Transcript The Genetics of Viruses and Bacteria

The Genetics of Viruses
and Bacteria
Microbial Models
Viruses come in many shapes and
sizes
Compare the size of a Eukaoryotic
cell, Bacterial Cell and a Virus
Discovery of the Virus
 Adolph
Meyer a German Scientist studied
the Tobacco Mosaic Virus.
 Thought
it was caused by a very small
bacteria because it could not be viewed
through the microscope.
Dimitri Ivanosky a Russian
Scientist

Filtered the sap to get rid of the bacteria.
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The plants still received the infection when
sprayed with the filtered sap.
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Still thought the pathogen were very small
bacteria.
Martinus Beijerink a Dutch Botanist
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Discovered that this infectious particle could
reproduce.
Sprayed plants with filtered sap and their sap
infected other plants.
Infection was not diluted on subsequent
infections.
Could not grow outside the host in culture
medium.
Could not be inactivated with alcohol like
bacteria
Wendell Stanley an American
Scientist

Finally crystallized
this infectious particle
and viewed it under
the electron
microscope.
Viral Composition
 Capsid
– protein coat
 Sometimes
an envelope – glycoproteins
acid – DNA or RNA. Never both.
Can be single or double stranded.
 Nucleic
 Some
have tail fibers – Bacteriophage T4
Viruses Are Obligate Intracellular
Parasites
 They
lack their own enzymes to perform
metabolism and reproduction.
 They utilize the host’s enzymatic
machinery to accomplish these tasks.
 Viruses have a host range or are host
specific.


Rabies infects more than one host
Eukaryotic viruses are usually tissue specific.
• Rhinoviruses, Adenoviruses, Herpes, HIV
Bacteriophage (phage virus)
Reproductive Cycles of Virus
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Lytic Cycle – destroys the host cell
The range of organisms that a virus can attack is
called the host range
Viral proteins are translated by host enzymes
and new viral particles are produced.
Viral particles are assembled and the host cell is
lysed. Host cell death occurs.
Bacterial cells possess restriction
endonucleases that destroy foreign DNA.
The bacterial DNA is methylated to protect from
destruction.
Lytic Cycle
Lysogenic Cycle Can Be Used For
Cloning
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Viruses can infect without destroying the host
cell.
They integrate their DNA into the host cell and
turn off their own genes.
These types of viruses are called temperate
viruses.
Bacterial cells that possess these viral genes are
celled prophages.
Viral DNA can be replicated along with the host
cell’s DNA.
Lysogenic Cycle
Lysogenic Viruses can be Triggered to
Become Lytic Viruses

Radiation, chemicals or any host cell stress can
cause the virus to enter the lytic cycle and
destroy the host cell.
 Some prophages (bacterial cells that possess
viral gene) express prophage genes that alter
the phenotype of the host cell.
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Bacteria produce endotoxins that originate from viral
genes.(Diptheria, Scarlet Fever and Botulism)
Genes can be inserted into bacterial cells using
viruses in a process called Transduction.
RNA as Viral Genetic Material

Recall: mRNA serves as the template for new genetic
material.
 Reverse transcriptase produces DNA from mRNA.
 transcribes RNA into DNA, retroactively “backward”,
integrating the viral DNA (provirus) into the hosts genome,
never to leave
 Newly made DNA integrates into the host chromosome.
 Viruses that do this are called retroviruses.
 HIV; human immunodeficiency virus
CAUSES
AIDS; acquired immunodeficiency syndrome
HIV Infection
HIV Infection
 The
host’s RNA polymerase transcribes
viral RNA from the DNA.
RNA serves as both a template and
mRNA.
RNA viruses mutate more rapidly because
replication of RNA does not have to
undergo the same proofreading steps as
replicating DNA.
Transduction
 When
phage viruses acquire bits of
bacterial DNA as they infect cells, resulting
in genetic recombination


Generalized-movement of a random piece of
bacterial DNA as the phage lyses the donor to
the recipient in the lytic cycle
Restricted/temperate/specialized- transfer of a
specific piece of DNA next to the prophage
site (bacterial cells that possess viral gene)
Prophage
When viral DNA is integrated into the bacterial chromosome (Plasmid)
Transduction
Viral Diseases in Animals
1- Viruses may damage or kill cells by causing the
release of hydrolytic enzymes from lysosomes
•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
2- Some viruses cause infected cells that produce toxins
that lead to disease symptoms
Prions and Viroids
The simplest Infectious Agents
Prions
•
Prions are infectious proteins. They contain no RNA or
DNA and have a long incubation period (~10 years)
- cause degenerative brain diseases like scrapes and
Creutzfeldt-Jakob disease.
- abnormal shaped brain proteins induce normal proteins
to assume an abnormal shape propagating itself.
Prions
–slow-acting, virtually indestructible infectious
–cause brain diseases in mammals
–Propagate by converting normal proteins into the prion version
Viroids
 Viroids
are tiny molecules of naked
circular RNA that infect plants.
- only several hundred nucleotides long.
- a molecule can be an infectious agent.
- disrupt metabolism by interfering with the
host genome.
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
Emerging Viruses
•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
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
BACTERIA 18.3
Bacteria
•Rapid reproduction, mutation, and genetic
recombination contribute to the genetic
diversity of bacteria
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 in both
directions from a
single point of origin
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, since meiosis
and fertilization do nor occur in bacteria
–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 (fertility) plasmid, designated F+
cells
–Function as DNA donors during conjugation
–F plasmid contains genes for the production of pili
(cytoplasmic bridges that serve as connection sites)
–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
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Chromosomal genes can be transferred during
conjugation when the donor cell’s F factor is
integrated into the chromosome
Hfr (high frequency of recombination) cell
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A cell with the F factor built into its chromosome
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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
-R plasmids make cells in which it is present
resistant to specific antibiotics
-ampicillin, tetracycline
-R plasmid can be transferred via conjugation
-Increase in resistant bacteria over time,
evolutionary advantage (concerning?!)
Other forms of genetic variability:
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
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
Other forms of genetic variability:
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
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
Enzyme 1
Feedback
Inhibition:
(b) Regulation of enzyme
production
Gene 1
Operons:
Adjusting the activity of
metabolic enzymes
already present
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
Regulating the
genes encoding
the metabolic
enzymes
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.
•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.
Operon Parts
•The regulatory metabolite is either the product of the
reaction or the reactant depending on the type of
operon.
•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