Micro Ch. 5 (Viruses)

Temperate, lysogenic viruses

"The Silent Virus Infection" Temperate phages undergo adsorption and penetration; do not undergo replication or release immediately. Viral DNA enters an inactive prophage state (inserted into bacterial chromosome). Copied during normal bacterial cell division

Release strategies of naked vs. enveloped viruses

*Enveloped virus*: METHOD 1) DIRECT MEMBRANE FUSION Virion attaches to host cell receptors by spikes. Envelope fuses with plasma membrane and capsid is released into cytoplasm. The nucleic acid then separates from the protein coat. METHOD 2) ENDOCYTOSIS enveloped virus adsorbs to the host cell by specific proteins on its surface, and virion is taken in by endocytosis. In the process the host cell plasma membrane surrounds the whole virion and forms a vesicle. The envelope of the virion then fuses with the plasma membrane of the vesicle and the nucelocapsid is released into the host cytoplasm. The capsid protein is then removed releasing the nucleic acid of the virus. *Naked virus*: also enters via endocytosis but cannot fuse with plasma membrane because there is no envelope. After engulfment, the viral nucleic acid is released from the endocytic vesicle. The nucleic acid then separates from the capsid.

Adsorption

A virus can invade its host cell only through making an exact fit with a specific host receptors. Host range: the limited range of cells that a virus can infect (hepatitis B: liver cells of humans; poliovirus: intestinal and nerve cells of primates; rabies: various cells of all mammals). Cells that lack compatible virus receptors are resistant to adsorption and invasion by that virus.

Prions and their class of infections

Agents of spongiform encephalitis infection. Deposited as long protein fibrils in the brain with no nucleic acid: composed entirely of protein. (Creutzfeldt-Jakob disease, Bovine spongiform encephalopathy [mad cow]). Questions still remain as to how the agent is propagated, since all other infectious agents require nucleic acid.

Protein spikes

Can be found on naked or enveloped viruses; project from the nucleocapsid or the envelope; allow viruses to dock with host cells.

Retroviruses

Carry their own enzymes to create DNA out of RNA. Retroviruses turn their RNA genomes into DNA.

Filterable viruses

Early researchers found that when fluids from host organisms passed through porcelain filters designed to trap bacteria, the filtrate remained infectious.

Nucleocapsid

The capsid together with the nucleic acid (naked viruses consist only of this).

Transformation infections

The effect of oncogenic, or cancer-causing viruses. Transformed cell qualities include: 1) Increased rate of growth, 2) Alterations in chromosomes, 3) Changes in cell surface molecules, 4) Capacity to divide indefinitely

Penetration and Uncoating

The flexible cell membrane of the host is penetrated by the whole virus or its nucleic acid. Penetration through endocytosis. Entire virus is engulfed by the cell and enclosed in a vacuole or vesicle. Uncoating: enzymes in the vacuole dissolve the envelope and capsid; virus fuses with the wall of the vesicle; viral nucleic acid is released into the cytoplasm. Direct fusion of the viral envelope with the host cell membrane (occurs with influenza and mumps viruses). Envelope merges directly with the cell membrane. Nucleocapsid is released to the cell's interior.

Assembly

This step actually puts together the new viruses using the "parts" manufactured in the synthesis process: new capsids and new nucleic acids.

Viruses have the capability to infect every cellular organism: true or false?

True: viruses have the ability to infect every cellular organism

Lytic viruses

Viral life cycle that culminates in the lysing of the cell to release genetic material and continue viral reproduction. Ex) T2 bacteriophage

Viral induction

Virus in a lysogenic cell becomes activated and progresses directly into viral replication.

Chronic latent infections

Virus periodically becomes activated under the influence of various stimuli. Herpes simplex and herpes zoster viruses.

Tropisms

specificities of viruses for certain tissues

Various compositions of viral nucleic acid

•DNA viruses -single-stranded (ss) -double-stranded (ds) -linear -Circular •RNA viruses -often single-stranded (sometimes double stranded) -positive-sense RNA: ready for immediate translation -negative-sense RNA: must be converted before translation can occur -segmented

Phases of an animal virus

1) adsorption, 2) penetration & uncoating, 3) synthesis, 4) assembly, 5) release. The length of the replication cycle varies from 8 hours in polioviruses to 36 hours in herpesviruses

Lysogeny (lysogenic viruses)

A condition in which the host chromosome carries viral DNA. Considered the least deadly form of parasitism because it allows the virus to spread without killing the host.

Virion

A fully formed virus capable of causing an infection.

Bacterial plaque assay

Areas where virus-infected cells have been destroyed and show up as a clear, well-defined patches in the cell sheet (microscopic manifestation of cytopathic effects). This technique is used to detect and count bacteriophages. These areas develop when grown in bacterial cell culture. Viruses released by an infected cell radiate out to adjacent host cells. New cells become infected, they die and release more viruses, and the process continues. Manifests as a macroscopic, round, clear space that correspond to areas of dead cells.

Persistent infections

Cell harbors the virus and is not immediately lysed: can remain for a few weeks or the remainder of the host's life, can remain in the cytoplasm. Provirus: Viruses incorporate into the DNA of the host (Measles virus).

Replication strategy of RNA containing viruses vs DNA viruses

DNA viruses enter the host cell's nucleus and are replicated and assembled there. RNA viruses are replicated and assembled in the cytoplasm.

Satellite viruses

Dependent on other viruses for replication. 1) Adeno-associated virus (AAV): originally thought that it could only replicate in cells infected with the adenovirus. Can also infect cells that are infected with other viruses or that have had their DNA disrupte through other means. 2) Delta agent: naked circle of RNA; expressed only in the presence of the hepatitis B virus; worsens the severity of liver damage.

Envelope

External covering of a nucleocapsid, usually a modified piece of the host's cell membrane. Can bud from: cell membrane, nuclear envelope, endoplasmic reticulum. Differs significantly from the host's membranes: some or all of the regular membrane proteins are replaced with viral proteins. Some envelope proteins attach to capsid proteins. Glycoproteins are exposed on the outside of the envelope. Pleomorphic.

T-even Bacteriophage (structure, function and life cycle)

Function: Infects Escherichia coli. Structure: icosahedral capsid head, central tube, collar, base plate, tail pins, fibers. Life cycle: 1) Adsorb to host bacteria using specific receptors on the bacterial surface 2) Nucleic acid penetrates after being injected through a rigid tube inserted through the cell wall and membrane. 3) Nucleic acid entry causes cessation of host cell DNA replication. 4) No uncoating is necessary Host machinery used for viral replication and synthesis of viral proteins 5) Parts spontaneously assemble into bacteriophages. 6) Cell lyses releasing mature virions. Upon release, virulent phages can spread to other susceptible bacterial cells and begin a new cycle of infection. An average-size Escherichia coli cell can contain up to 200 new phage units.

Pleomorphic

Has a variable shape and ranges from spherical to filamentous.

3 different capsid shapes

Helical (naked, enveloped), Icosahedral (naked, enveloped), Complex (never enveloped).

What characteristics are used to name viruses?

Hosts and diseases they cause, structure, chemical composition, similarities in genetic makeup. International Committee on the Taxonomy of Viruses has 3 orders, 73 families, 283 genera.

General treatment strategy for viral infections (and its complications)

Most antiviral drugs block viral replication by targeting the function of host cells, but with severe side effects. DRACO: double-stranded RNA activated capsase oligomizer (Causes virus-infected cells to destroy themselves, no matter what the virus is). Vaccines are valuable, but there are a limited number available.

Capsid

Protein shell that surrounds the nucleic acid. Composed of identical protein subunits called capsomeres. Capsomeres spontaneously self-assemble into the finished capsid. Three different types: helical, Icosahedral, Complex (bacteriophage only).

Synthesis

Replication and Protein Production (DNA viruses enter the host cell's nucleus and are replicated and assembled there. RNA viruses replicated and assembled in the cytoplasm. Retroviruses turn their RNA genomes into DNA).

Size of viruses as compared to other microorganisms

Smallest infectious agents. Smallest viruses: parvoviruses around 20 nm in diameter. Largest viruses: mimiviruses around 450 nm in length. Some cylindrical viruses can be relatively long (800 nm) but are so narrow in diameter (15 nm) that their visibility is limited without an electron microscope.

Oncogenic infections

Some viruses carry genes that directly cause cancer. Other viruses produce proteins that induce a loss of growth regulation, leading to cancer. Oncoviruses are mammalian viruses capable of initiating tumors (Papillomaviruses, Herpes virus, Hepatitis B virus, HTLV I).

Host range

The limited range of cells that a virus can infect (hepatitis B: liver cells of humans; poliovirus: intestinal and nerve cells of primates; rabies: various cells of all mammals). Cells that lack compatible virus receptors are resistant to adsorption and invasion by that virus.

Release (esp. # released)

The number of viruses released by infected cells is variable, controlled by: Size of the virus, Health of the host cell (Poxvirus-infected cell: 3,000 - 4,000 virions; Poliovirus-infected cell: 100,000 virions). Immense potential for rapid viral proliferation.

Cytopathic Effects (CPEs)

Virus-induced damage to the cell that alters its microscopic appearance. Types include: gross changes in shape and size, development of intracellular changes, inclusion bodies: compacted masses of viruses or damaged cell organelles in the nucleus and cytoplasm. Syncytia: fusion of multiple host cells into single large cells containing multiple nuclei. Accumulated damage from a virus infection kills most cells.

Acellular

Viruses bear no resemblance to cells and lack any of the protein-synthesizing machinery found in cells.

Why are viruses obligate intracellular parasites?

Viruses cannot multiply unless they invade a specific host cell and instruct its genetic and metabolic machinery to make and release new viruses.

The role viruses have played in the evolution of all organisms

Viruses infect cells and influence their genetic makeup. They shape the way cells, tissues, bacteria, plants, and animals have evolved. 10% of the human genome consists of sequences that come from viruses. 10 - 20% of bacterial DNA contains viral sequences.

Virioid

Viruslike agents that parasitize plants. About one-tenth the size of an average virus. Composed of naked strands of RNA, lacking a capsid or any other type of coating. Significant pathogens in economically important plants: tomatoes, potatoes, cucumbers, citrus trees, chrysanthemums.

Lysogenic conversion

When a bacterium acquires a new trait from its temperate phage (Corynebacterium diphtheriae - diphtheria toxinl Vibrio cholerae - cholera toxin; Clostridium botulinum - botulinum toxin).

"In vivo" vs. "in vitro" viral culturing techniques

laboratory-bred animals and embryonic bird tissues vs. cell or tissue culture methods