D. Blicq dblicq@rrc.mb.ca June 2006 (update 01/04/2010) DIRECTORY I BIO I NOTICE BOARD
Viruses are obligate parasites requiring a host cell's metabolic resources for reproduction. Viruses are the causative agents a large number of diseases, ranging from the common cold to the pandemic of HIV / AIDS which has decimated many communities. There is some debate as to whether or not viral particles are technically alive - being comprised of a protein outer coat and an RNA core whose sole function is to infiltrate an appropriate host and assume control of the host's genetic apparatus to create more viral particles. This section will examine and describe a few basic concepts in virology.
As obligate parasites - organisms which require the cellular resources of a host to propagate - viruses occupy a unique niche of the microbial world. One of the perennial philosophical questions is simply this - are viruses technically alive? Does an outer coat and and an injectable RNA molecule meet the requirements for "life"? Alive or not, viruses are certainly implicated in many infectious diseases and have been characterized in a number of ways.
Viral Composition
Size - typically 20-300 nm (small enough to penetrate cellular membranes)
Physical Composition -
Outer protein Coat / Capsid - serves to protect genetic information inside viral particle and permit attachment to appropriate host.
Inner Core - contains genetic information in the form of RNA or DNA
Outer Envelope - some viruses also have a quasi-membrane or envelope (proteins and phospholipids) around the Capsid to enhance adhesion to an appropriate host
Morphology / Shape - viruses have a number of remarkably geometric shapes:
Polyhedral - the most common for human and zoonotic diseases
Helixes / helical
Other
Capsid Construction - made of identical protein subunits termed "capsomeres"
Virus as Parasite
Viral particles require an appropriate host cel
l to propagate. The host cell provides the raw materials to construct new viral particles and is a source of energy in the form of ATP (adenosine triphosphate). The genetic material of the virus quickly assumes control of the host's ribosomes (protein / enzyme constructing systems) and uses them to assemble viral proteins / enzymes, new capsomeres and capsids to create new viral particles.
Viral Replication - Lytic or Lysogenic Cycles
Upon entering and assuming enzymatic control of an appropriate host cell viral particles there are two possible outcomes.
http://mining.ubc.ca/cerm3/lytic%20vs%20lysogenic.gif
Lytic - the virus may be "lytic" (assembling new viral particles from the host's biomolecules) and eventually lysing the host to release the newly-assembled viral population. Lytic stages include:
 |
Adsorption - virus attaches to host's cellular membrane |
|
Penetration - virus injects genetic material through membrane |
|
Uncoating - viral coat digested releasing viral genetic material |
|
Synthesis - virus commences cellular take-over and assembly |
|
Release - newly-made viral particles released, cell lysed. |
Lysogenic / Latent - the virus may be latent / dormant, incorporating it's genetic blueprint into the hosts genome as a "provirus". This type of infection is often invisible to the host cell, which will now be inadvertently copying the latent viral genome / provirus into each and every daughter cell during normal replication. Eventually, in response to some environmental stimulation or chemical opportunity the viral genome becomes activated and commences lytic behaviour in all the host cells. Lysogenic viral behaviour can be a source of latent or recurring infections.
Viral Replication
RNA Viruses - viral RNA stays in the cytoplasm of the host cell. Viral RNA directs synthesis of viral enzymes / proteins by host's ribosomes in cyotplasm where the new viruses are assembled.
DNA Viruses - viral DNA enters the host cell's nucleus and produces viral mRNA which assumes control of the host's ribosomes to synthesize viral proteins / enzymes. Mature viral particles are assembled in the host cell's nucleus.
Retroviruses - employ the enzyme reverse
transcriptase to produce DNA from the viral RNA. The newly produced
RNA enters the nucleus and begins the proce
ss
of viral replication.
Bacteriophage - are viruses specific for bacterial species. Bacteriophage can cause lytic or lysogenic effects similar to those described above. In the bacteriophage lysogenic stage, the dormant genetic material is termed a prophage rather than a provirus.
Mutation
In addition to viral particles causing a number of infections directly, there is also a significant risk of viral mutation. With a short lifespan and continual natural selection and genetic variation occurring, viruses are a significant threat to mutate into new variants. Such mutation can have significant consequences, affecting any number of parameters:
Virulence - changes in infectivity, transmission patterns and persistence
Antigenic Variation - changes in the immunological composition which may significantly change virulence, detection and therapeutic options.
As well, induced mutation techniques may be applied to strains to create harmless variants as a source of vaccines to protect against more virulent populations.
Viral Infections
There are a number of variations which may occur during viral infections depending on a number of factors. Infections may be acute, chronic or latent
Acute Infections (influenza, measles, mumps, etc.) typically short duration (days / weeks) with recovery mitigated through the host's immunological response.
Chronic Infections (hepatitis B, C) infections are prolonged, often over long periods. Symptoms may be mild or severe (organisms dependant) but the viral population is continually present.
Latent Infections (Herpes) are caused by lysogenic (latent) infections which may be asymptomatic until virus is activated. Causes recurrent manifestation of disease and symptoms.
Long-term Infections (HIV, AIDS) are very slow-growing viral infections which may take many years to fully manifest. Viral numbers slowly increase over time although there are often few symptoms early in the earlier stages of the infection. For some viral infections, the condition is often lethal to the host organism.
http://www.merck.com/media/mmhe2/figures/fg199_1.gif
Viroids and Prions
Viroids and Prions are"amyloids" which are remarkably tough, waxy glycoproteins often implicated in serious diseases. These particles have been implicated in a number of diseases including:
Spongiform Encephalitlis (Creutzfeldt-Jacob disease) Kuru Leprosy Alzheimer's? Others? Prions are infectious, physically resilient and contain no nucleic acids, only protein. They are small (~ 30 kD) and some are stunningly heat resistant - able to survi
ve the cooking process for meal preparation. While many debate whether or not these infectious particles are technically alive, it is certain that are highly infectious and represent a significant challenge for medical practitioners. Prion particles have no overt immunological response - they are identical in amino acid sequence to normal proteins found in human cells. They can survive being cooked, can exist in dry animal feed (possibly for years) and often cause slow asymptomatic infections until full onset. Transmission is described as "horizontal" - person-to-person through ingestion of contaminated meat. Other forms of transmission may also be possible but unknown.
Example Viral Infectious Diseases:
AIDS Chickenpox Common Cold Cytomegalovirus Dengue Fever Epidemic parotitis Hand, foot and mouth disease Hepatitis Herpes simplex / zoster Infuenza Lassa Fever Measles Marburg Fever Mononucleosis Mumps Polio Rabies Rubella SARS Smallpox Viral encephalitis Viral meningitis Viral pneumonia West Nile Virus Yellow Fever
