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 cell has a host range


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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



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
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

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Jump to a new type of host

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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