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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
4a. Virus Reproduction: BASIC REPRODUCTION
host
cell
B
E
Basic reproduction ocurs in viruses that do not have an
envelope.
A
C
D
A. Host enzymes transcribe the viral genome into viral mRNA,
which other hosts enzymes use to make more viral proteins.
B. Virus enters cell and is uncoated, releasing viral DNA and
capsid proteins.
C. Host ribosomes translate viral mRNA into capsid proteins.
D. Viral genomes and capsid proteins self-assemble into new
virus particles, which exit the cell.
E. Host 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
1. Attachment
2. Entry
3. Replication
4. Assembly
5. Lysis
Description of Stage of Lytic Cycle – What is
happening?
* virus attaches to a host cell
* locks on to a specific receptor on the surface of the
host cell (lock and key)
* 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
* 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
* newly produced viral components are assembled
into complete new virus particles
* 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?
a) 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.
b) (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
Measles
* nose/throat of a child or adult
Shingles
* nerve cells in a select area of the
body
How it Affects the Organism,
Tissue, Organ or System
* causes a rash on the skin
* runny nose, swelling of the
eyes/eyelids, watery eyes, coughing,
fever
* causes a rash, pain and itching in
the infected area
7. Case Study – HIV/AIDS
a) HIV virus
Glycoprotein
Viral Envelope or Membrane
Capsid
RNA (two identical strands)
Reverse Transcriptase
b) 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.