Chapter 13 Viruses, Viroids, and Prions

Chapter 13
Viruses, Viroids, and Prions

VIRUSES

I General Characteristics
A. Virus definition: viruses are obligatory intracellular parasites that require a living host in order to multiply. Latin for “poison”.
1. host range: the spectrum of host cells the virus can infect
2. bacteriophages (phages): viruses that infect bacteria.
B. A filterable agent: passes through filters that retain bacteria.
1. range in size from 20 to 1000 nanometers in length.
II Viral Structure
A. Viron: a complete, fully developed, infectious viral particle composed of nucleic acid and surrounded by a protein coat that protects it from the environment and is a vehicle of transmission from one host cell to another. A virion is an infectious stage. Protein and nucleic acid make a viron that is infectious.
B. Nucleic Acid: May either be DNA or RNA in a double stranded or single stranded form. Can be linear or several separate segments.
C. Capsid and envelope
1. capsid: protein coat made up of protein subunits (capsomeres)
2. envelope: may cover a capsid
a. made up of some combination of lipids, proteins, and carbohydrates.
b. may be covered with spikes (most envelopes are covered with spikes)
i. spikes: carbohydrate-protein complexes that project from the surface of the envelope
ii. can be used as a means of identification
iii. influenza virus is spiked to attach to red blood cells
i. hemagglutination: when viruses use spikes to adhere to red blood cells. Causes clumping. Useful for lab tests.
c. Nonenveloped virus: not covered by an envelope. (never have spikes.
D. General morphology
1. Helical viruses: resemble long rods. Their capsids a hollow cylinder with a helical structure. (tobacco mosaic virus or bacteriophage M13, ebola)
2. Polyhedral viruses(icosohedral): usually have a capsid in the shape of an icosahedron. (adenovirus and poliovirus)
3. Enveloped viruses: have their capsid covered by an envelope.
a. roughly spherical but pleomorphic
b. enveloped helical (influenza)
c. enveloped polyhedral (herpes simplex) with a capsule.
4. Complex viruses: (poxviruses ex: small pox and cow pox. *chicken pox is a herpes virus not a pox virus)
a. bacteriophage: looks like a spaceship
b. pox viruses: do not contain clearly identifiable capsids, but have several coats around the nucleic acid.

III Taxonomy of Viruses
A. family: -viridae
B. genus: -virus
C. viral species: a group of viruses sharing the same genetic information and ecological niche.
1. Specific epithets are not used
2. Designated by descriptive common names.
a. ex: HIV-1, HIV-2
D. Herpesviridae simplex virus. Human herpes virus 2
IV Isolation, Cultivation, and Identification of viruses (omit except for highlighted, just need to identify these three) Three ways of culturing:
A. Growing bacteriophages in the lab
B. Growing animal viruses in the lab. In live animals.
1. embryonated eggs
C. In cell cultures
1. monolayer: describes growth characteristics of normal cell cultures in glass or plastic containers.
2. cytopathic effects (CPE)
3. primary cell line: derived from tissue slices, tend to die out after only a few generations.
4. diploid cell line: developed from human embryos can be maintained for about 100 generations and are widedly used for culturing viruses that require a human host.
5. continuous cell line: transformed cancerous cells that can be maintained throught an indefinite number of generations. (aka immortal) HeLa.
C. Identification
V Viral Multiplication (omit)
A. Multiplication of bacteriophages
1. lytic cycle of T-even phages
a. phage lysozyme
b. eclipse period
c. maturation period
i. lyses
ii. releases
d. burst time
e. burst size: the number of bacteriophages produced by one bacterial host cell.
f. one-step growth experiment (13.11)
2. Lysogeny
a. prophage: phage DNA inserted into a host’s DNA
b. lysogenic (temperate) phages
c. lysogenic cells
d. phage conversion
e. specialized transduction
B. Multiplication of Animal Viruses
1. Attachment
a. endocytosis
b. receptor site
2. Entry
a. Non-enveloped virions:
i. direct penetration



b. Enveloped virions:
i. membrane fusion: a method of entry for enveloped viruses in which the viral envelope fuses with the plasma membrane and releases the capsid into the cell’s cytoplasm. (HIV)
ii. membrane fusion and endocytosis (together)
iii. pinocytosis: an active cellular process by which nutrients and other molecules are brought into a cell. The plasma membrane folds inward to form vesicles.
3. Uncoating: the separation of the viral nucleic acid from its protein coat once the virion is enclosed within the vesicle.
a. the capsid is digested when the cell attempts to digest the vesicle’s contents, or the nonenveloped capsid may be released into the cytoplasm of the host cell.
4. Biosynthesis of DNA containing viruses:
a. Adenoviridae (common cold)
b. Poxviridae (small pox and cow pox)
c. Herpesviridae
d. Papovaviridae (papillomas)
e. Hepadnaviridae (hepatitis)
5. Biosynthesis of RNA containing viruses
a. Picornaviridae (pico=small RNA) (polio)
b. Togaviridae (toga=covering)
c. Rhabdoviridae (rhabdo=rod) (rabies)
d. Reoviridae (respiratory, enteric, orphan)
e. Retroviridae
i. AIDS
ii. reverse transcriptase
iii. provirus
6. Maturation and release: the time during which the capsids and DNA of a phage, already formed, are now assembled into complete viruses.
a. Enveloped viruses released by
i. budding: the envelope develops around the capsid
b. Non-enveloped viruses released by:
i. lysis: cell bursts, viruses released, cell dies
VI Viruses and Cancer (omit)
A. Cancer
1. tumor
a. malignant
i. leukemias
ii. sarcoma
iii. adenocarcinoma
b. benign
B. Transformation of normal cells into tumor cells
1. oncogenes
2. transformation
3. oncogenic viruses (oncoviruses)
4. TSTA: tumor specific transplantation antigens (T-antigens)
C. DNA oncogenic viruses
D. RNA Oncogenic viruses

VII Latent Viral Infections
A. Latent infection: Virus remains in asymptomatic host cell for long periods. (inactive)
1. shingles, HIV, Herpes
2. often the viral DNA is integrated into the host DNA
3. This is termed a provirus.
4. Can be reactivated to cause disease.
VIII Persistent Viral Infections
A. Persistent (chronic) viral infection: a disease process that occurs gradually over a long period.
1. aka: slow viral infection
2. budding viruses often cause persistent infections

PRIONS (omit)
I Prion
A. proteinaceous infectious particle.
1. scrapie
2. mad cow disease
3. CJD

VIROIDS (omit)
I Viroid
A. Short pieces of naked RNA with no protein coat
B. causes plant disease




NOTES
Rickettsias and Chlamydias are bacteria NOT viruses. But all 3 are intracellular parasites.

Page 401 table 13.3


Biosynthesis of DNA containing viruses:
I. Following attachment, entry and uncoating, the viral DNA is released into the nucleus of the host cell.
II Transcription of a portion of the viral DNA occurs, then translation.
III Remaining late viral genes occur.
IV Synthesis of capsid proteins in the cytoplasm of the host cell.
V After the capsid proteins migrate into the nucleus of the host cell, maturation occurs. The viral DNA and capsid proteins assemble to form complete viruses.
VI The viruses are then released from the host cell.

Biosynthesis of RNA containing viruses:
Essential the same as DNA viruses.
Multiply in the host cell’s cytoplasm.

Wendell Stanley, an American chemist isolated tobacco mosaic virus in 1935.


Viruses are harder to kill because they use the host’s own cells. Killing the virus may kill the host.

*Know the 4 major virus types and an example of each one. See general morphology above.
Helical: Ebola
Polyhedral: Adenovirus, Polio
Enveloped : Influenza, Herpes simplex
Complex: Small pox, bacteriophages


*Know 5 stages of viral replication and what happens at each stage. Focus on animal viruses.
Page 400.
1. attachment
2. penetration
3. uncoating
4. synthesis and maturation
5. release or budding

All of our major viral diseases are enveloped. (HIV)
Capsules mutate, that is another reason that it is hard to treat viruses.

Provirus definition: Viral DNA integrated into a host cell chromosome. It never comes out of the chromosome.

Viruses use the least number of enzymes. Humans use many.

Recognize shape, picture of Ebola. A helical virus in our PowerPoint notes. Pg 390

Poxvirus’s synthesize DNA in the cytoplasm.
Herpes, papo, adeno, hepadna synthesize in the nucleus.

Reverse transcription: RNA to DNA

Transduction: the capacity for lysogenic bacteriophages to transfer a piece of cell DNA to the prophage of an adjacent cell.

See slides on Herpesvirus, Picornavirus, and Retrovirus Families. Pg 383 & 404

Herpesviridae:
Large
Enveloped icosehedral
DNA virus
Often establish latent infections

Picornaviridae:
Small
Non enveloped Icosohedral
RNA virus
Environmentally resistant
Many different types
Common cold, poliomyelitis, Hepatitis A

Retroviridae:
Enveloved icosohedral
RNA virus
Reverse transcriptase
HIV 1 & 2