Download Chapter 9- Taxonomy and the World of Microorganisms and Viruses

Diversity of Living Things
2.2 Viruses
What is a Virus?
• Microscopic particles
capable of reproducing
only within living cells
• Lifeless outside of living
cells, in living cells (host
cell) they can
reproduce.
• Virus means poison in
Latin
http://www.humanillnesses.com/original/images/hdc_0001_0003_0_img0280.jpg
Viruses
• Classified as non-living matter
• But has many characteristics
of living matter
• 1934: early electron
microscope allowed scientist
to first see viruses
• Less than 0.1 micrometers in
diameter (1 micrometer=
10 -6 m)
• 5000 flu viruses fit on the
head of a pin
Structure of Virus
• Basic structure:
Nucleic acid (DNA or
RNA) in core,
surrounded by capsid
(protein coat)
• Some viruses have
lipid membrane
around capsid (HIV)
http://www.microscopy.fsu.edu/cells/viruses/images/virus.jpg
Structure of Viruses
• Bacteriophages or
phages (category of
viruses that invade and
destroy bacteria cells)
have unique shape and
distinct head and tail
regions
• The capsid can display
various shapes (See
Figure 3 and 4 on p. 55)
http://www.emc.maricopa.edu/faculty/farabee/biobk/T2phage.gif
Viral Diversity and Specificity
• Most viruses are selective and host-specific
- Bacteriophages have a very restricted host range
- Most plant viruses can infect a wide range of host
plants
- Some animal viruses have a broad range while
others have a very narrow host range
swine flu - hogs & humans
rabies - many mammals and birds
common cold - only cells in the upper respiratory tract
of humans
HIV - only certain types of human white blood cells
Classification and Phylogeny
• Classified into orders,
families, genera and
species
• Classified based on size,
shape and type of
genetic material
http://www.goalfinder.com/product.asp?pr
oductid=99
Origin of Virus
• Many theories about origins of viruses
• Could have been parasitic organisms that
depended less and less on their own cell
components
• Could have come from fragments of genetic
material of other organisms
• Another hypothesis is that virus-like particles
existed before the first cells
Importance of viruses
• Cause many human diseases
– Mild: common cold, chicken pox
– Serious: AIDs, cholera and
rabies
• Ability to spread from person to
person. Ex. Influenza can infect
millions of people in a short
amount of time.
– Epidemic: large-scale outbreak
of disease in a particular region
– Pandemic: epidemic occurring
on global scale
http://nursing-resource.com/influenza/
Importance of viruses cont’d
http://www.topnews.in/trials-start-potentnew-hepatitis-c-drug-developed-cardiff2261786
• Small number of viruses have been
linked to cancer. This can happen if
the virus alters the host cell’s DNA
leading to uncontrolled cell
division. Hepatitis C has been
shown to produce this effect in
liver cells.
• Viruses also cause disease in
animals and plants
• They can be useful in ecosystems
by controlling populations of
certain organisms
Viral Replication
• 4 basic steps in the “lytic cycle”
1. Attachment
2. Synthesis
3. Assembly
4. Release
• See video: http://www.youtube.com/watch?v=wLoslN6d3Ec
• Some viruses have a lysogenic cycle, where host cells are not
destroyed
• Cancer-causing viruses can act by adding specific genes to a
host cell’s DNA, causing it to become a cancer cell
• Transduction is when a virus transfers DNA from one bacteria
cell to another. This only happens on rare occasions.
Lytic Cycle
Lytic Cycle
• Lytic cycle can take as
few as 25-45 minutes to
produce as many as 300
new viruses
http://www.oralchelation.com/viewpoint/images/virus1.gif
Lysogenic cycle & Transmission
• Herpes virus remains dormant in body cells
• During stress, virus can be activated and go through lytic
cycle causing cold sores to form
• Virus can then go in dormant stage, therefore person
remains infected with the virus
• Virus can be transmitted in many ways such as air,
physical contact, insect bites. See Table 2 on p. 57
Vaccinations and Human Health
• Viruses do not respond to
treatment by antibiotics or
other drugs, but some can
be prevented by vaccines
• Vaccines are mixtures that
contain weakened or dead
forms of a virus. B-cells
retain memory of the
disease so immune system
can react quickly when
exposed to real virus.
http://www.scienceclarified.com/scitech/Bacteria
-and-Viruses/Fighting-an-Invisible-Enemy.html
More on vaccinations
• Vaccinations have dramatically improved
human health
• Some diseases have been completely
eliminated such as small pox
• It is not possible to create vaccines for all
viruses. For example, there is no vaccine
against HIV due to the virus structure and
characteristics of the infection
Applications of viruses
• Viruses are used in genetic engineering:
– to treat diseases through gene therapy by
inserting gene into individuals suffering from
genetic disorder
– to insert gene from one species to another species
(to create GMOs and for genetic engineering of
plants)
– as capsules to deliver drugs to target cells in the
body such as cancerous tumour cells
Viral Vectors
• Viruses can be used as vectors (carriers) of genes into
cells.
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Images from:
http://stemcells.nih.gov/info/2006report/2006Chapter4.htm
http://news.haverford.edu/blogs/nicu/2010/06/20/the-worth-of-viral-infection/