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Chapter 13 Viruses, Viroids, and Prions

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Thomas Doetsch

on 17 December 2013

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Transcript of Chapter 13 Viruses, Viroids, and Prions

Chapter 13 Viruses, Viroids, and Prions
What are viruses? Viroids? And Prions?

How do they reproduce?

What are the different kinds of Viruses?
Viruses
Viruses were originally distinguished from other infectious agents because they were especially small and because they are obligatory intracellular parasites.
Obligatory intracellular parasites
- parasites that absolutely require living host cells in order to multiply.
Viruses-
entities that contain
-Either DNA or RNA
-A Protein coat that surrounds the nucleic acid
-Multiply inside living cells
-Are able to transfer viral nucleic acid to other cells
Host Range and Phages
Host Range- the spectrum of host cells the virus can infect.
-Although most host ranges are able to infect specific types of cells of one particular species there are some viruses that can infect invertebrates, vertebrates, plants, portists, fungi, and bacteria.
Bacteriophages- Viruses that can infect bacteria
Viral Size
Viral sizes are determined with the aid of electron microscopy.
This is the same picture that is on page 369 in the textbook
Although most viruses are quite a bit smaller than bacteria some are about the same size.
Viruses range from 20 to 1000 nm in length.
Viral Structure
Virion-
a complete fully developed, infectious viral particle composed of nucleic acid and surrounded by a protein coat that protects it from one host to another
Nucleic Acid
The nucleic acid of a virus can be single or double-stranded. Depending on the virus, the nucleic acid, can be linear or circular, and in some viruses the nucleic acid can be in separated into several segments.
Viral Structure Cont.
The structure of a virus is composed of separate parts: capsid, capsomeres, envelop, and spikes,
Capsid-
The protein coat that protects the nucleic acid
Capsomeres-
Protein subunits that compose a capsid
Envelop-
only exist in some viruses, but it covers the capsid and is composed of lipids, proteins, and carbohydrates.
Spikes-
these cover some envelops, they are composed of carbohydrate-protein complexes that
project from the surface of the envelope.
Some viruses attach to host cells by means of spikes
Here are some nice diagrams
General Morphology
Viruses have 4 main shapes
Helical, Polyhedral, Enveloped, and Complex
Helical
-Resemble long rods that maybe rigid or flexible
-The capsid in this particular type of virus is hollow and cylindrical
-The viruses that cause rabies and Ebola hemorrhagic fever are helical viruses
Polyhedral
-The capsid of most polyhedral viruses is in the shape of an
icosahedron
, aka a regular polyhedron with 20 triangular faces and 12 corners
-Adenovirus and the Poliovirus are examples of an icosahedral viruses
Enveloped
-Roughly spherical
-When helical and polyhedral viruses are enclosed by envelops they are called enveloped helical or enveloped polyhedral viruses
-The influenza virus is an enveloped helical virus
-Herpes simplex virus is an example of a enveloped polyhedral virus
Complex
-Bacteriophages are complex viruses
-Complex viruses most times have the capsid or (head) one shape such as polyhedral and the tail sheath another size such as helical
Taxonomy of Viruses
Symptomatology is the oldest classification of virus, it was nice but not scientifically acceptable because a virus may cause more than one disease depending on the tissue(s) affected and symptomatology didn't account for this.
Viral species are a group of viruses sharing the same genetic information and ecological niche (host range)
Thus the viruses that grow in the same general environment, when classified will be grouped closer together
Isolation, Cultivation, and Identification of Viruses
Much of our understanding of viral multiplication has come from bacteriophages, because those culture are relatively inexpensive and easier to maintain.
Viruses and bacteria are mixed in liquid agar then poured into a petri dish on top of hardened agar where it solidifies and forms a very thin layer layer of new agar.
When a virus infects a bacteria it multiplies and releases several hundred new viruses, which then go out and infect more bacteria and soon enough all the bacteria have been infected and have died
Plaques and PFUs
Plaques- A visible lawn of bacterial growth against the surface of the agar
PFUs- the concentrations of viral suspensions measured by the number of plaques
Cell Cultures
Cell cultures have become more popular amongst viruses as opposed to embryonated eggs because of the homogenous layout of the culture
Primary cell lines- derived from tissue slices, are lines in which viruses grow over a couple generations.
Diploid cell lines- develop from human embryos and can be maintained for about 100 generations and are widely used for culturing viruses that require a human host cell
A major problem with cell culture is that the cell lines must be kept free of microbial contamination
Viral Identification
Identifying viruses is not an easy task, viruses cannot be seen at all without the use of an electron microscope.
Serological methods similar to western blotting are the most commonly used means of identification.
Virologists can identify and characterize viruses by using such modern molecular methods as use of restriction fragment length polymorphisms (RFLPs) and polymerase chain reaction (PCR)
Viral Multiplication
A virus can only multiply when it is inside a host cell.
Once a virus breaks into a cell it takes over the host's metabolic machinery
A single virion can give rise to anywhere from several to thousands of similar viruses in a single host cell.
A
one-step growth

curve
shows the multiplication of viruses within a host over a certain period of time.
Multiplication in Bacteriophages
Bacteriophages multiply in two main ways, the lytic or lysogenic cycle.
The lytic cycle ends with the lysis and death of the host cell
The lysogenic cycle is when the bacteriophages penetrates the cell but the cell may still be alive after having been infected and the bacteriophage multiplies
The Lytic Cycle
This cycle happens in five distinct stages: Attachment, penetration, biosynthesis, maturation, and release
Attachment
is when a virus attaches to complementary receptor site on the bacterial cell
Penetration
occurs when the bacteriophage injects it's nucleic acid into the bacterium. In order to do this the bacteriophages tail releases an enzyme called phage lysozyme, which breaks down a portion of the bacterial cell wall.
Biosynthesis
happens when the nucleic acid of the virus has reached the cytoplasm of the host cell. This this is when the biosynthesis of viral nucleic acid and protein occurs.
The Lytic Cycle Cont.
Biosynthesis Cont.
Initially, the phage uses the host cell's nucleotides and several of it's enzymes to synthesize many copies of it's nucleic acid.
For several minutes following infection, complete phages cannot be found in the host. Only separate components DNA and protein can be detected, this is called the
eclipse period
Maturation
is the net step in the process, and this is when bacteriophages nucleic acid and capsids are assembled into complete virions.
Release
is the final lytic stage and can anyone guess what happens in this stage? Correct! This is when plasma membrane lysis and
releases
all newly formed virions.
The released bacteriophages travel out of the host and go to infect other susceptible cells in the vicinity and the viral multiplication cycle is repeated within the newly infected cells
Next up the Lysogenic Cycle!
The Lysogenic Cycle
Some viruses do not cause lysis and death of the host when they multiply
These lysogenic phages may go through the lytic cycle but they are also capable of incorporating their nucleic acid into the host cell's nucleic acid to begin a lysogenic cycle
Lysogeny is when the phage is inactive so when a host has a lysogenic phage then it is called a lysogenic cell
When the phage implants it's nucleic acid into the host's DNA it's now called a prophage
Although most of the prophage genes are repressed by two repressor protiens that are produced by the phage genes. These repressors stop the transcription of all the other phage genes
The Lysogenic Cycle
Cont.
There are three important parts of lysogeny:
1 The lysogenic cells are immune to reinfection by the same phage
2 Phage Conversion
3 Specialized Transduction
Phage Conversion
is when the host cell exhibits new properties. Such as producing a specific toxin or transforming into a pathogen.
The cell/host can only perform these when it has a lysogenic phage because of the difference in it's bacteria
Specialized transduction
is mediated by a lysogenic phage which packages bacterial DNA along with it's own DNA in the same capsid.
Multiplication of Animal
Viruses
The multiplication of animal viruses follows the basic pattern of bacteriophage multiplication, but has differences in the process of attachment, entry, uncoating, and release. These processes are shared by both DNA and RNA containing animal viruses.
Attachment
Multiplication of Animal Viruses:
Like bacteriophages, animal viruses have attachment sites that attach to complementary receptor sites on the host cell's surface.
Animal viruses do not possess appendages like the fibers of some bacteriophages.
Attachment is completed when many sites are bound.
Understanding the nature of attachment can lead to the development of drugs that prevent viral infections.
Multiplication of Animal Viruses:
Entry
Viruses enter into eukaryotic cells by pinocytosis
Vesicles contain elements that originate outside the cell and are brought into the interior of the cell to be digested.

Enveloped viruses can enter by an alternative method called fusion.
Multiplication of Animal Viruses:
Uncoating
Viruses disappear during the eclipse period of an infection.

Uncoating is the separation of the viral nucleic acid from its protein coat once the virion is enclosed within the vesicle.

Some animal viruses accomplish uncoating by the action of lysosomal enzymes of the host cell.
The Biosynthesis of DNA Viruses
DNA containing viruses replicate their DNA in the nucleus of the host cell by using viral enzymes, and they synthesize their capsid and other proteins in the cytoplasm by using host cell enzymes.
The virions are transported along the endoplasmic reticulum to the host cell's membrane for release.
The Transformation of Normal Cells into Tumor Cells
These cancer causing alterations to celllular DNA affect parts of the genome called oncogenes.

American microbiologists, J. Michael Bishop and Harold E. Varmus received the 1989 Nobel Prize in medicine for proving that the cancer inducing genes carried by viruses are derived from animal cells.

Viruses capable of inducing tumors in animals are called oncogenic viruses, or
oncoviruses.

All oncogenic viruses genetic material integrates into the host cell's DNA and replicates along wit the host cell's chromosome.

Tumor cells undergo transformation.

Many tumor cells contain a virus specific antigen on their cell surface called
tumor-specific transplantation antigen (TSTA)
, or an antigen in the nucleus called the
T antigen
.
DNA Oncogenic
Viruses
:
Adenoviridae

Herpesviridae

Poxviridae

Papovaviridae

Hepadnaviridae
RNA Oncogenic
Viruses
:
Among the RNA viruses, only the oncoviruses in the family Retroviridae cause cancer.

The human T-cell leukemia viruses (HTLV-1 and htlv-2) are retroviruses that cause adult T-cell leukemia and lymphoma in humans.
Latent Viral Infections
Classic example of a latent infection in viruses is the infection of the skin by herpes simplex virus, which produce cold sores.
In some individuals, viruses are produced, but symptoms never appear.

Only 10 to 15% of people carrying the virus exhibit the disease.
ex. Chickenpox virus, the shingles rash, AIDS, and etc
Persistent Viral Infections
A persistent or chronic viral infection occurs gradually over a long period.

Persistent viral infections have been shown to be caused by conventional viruses.

Unlike latent viral infections, most persistent viral infections gradually build up over a long period rather than suddenly appearing.
Prions
In 1982, American neurobiologist Stanley Prusiner proposed that infectious proteins caused a neurological disease in sheep called scrapie.
Prusiner coined the name prion for proteinaceous infectious particle.
All nine are neurological diseases called spongiform encephalopathies.
These diseases are caused by the conversion of a normal host glycoprotein called PrPc into an infectious form called PrPsc.
Plant Viruses and Viroids
Plant viruses resemble animal viruses in many ways.
Viruses can cause color change, deformed growth, wilting, and stunted growth in their plant host.
Plant cells are generally protected from disease by an impermeable cell wall.

Once a plant is infected, it can spread the infection to other plants in its pollen an seeds.
Some plant diseases are caused by viroids.
Tortora, Gerard J., Berdell R. Funke, and Christine L. Case. "Chapter 13/ Viruses, Viroids, and Prions." Microbiology: An Introduction. 10th ed. Redwood City, CA: Benjamin/Cummings Pub., 1995. 367-94. Print.
Bibliography:
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