Download Viruses and Prions and Bacteria, OH MY!

Viruses and Prions and
Bacteria, OH MY!
Genetics of Viruses and Bacteria
Introduction to the Genetics of
Viruses

Work on the tobacco mosaic virus (TMV)
provided the first evidence for the
presence of viruses, a pathogen smaller
than bacteria.
Structure of viruses



Smallest viruses are 20 nm in diameter
Largest can barely be seen by a light
microscope
Not a cell!


Is it living? We can’t decide!
Infectious particles consisting of at least a
nucleic acid (RNA or DNA) enclosed in a
protein coat
Structure of Viruses

Viral Genomes

Can be :
Double
Stranded
Single Stranded
DNA
X
X
RNA
X
X
Structure of Viruses

Capsids and Envelopes


Capsid= protein shell enclosing the viral genome
 Rod-shaped, polyhedral, helical, or more complex
 Large number of protein subunits (but generally similar)
 The most complex are found in phages (bacteriophages)
Viral envelopes
 Derived from host cell
 Help virus infect host
Structure of Viruses
Viral envelopes
General Viral Reproductive Cycles

Obligate intracellular parasites:


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Viruses lack metabolic enzymes, ribosomes, and other
cellular machinery
Each virus has a host range



Can only reproduce in a host
Limitations of what the virus can infect
Due to the lock-and-key specificity of the virus and the cell’s
receptor molecules
Genome replication



DNADNA
RNA RNA
RNADNARNA
General Viral Reproductive Cycles

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Virus injects its genome
into the host cell
Viral genome reprograms
the cell to perform
reproductive work for the
virus using the cell’s
resources
Assembly of viral proteins
and genomes is often
spontaneous after
synthesis
Viral progeny exit the cell
Reproductive Cycles of Phages
Lytic Cycle
vs.
OR
Lysogenic Cycle
Reproductive Cycles of Phages

Lytic Cycle





Phage infects a healthy cell
Bacteria copies virus
Progeny exit bacteria by lysing it
Viruses in this cycle are considered
virulent
Bacterial defenses:

Natural selection favors bacterial
mutants with unrecognizable receptor
sites


Although, natural selection also favors
phage mutants
Phage DNA often recognized and
dismantled by the cell’s restriction
enzymes
Reproductive Cycles of Phages

Lysogenic Cycle



Viral genome replicated without destroying host
Phages that use both types of cycles are called
temperate phages
Viral DNA is incorporated into bacterial genome


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Bacteria reproduces normally, but also copies viral genome
Viral DNA called a prophage at this point
Prophages genes can regulate host cell
Environmental signal triggers the switch from
lysogenic cycle to lytic cycle
Lytic and Lysogenic cycles
Reproductive Cycles of Animal
Viruses

Many animal viruses with
RNA genomes have a
viral envelopes derived
from the plasma
membranes


Enveloped viruses can exit
the cell without destroying
the cell: contrasts with lytic
cycle of phages
Some animal viruses with
DNA genomes have viral
envelopes derived from
nuclear membrane
Reproductive Cycles of Animal
Viruses

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Attachment
Entry
Uncoating
RNA and protein
synthesis
Assembly and exit
Reproductive Cycles of Animal
Viruses

RNA Viruses


Contains a viral enzyme that can be used for
RNARNA synthesis
In different viruses, original RNA genome can:
Be used as mRNA
 Be used as a template for mRNA

Reproductive Cycles of Animal
Viruses

Most complicated RNA
animal viruses:
Retroviruses (An example
is HIV)



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Contain reverse
transcriptase
RNADNA
New viral DNA integrates
into cell DNA and stays in
the cell’s DNA as a provirus
(never leaves)
Cell produces viral RNA to
function both as mRNA and
as genomic RNA
Evolution of Viruses

Not likely to have originated prior to the
first cells

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Why? Because they are obligate intracellular
parasites
Believed to have originated from
fragments of cellular nucleic acids
High mutation rate in many viruses
Vaccines


Harmless variations of viruses used to
elicit an initial immune response by the
body
The main way to fight viruses

Why can’t we treat viral diseases?

Drugs that would attack the virus would have to
also attack the host cell
Emerging Viruses


Viruses that seem to appear suddenly
Mutations of pre-existing viruses


Jump to a new type of host


RNA viruses particularly susceptible because
they lack proofreading in replication
e.g. Hantavirus
Ebola, HIV, SARS, influenza
Viruses and Cancer

Some viruses have been shown to cause cancer


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e.g. Hepatitis B and liver cancer
Viruses can trigger cancerous genetic conditions
Oncogenes- a gene found in viruses or as a part
of the normal genome that is involved in
triggering cancerous conditions
Proto-oncogenes- a gene that has be potential
to cause cancer pending alteration into an
oncogene
Viroids and Prions

Viroids


Naked circular RNA molecules that infect plants
Prions

Infectious proteins

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e.g. scrapie in sheep, mad cow, Creutzfeldt-Jakob disease
Slow acting
No cures
Cannot replicate itself, but can convert normal proteins
Bacterial Genetics
Bacterial Genome

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One, double stranded, circular DNA
molecule.
Referred to as a bacterial chromosome
Supercoiled into a dense region called the
nucleoid
Some bacteria also have plasmids, smaller
circles of DNA
Bacterial Genome

Bacterial cells divide by binary fission (asexual)
Bacterial Genome Variation


Due to asexual reproduction, much
variation is due to spontaneous mutation
Other variation can result from


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Transformation
Transduction
Conjugation
Mechanisms for Gene
Transfer/Genetic Recombination

Transformation: the uptake of naked, foreign DNA from
the surrounding environment which results in a change
in the bacteria’s genotype and phenotype.
 e.g.
Mechanisms for Gene
Transfer/Genetic Recombination

Transduction: the random transfer of DNA from one
bacterial cell to another via virus (phage) transport
Mechanisms for Gene
Transfer/Genetic Recombination

Conjugation: the direct, one way transfer of genetic
material between two bacterial cells that are temporarily
joined.

Mechanisms for Gene Transfer/Genetic
Recombination
Conjugation continued:
 Donor (F+) cell extends a sex pilus to the recipient (F-) cell
 Ability to make sex pili due to the presence of an F factor
 F factor can be a plasmid, or part of the bacterial
chromosome
 One strand of the F factor is donated to the F- cell then both
cells synthesize (make) a second strand using the first as a
template
 F Factor received by recipient cell may confer (give) advantages
to the cell (because it increases genetic variation)
Figure 18.18 page 350
Genetic Recombination


Can also occur within a single cell
Transferable elements

Insertion sequences

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Contains a single gene that codes for enzyme
called transposase (catalyzes movement within the
genome)
Transposons

Contains insertion sequences and an additional
gene
Figure 18.19 page 352
Antibiotic Resistance

Some bacteria are resistant to antibiotics


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The is the result of the presence of an R plasmid (R for
Resistance)
Contains genes that counteract the antibiotic
When these and non-resistant cells are exposed to the
antibiotic, non-resistant cells die, but resistant cells can
still proliferate (grow/reproduce)
 Leads to a population of bacteria that is resistant to
antibiotics
 A constant problem in the medical field
Regulation of Gene Expression

Operons!