What is a virus Viruses are not alive A virus in an obligate intracellular parasite Requires host cell to reproduce Can be seen at magnifications provided by the electron microscope they are microscopic ID: 275661
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Slide1
VirusesSlide2
What is a virus?
Viruses are not alive
A virus in an obligate intracellular parasite
Requires host cell to reproduce
Can be seen at magnifications provided by the electron microscope (they are microscopic)Slide3
Characteristics of Viruses
1.) Contains
a single type of nucleic acid:
either DNA
or RNA but not both2.) Has a protein coat (capsid) surrounding
the nucleic
acid, some also have a
lipid envelope
around the
capsid
3
.)
multiply inside living cells by using
the synthesizing
machinery of the host
cell
4
.) Cause
the synthesis of specialized
viral structures
that can transfer the viral
nucleic acid
to other
cells
5
.) Have
a specific host rangeSlide4
Size of a Virus
Usually much smaller than bacteria
must
be smaller than the cells they infect:
20-14,000nm in lengthSlide5
Size ContinuedSlide6
Structure of viruses
Virion = infectious viral particle: completely assembled with a protein coat surrounding the nucleic acid
All viruses are made of at least 2 parts
Inner core of
nucleic acid
Enclosed in
protein capsid
* Some
also contain
lipoprotein envelopeSlide7
Structure of viruses
1.)
Nucleic Acids
:
Either DNA or RNA, but not bothSingle or Double Stranded (SS or DS)
if RNA, it can be
plus sense
strand (
has codons
) or minus/antisense (need to
make complement
sense strand for translation
)
If DNA- usually double stranded
Linear or circular
Genome is SMALL
Only a few genes (most have 6-10 genes)Slide8
Structure of viruses continued
2. Capsid – protein coat (protein shell)
Surrounds the nucleic acid
protects
the virion in the external environmentAids
in transfer between host
cells
Composed of subunits called
capsomeres
some
capsids have
protein-carbohydrate pointed
projections called pentons
if
pentons are present they
are used
for attachment to
the host cellSlide9
Structure of viruses
*3
. Envelope
(not all viruses
)
Function is to protect the
virion
some
viruses have an envelope around
the capsid
consisting of lipids, proteins
and carbohydrates
(cell membrane like
)
with
envelope =
enveloped
virus
the
envelope may be coded for by the virus or taken from the host cell plasma membrane
some
envelopes have carbohydrate-protein complexes called spikes which are used for attachment to the host cell
if
a virus does not have an envelope it
is called
a
non-enveloped
virus, “naked”Slide10
Enveloped virusSlide11
Non-enveloped virus
Capsomere
proteinSlide12
Morphology/symmetry
The capsid can be distinct and sometimes identifies a particular virus. It is constructed in a highly symmetrical manner
Helical
Cylindrical capsid, hollow
Can be rigid or flexible
Made up of a helical structure of
capsomeres
with the nucleic acid wound up inside
Examples:
Rabies virus, Ebola virus, tobacco mosaic virus (TMV)
Rabies VirusSlide13
Morphology/Symmetry
Polyhedral
Most are icosahedrons (
icosohedral
)20 equilateral triangle faces and made from capsomeres
12 corners made form
capsomeres
called pentons which contain 5
protomers
each
Appear spherical
Examples:
Adenovirus, Polio virus
Polio virusSlide14
Morphology/Symmetry
Complex
Several types of symmetry in one virus
Unique shape
Examples:
Bacteriophage: capsid and accessory structure
Pox virus: no clear capsid, just several protein layers around the nucleic acid
Glass sculpture of pox virusSlide15
Replication
Replication must occur in a host cell (multiply only when inside a living cell)
The viral genome codes for viral structural components and a few viral enzymes needed for processing the viral enzymes
Everything else is supplied by the host:
Ribosomes,
tRNA
, nucleotides, amino acids, energy etc.
The DNA or RNA of the virus takes control of the host cell' metabolic machinery and new viral particles are produced utilizing the raw materials from the host cell.Slide16
Replication
Replication of viruses is studied in great detail in
bacteriophages
Bacteriophages are viruses that infect a specific bacteria
Two possible types of infection cycles:1.) Lytic cycle (
virulent
)
Ends with the
lysis
and death of the host bacterial wall
2.) Lysogenic cycle
Host cell remains alive, but carries the virus in its genomeSlide17
Lytic Cycle
1.)
Attachment
- phage contacts a bacterium (attachment to host) and uses the tail fibers to attach to proteins on the bacterial cell wall
http://sites.fas.harvard.edu/~
biotext/animations/lyticcycle.htmlSlide18
Lytic Cycle
2.)
Penetration/Entry
- the phage injects its DNA into the bacterium
The phage tail releases lysozyme to break down the bacterial cell wallThe sheath contracts to drive the tail core through the weakened cell wall and plasma membrane
The DNA is injected into the bacterium through the tail core
Uncoating
-
During or before penetration
3.)
Synthesis of new virus particles (Multiplication)
Once inside, host protein synthesis is stopped
Virus has host make proteins and nucleic acid
Virus directs viral nucleic acid replication and transcriptions and translation of viral genes (host’s cell transcription stops)
This results in a pool of viral genomes and capsid partsSlide19
Lytic Cycle
4.)
Assembly
“eclipse period” – the time of viral entry
The bacteriophage DNA and capsid spontaneously assemble into complete virons
5-10
hrs
DNA viruses
2-10
hrs
RNA virusesSlide20
Lytic Cycle
5.)
Lysis
- release of virus and death of host cell
A single virus can give rise to up to 1000 new virus particles from on host cell
Virions
will leave bacteria (host)
Lysozyme encoded by viral genes causes the cell wall to break down
The bacteria lyses releasing the
virions
Cycle will then repeat with new phagesSlide21
The Lysogenic Cycle
The
lysogenic phage
infects the cell, but remains inactive in a stage called
lysogeny1.) the phage attaches to the host cell and injects DNA
2.) the phage genome circularizes
At this point, the phage could begin a normal lytic cycle or it can begin the lysogenic cycle/
lysogeny
http://sites.fas.harvard.edu/~
biotext/animations/lysogeny.htmlSlide22
The Lysogenic Cycle
Latency- “dormant” state- unpredictability
Viral DNA/RNA
integrated into DNA of host = hidden DNA=provirus
Can be reactivated in the future
Factors that influence: stress, other viral infections, UV light
Example: fever blisters, chicken pox, HIV 2+ yearsSlide23
Cultivation of Viruses for Study
Embryonated
eggs
Refer to handout given in classSlide24
Cell Cultures
Refer to handout given in classSlide25
Animal Models
Refer to handout given in class