Hepatitis
Varicella Zoster
Mumps
Measles - Rubeola
Other Viruses that affect humans
•Influenza Virus
•Rubella
•Parvo-virus
•Epstein Barr Virus
•Hanta Virus
•(HPV) Human Papilloma Virus
•(RSV) Respiratory Syncytial Virus
•Rabies
•Rhinovirus
•Rotavirus
•West Nile Virus
Viral Diseases in Plants
•More than 2,000 types of viral diseases of
plants are known
•Common symptoms of viral infection include
–Spots on leaves and fruits, stunted
growth, and damaged flowers or roots
Viral Diseases in Plants
•Plant viruses spread disease in two major
modes
–Horizontal transmission, entering through
damaged cell walls
–Vertical transmission, inheriting the virus
from a parent
Viroids -The Simplest Infectious Agent
–Are circular RNA molecules that infect
plants and disrupt their growth
Bacteria
•Rapid reproduction, mutation, and genetic
recombination contribute to the genetic
diversity of bacteria
•Bacteria allow researchers
–To investigate molecular genetics in the
simplest true organisms
The Bacterial Genome and
Its Replication
•The bacterial chromosome
–Is usually a circular DNA molecule with
few associated proteins
•In addition to the chromosome
–Many bacteria have plasmids, smaller
circular DNA molecules that can replicate
independently of the bacterial chromosome
The Bacterial Genome and
Its Replication
•Bacterial cells divide
by binary fission
–Which is preceded
by replication of the
bacterial
chromosome
Replication
fork
Origin of
replication
Termination
of replication
Binary Fission
Mutation and Genetic Recombination as
Sources of Genetic Variation
•Since bacteria can reproduce rapidly
–New mutations can quickly increase a
population’s genetic diversity
•Genetic diversity
–Can also arise by recombination of the
DNA from two different bacterial cells
–Remember that prokaryotes don’t undergo
meiosis or fertilization
Recombination in Bacteria
•Three processes bring bacterial DNA from
different individuals together
–Transformation
–Transduction
–Conjugation
Transformation
Is the alteration of a bacterial cell’s genotype and
phenotype by the uptake of naked, foreign DNA from
the surrounding environment
Transduction
Phages carry
bacterial genes
from one host cell
to another
Conjugation and Plasmids
•Conjugation
–Is the direct transfer of genetic material
between bacterial cells that are temporarily
joined
DNA
transfer is
one way
Figure 18.17
Sex pilus
1 m
The F Plasmid and Conjugation
•Cells containing the F plasmid, designated F+ cells
–Function as DNA donors during conjugation
–Transfer plasmid DNA to an F recipient cell
F Plasmid
Bacterial chromosome
F+ cell
F+ cell
Mating
bridge
F– cell
2
1
A cell carrying an F plasmid
(an F+ cell) can form a
mating bridge with an F– cell
and transfer its F plasmid.
Figure 18.18a
F+ cell
Bacterial
chromosome
3
A single strand of the
F plasmid breaks at a
specific point (tip of blue
arrowhead) and begins to
move into the recipient cell.
As transfer continues, the
donor plasmid rotates
(red arrow).
4
DNA replication occurs in
both donor and recipient
cells, using the single
parental strands of the
F plasmid as templates
to synthesize complementary
strands.
The plasmid in the
recipient cell
circularizes. Transfer
and replication result
in a compete F plasmid
in each cell. Thus, both
cells are now F+.
F Plasmid recombination
• Chromosomal genes can be transferred during
conjugation when the donor cell’s F factor is
integrated into the chromosome
Hfr cell
• A cell with the F factor built into its chromosome
• The F factor of an Hfr cell
• Brings some chromosomal DNA along with it when it is
transferred to an F– cell
R plasmids and Antibiotic Resistance
Confer resistance to various antibiotics
Transposition of Genetic Elements
•Transposable elements
–Can move around within a cell’s genome
–Are often called “jumping genes”
–Contribute to genetic shuffling in bacteria
by folding the DNA
Insertion Sequences
•An insertion sequence contains a single
gene for transposase
–An enzyme that catalyzes movement of
the insertion sequence from one site to
another within the genome
Insertion sequence
3
A T C C G G T…
A C C G G A T…
3
5
TAG G C CA…
TG G C CTA…
5
Transposase gene
Inverted
Inverted
repeat
repeat
(a) Insertion sequences, the simplest transposable elements in bacteria, contain a single gene that
encodes transposase, which catalyzes movement within the genome. The inverted repeats are
backward, upside-down versions of each other; only a portion is shown. The inverted repeat
sequence varies from one type of insertion sequence to another.
Figure 18.19a
Transposons
•Bacterial transposons
–Also move about within the bacterial genome
–Have additional genes, such as those for
antibiotic resistance
Transposon
Insertion
sequence
Antibiotic
resistance gene
Insertion
sequence
5
5
3
3
Inverted repeats
Transposase gene
(b) Transposons contain one or more genes in addition to the transposase gene. In the transposon
shown here, a gene for resistance to an antibiotic is located between twin insertion sequences.
The gene for antibiotic resistance is carried along as part of the transposon when the transposon
is inserted at a new site in the genome.
Figure 18.19b
Prokaryotic Gene Expression
•Individual bacteria respond to
environmental change by regulating their
gene expression
•E. coli, a type of bacteria that lives in the
human colon
–Can tune its metabolism to the changing
environment and food sources
Response to the environment
•This metabolic control occurs on two levels
–Adjusting the activity of metabolic
enzymes already present
–Regulating the genes encoding the
metabolic enzymes
(a) Regulation of enzyme
activity
Precursor
Feedback
Inhibition
Feedback
inhibition
(b) Regulation of enzyme
production
Enzyme 1
Gene 1
Enzyme 2
Gene 2
Enzyme 3
Gene 3
Regulation
of gene
expression
–
Enzyme 4 Gene 4
–
Enzyme 5
Tryptophan
Figure 18.20a, b
Gene 5
Operons: The Basic Concept
•In bacteria, genes are often clustered into operons,
composed of
–An operator, an “on-off” switch
–A promoter
–Genes for metabolic enzymes
•An operon
–Is usually turned “on”
–Can be switched off by a protein called a repressor
Operon Parts
•The regulatory gene codes for the repressor
protein.
•The promoter site is the attachment site for RNA
polymerates.
•The operator site is the attachment site for the
repressor protein.
•The structural genes code for the proteins.
Operon Parts
•The repressor protein is different for each operon
and is custom fit to the regulatory metabolite.
Whether or not the repressor protein can bind to
the operator site is determined by the type of
operon.
•The regulatory metabolite is either the product of
the reaction or the reactant depending on the type
of operon.
The trp operon: regulated
synthesis of repressible enzymes
trp operon
Promoter
DNA
Promoter
Genes of operon
trpD trpC
trpE
trpR
Regulatory
gene
mRNA
5
3
Operator
Start codon
RNA
polymerasemRNA 5
Inactive
repressor
trpA
Stop codon
E
Protein
trpB
D
C
B
A
Polypeptides that make up
enzymes for tryptophan synthesis
(a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the
DNA at the
promoter and transcribes the operon’s genes.
Figure 18.21a
DNA
No RNA made
mRNA
Protein
Active
repressor
Tryptophan
(corepressor)
(b) Tryptophan present, repressor active, operon off. As tryptophan
accumulates, it inhibits its own production by activating the repressor protein.
Figure 18.21b
Trp Operon
Repressible and Inducible Operons:
Two Types of Negative Gene Regulation
•In a repressible operon
–Binding of a specific repressor protein to
the operator shuts off transcription (Found
in anabolic pathways)
•In an inducible operon
–Binding of an inducer to an innately inactive
repressor inactivates the repressor and
turns on transcription (Found in catabolic
pathways)
Lac Operon
The lac operon: regulated synthesis of
inducible enzymes
Promoter
Regulatory
gene
DNA
Operator
lacl
lacZ
3
mRNA
Protein
No
RNA
made
RNA
polymerase
5
Active
repressor
(a) Lactose absent, repressor active, operon off. The lac repressor is innately active, and in
the absence of lactose it switches off the operon by binding to the operator.
Figure 18.22a
Lac Operon “off”
lac operon
DNA
lacz
lacl
3
mRNA
5
lacA
RNA
polymerase
mRNA 5'
5
mRNA
-Galactosidase
Protein
Allolactose
(inducer)
lacY
Permease
Transacetylase
Inactive
repressor
(b) Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses
the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced.
Figure 18.22b
Lac Operon “on”
Types of Operons
• Inducible enzymes
• Usually function in catabolic pathways
• Repressible enzymes
• Usually function in anabolic pathways