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Chapter 18
Microbial Models:
The Genetics of
Viruses and Bacteria
What is a virus?
An
infectious particle
consisting of nucleic acid
enclosed in a protein coat.
Virus Characteristics
1.
2.
3.
4.
5.
Small size
Infectious
Reproduction
Alcohol Resistance
Crystallization
Size
Large
- barely visible with the
light microscope.
Small - down to 20nm.
Comment - Much smaller
than cells including bacteria.
Infectious
Viruses
cause many
diseases. They can be
spread from one organism to
another.
Reproduction
Can
not reproduce without a
host.
Viruses are obligate
parasites.
Alcohol Resistance
Not
immediately killed by
alcohol like cells are.
Reason - resistant to
dehydration by osmosis.
Crystallization
Can
form crystals.
Cells can't do this.
Viral Structure
1. Genome - the genetic
information.
2. Capsids and Envelopes the outer covering.
Viral Genomes
Are
used to classify virus
types.
I - ds DNA
II - ss DNA
III - ds RNA
IV-VI - ss RNA
Capsid
Protein
shell that encloses
the viral genome.
Made from a large number of
protein subunits.
Number of kinds of subunits
usually small.
Often geometric in shape.
Envelopes
Membranes
cloaking the
capsids of some viruses.
Made from host membrane,
but may have virus originated
glycoproteins added.
General Steps for
Viral Replication
1.
2.
3.
4.
5.
Cell Entry
Genome Replication
Capsid Protein formation
Self-assembly of offspring
Exit from Host
Cell Entry
Locate
host by "Lock-andKey” fit between virus
proteins and cell receptor
molecules.
Infection - Begins when viral
genome enters the cell.
Genome Replication
Host
is reprogrammed to
copy virus genome.
Use host DNA polymerase.
Use virus enzymes to copy
RNA to DNA.
Use host's resources to make
copies.
Capsid Protein
Formation
Host's
protein synthesis
machinery used to make
virus proteins.
Self-Assembly of
Offspring
genome
+ capsid -----> virus
Particles usually assemble
spontaneous.
Exit From Host
Host
cell may burst or lyse,
releasing the virus.
May "bud-off" host
membrane, forming
envelopes around the
capsids.
Bacterial Viruses
Best
understood of all
viruses.
May show two types of virus
life cycles:
Lytic
Cycle
Lysogenic Cycle
Lytic Cycle
Virus
reproductive cycle that
kills the host cell.
Note - the previously
described virus life cycle was
a Lytic Cycle.
Ex: T4 Virus on E. coli
Uses
Lytic Cycle.
Example of an Virulent Virus
(a virus that only uses the
lytic cycle).
Has about 100 genes.
Completes cycle in 20-30
minutes.
Lysogenic Cycle
Virus
reproduction that
doesn't immediately kill the
host cell.
Viral DNA is inserted into the
host DNA, but not expressed.
Movie – Lysogenic Cycle
Temperate Viruses
Viruses
with both lytic and
lysogenic cycles.
l phage - Temperate virus on
E. coli. (l = Lambda )
Prophage
Virus
DNA inserted into the
host's DNA.
Viral DNA is reproduced with
host DNA.
"Timebomb" for lytic cycle.
Lytic Mode Triggers
Switches
the host from
Lysogenic to Lytic Cycles
ex: radiation, chemicals,
stress
Animal Viruses
Belong
to several viral
classes.
Specific Interests:
1. Viruses with envelopes
2. RNA viruses
Envelopes made with
Host Membranes from:
1. Plasma Membrane
2. Nuclear Membrane
Herpes Viruses
Use
nuclear membrane.
Viral DNA integrated into
Host DNA as a provirus.
Shows both lytic and
lysogenic life cycles.
RNA Viruses
Classes
III - VI
Class VI - Retrovirus use Reverse Transcriptase to
make DNA from an RNA
template.
Human
Immunodeficiency
Virus
HIV
- causes AIDS.
Retrovirus from chimps.
Destroys the body’s immune
system, allowing other
diseases to kill.
Movie - HIV
Other Viral Diseases
Measles
Polio
Smallpox
Influenza
Treatments for Viruses
Vaccines
(preventative).
Some Drugs
Ex:
Ara-A
Comment
Acyclovin
- some treatments
are working on the reverse
transcriptase.
Problem
Since
viruses work within a
host cell, they are very
difficult to treat.
Best Treatment - prevention.
Viruses
Can
also cause cancer
Ex: HTLV-1
Papilloma Viruses
Plant Viruses
Can
be an important
agricultural problem.
Ex: Tobacco Mosaic Virus
Tulip Flower Breakage
Viroids
Infectious
particles of naked
RNA.
Similar to viruses, but lack a
capsid.
Ex:
Coconut
Blight
Chrysanthemum Wilt
Viroids
Sequences
are similar to
Introns.
Connection?
Prions
Infectious
protein particles.
Cause “mad-cow” and
Creutzfeldt-Jakob disease.
Protein folding problem?
Prion Action
Bacteria Genomes
DNA
circle (Chromosome).
Plasmids. - Small circle of
DNA that is independent of
the chromosome.
Carries a small number of
traits.
E. coli Mutations
Binary
Fission is not always
perfect. Mistakes or
mutations are made.
Rate: 1x10-7 cell divisions.
Rare, but impact is high with
the short generation time.
Importance
Mutations
are the prime
source of new alleles in
bacteria.
Bacterial Genetic
Recombinations
1. Transformation
2. Transduction
3. Conjugation
4. Plasmids
5. Transposons
Be able to discuss a few of
these methods.
Genetic Recombination
Mixes
the genetic material
into new combinations.
Will move new mutations
through a population.
Transformation
Alteration
of a cell's DNA by
the uptake of foreign DNA.
Ex: Griffith's experiment and
our lab last semester.
Transduction
Transfer
of genes through
phage infections.
Conjugation
Direct
transfer of genetic
material between two
bacterial cells.
Bacterial "sex”.
Used to map genetic
sequences in bacteria.
Plasmids
Small
circular piece of DNA.
Carry many important traits.
Ex: Fertility Factor
Antibiotic Resistance
(R Plasmids)
Episome
Plasmid
that has been
incorporated into the
bacterial chromosome.
Transponsons
Transposable
genetic
elements.
Also called "Jumping Genes”.
Transposons
May
change locations within
the chromosome, or they may
copy into a new location.
Don't usually attach to DNA
at specific locations, allowing
them to scatter genes
throughout the genome.
Gene Expression in
Eukaryotes
Important
for cellular control
and differentiation.
Understanding “expression”
is a key area in Biology.
More details in a future
chapter.
General Mechanisms
1. Regulate Gene Expression
2. Regulate Enzyme Activity
Operon Model
Jacob
and Monod (1961) Prokaryotic model of gene
control.
Always on the national AP
Biology exam !
Operon Structure
1. Regulatory Gene
2. Operon Area
a. Promoter
b. Operator
c. Structural Genes
If Lactose is absent
Regulatory Gene
Makes
Repressor Protein
which may bind to the
operator.
Promoter
Attachment
sequence on the
DNA for RNA polymerase.
Operator
The
"Switch”, binding site for
Repressor Protein.
If blocked, will not permit
RNA polymerase to pass,
preventing transcription.
Structural Genes
Make
the enzymes for the
metabolic pathway.
Lac Operon
For
digesting Lactose.
Inducible Operon - only
works (on) when the
substrate (lactose) is present.
If no Lactose
Repressor
binds to operator.
Operon is "off”,
no transcription,
no enzymes made
If Lactose is absent
If Lactose is present
Repressor
binds to Lactose
instead of operator.
Operon is "on”,
transcription occurs,
enzymes are made.
If Lactose is present
Enzymes
Digest
Lactose.
When enough Lactose is
digested, the Repressor can
bind to the operator and
switch the Operon "off”.
Net Result
The
cell only makes the
Lactose digestive enzymes
when the substrate is
present, saving time and
energy.
trp Operon
Makes
Tryptophan.
Repressible Operon.
If no Tryptophan
Repressor
protein is inactive,
Operon "on”
Tryptophan made.
“Normal” state for the cell.
Tryptophan absent
If Tryptophan present
Repressor
protein is active,
Operon "off”,
no transcription,
no enzymes
Result - no Tryptophan made
If Tryptophan present
Repressible Operons
Are
examples of Feedback
Inhibition.
Result - keeps the substrate
at a constant level.
CAP - positive gene
regulation
Catabolite
Activator Protein
Accelerates the level of
transcription by working with
the RNA polymerase.
Uses cAMP as a secondary
cell signal.
CAP - Mechanism
Binds
to cAMP.
Complex binds to the
Promoter, helping RNA
polymerase with transcription.
Result
If
the amount of glucose is
low (as shown by cAMP) and
lactose is present, the lac
operon can kick into high
gear.
Summary
Know
the general structure of
a virus and how a virus
reproduces.
Know the components of
bacteria genomes and some
methods for genetic
recombination.
Summary
Know
Operon theory !
Know the difference between
inducible and repressible
enzymes.