Virus Scavenger Hunt Solutions The Basics of a Virus – “What is a Virus? What is a Viral Infection?”




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Virus Scavenger Hunt Solutions
1. The Basics of a Virus – “What is a Virus? What is a Viral Infection?”
a) A virus is a microscopic organism that cannot reproduce or carry out any other life processes without a host cell.
b) The term “virus” comes from the Latin word virus meaning toxin or poison.
c)


Glycoproteins

Capsid




Viral Genome (DNA or RNA)

Viral Envelope or Membrane




Glycoproteins – protein that have a carbohydrate attached, bind to specific receptors on the surface of the host cell which helps it to “recognize” its host cell (determines the specificity or host range of the virus)
Capsid – protein coat that covers viral genome, the shape of the capsid determines the overall shape and type of virus
Viral Genome (could be DNA or RNA depending upon the type of virus)
Viral Envelope or Membrane – membrane which surrounds viral genome, made up of phospholipids that are derived from the host cell
d) Viruses can spread from mother to child or person to person. They can also spread from animals to humans. Viruses can spread through simple contact, sexual contact, contaminated food/water or through mosquitoes.

2. Characteristics of a Virus – “What are the Essential Characteristics of Viruses?”
a) They can be crystallized (solidify it to make it like a crystal), they are dormant/inactive outside of a cell, they do not show growth, development, reproduction or nutrition.
b) They multiple within host cells, they possess genetic material (RNA or DNA), they are different strains/races, they exhibit mutations.
c) Even though they reproduce and possess genetic materials, they lack the ability to carry out metabolic processes so they are considered to be non-living entities. They need a host cell in order to reproduce and survive and do not have the means to do so on their own. This is why viruses are considered to be obligate parasites.


3. Virus Morphology – “Four Shapes”


Shape of Virus

Diagram of Virus

Example of Virus

Helical







Tobacco Mosaic Virus



Icosahedral








Coronavirus



Enveloped








HIV


Complex







Bacteriophage (phage)




4. Virus Reproduction
4
host

cell
a. Virus Reproduction: BASIC REPRODUCTION


B


Basic reproduction ocurs in viruses that do not have an envelope.


  1. H
    E

    A
    ost enzymes transcribe the viral genome into viral mRNA, which other hosts enzymes use to make more viral proteins.

  2. V
    C
    irus enters cell and is uncoated, releasing viral DNA and capsid proteins.

  3. Host ribosomes translate viral mRNA into capsid proteins.

  4. Viral genomes and capsid proteins self-assemble into new virus particles, which exit the cell.

  5. H
    D
    ost enzymes replicate the viral genome.


4b. Lytic Cycle
The LYTIC CYCLE is a viral reproductive cycle, during which a virus takes over all metabolic activities of a cell and causes the host cell to die. Bacteriophages that ONLY reproduce using the lytic cycle are called a VIRULENT PHAGES.


Stage of Lytic Cycle

Description of Stage of Lytic Cycle – What is happening?



  1. Attachment




* virus attaches to a host cell

* locks on to a specific receptor on the surface of the host cell (lock and key)




  1. Entry

* virus injects its DNA into the host cell

* the empty coat remains outside the host cell

* inside the cell, the viral DNA breaks down the host cell DNA






  1. Replication

* virus takes over total metabolic activities of the host cell

* using the raw materials in the cell, viral DNA directs production of new virus parts


4. Assembly


* newly produced viral components are assembled into complete new virus particles





  1. Lysis

* the host cell bursts open and releases new virus particles

* these particles begin another cycle by infecting nearby cells



4c. Lysogenic Cycle
The Lysogenic Cycle is another type of viral reproductive cycle in which the genome of the phage is replicated without destroying the host. Phages capable of using both modes of reproduction (lytic and lysogenic) are referred to as TEMPERATE PHAGES.

a) When the viral DNA is incorporated into the host cell’s chromosome, the viral DNA is referred to as a PROPHAGE. One gene on the prophage codes for a protein that prevents transcription of most of the other prophage genes. This explains why the phage genome is mostly silent, but this also explains why the viral DNA does not direct production of more virus, which eventually prevents the cell from lysing or breaking in the lysogenic cycle.


b) The term “lysogenic” implies that prophages are capable of giving rise to active phages that

lyse their host cells. This occurs when the viral genome exits the bacterial chromosome and

initiates the lytic cycle. The triggers for this switch-over are usually environmental factors such

as mutagens (e.g. radiation and presence of certain chemicals)




5. The Immune System of Bacteria – Why haven’t phages exterminated all bacteria?


  1. Restriction Endonucleases are enzymes found within bacteria that cut up foreign DNA (in this case, viral DNA). Why is the bacteria’s own DNA not cut by restriction enzymes? The bacterial cell’s own DNA is chemically modified in such a way that it prevents attacks by restriction enzymes.




  1. (1) The presence of restriction enzymes enable bacteria to “fight back” against viruses

(2) Natural selection favours bacterial mutants with receptor cites that are no longer recognized by a particular type of phage.
However…. Natural selection also favours phage mutants that are resistant to certain bacteria’s restriction enzymes. Therefore, the phage-host relationship is in constant evolutionary flux.
6. Viral Infections – “What is a Viral Infection?”
Viruses are very specific. That is, they only infect a very limited range of species and target certain organs and cells. These specific species/organs/cells are referred to the virus’ HOST RANGE.


Name of Virus

Organism, Tissue, Organ, or System Affected

How it Affects the Organism, Tissue, Organ or System

Measles


* nose/throat of a child or adult



* causes a rash on the skin

* runny nose, swelling of the eyes/eyelids, watery eyes, coughing, fever



Shingles


* nerve cells in a select area of the body


* causes a rash, pain and itching in the infected area








7. Case Study – HIV/AIDS


  1. HIV virus


Capsid

Glycoprotein


Viral Envelope or Membrane


RNA (two identical strands)

Reverse Transcriptase






  1. HIV is a retrovirus because its RNA is transcribed into DNA (RNA  DNA). Normally (in all cells), DNA serves as the template to produce RNA (DNA  RNA). This is why it is called a “retro”virus because transcription is, in effect, going “backwards”. The enzyme responsible for transcribing RNA into DNA is called REVERSE TRANSCRIPTASE.


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