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porcupineideaBiotechnology

Dec 16, 2012 (4 years and 7 days ago)

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Diversity of Living Things

2.2 Viruses

p. 54
-
59 Nelson Biology 11

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 Viruses


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 2 on p. 335)

http://www.emc.maricopa.edu/faculty/farabee/biobk/T2phage.gif

Importance of viruses


Cause many human diseases


Some have only mild
symptoms such as the
common cold and chicken pox
while others are more serious
such as AIDs, cholera and
rabies


Differ in terms of their 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


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


http://www.topnews.in/trials
-
start
-
potent
-
new
-
hepatitis
-
c
-
drug
-
developed
-
cardiff
-
2261786

Classification


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

Phylogeny


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



Viral Diversity and Specificity


Most viruses are selective and
host
-
specific (
e.g
: only plants,
animals etc.)


Bacteriophages

have a very
restricted
host range
, while
most plant viruses can infect a
wide range of host plants


Host range
: limited range of
host cells that a virus can
infect. (Example: polio virus
only infects human nerve and
intestinal cells)

http://micro.magnet.fsu.edu/cells/viruses/images/bacteriophage.jpg

Viral Diversity and Specificity


Some animal viruses
have a broad range


ex. Swine flu virus (hogs
and humans)


Rabies (many
mammals)


http://www.miamidade.gov/asd/images/rabies.jpg

Viral Diversity and Specificity


Other animal viruses
have a very narrow host
range


Ex. Cold virus only
infects cells in the
human upper
respiratory tract


AIDS affects certain
types of white blood
cells

http://www.teenaids.org/Portals/0/images/whatIsAIDS
-
pic3.gif

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 on bacteria
cell to another. This only happens on rare occasions.



Lytic Cycle

Viral Replication
-

Attachment and
Synthesis

Attachment
:


Virus attaches to a specific
receptor site on the host
cell membrane


Lock and key model


W
hole
virus or just
DNA/RNA enters the host
cell

Synthesis
:


Viral
DNA/RNA directs host
cell in replicating virus parts

http://www.pickens.k12.sc.us/phsteachers/edmunds/lytic%20cycle%201%20animated.gif

Viral Replication
-

Assembly and
Release

Assembly
: Viral parts are
assembled into new virus
particles


Release
:


New virus particles are
released from host cell


Lysis

(destruction/bursting)
of host cell

http://www.microphage.com/images/lysis2.jpg

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
-
Viral Replication


4 basic steps in the “lysogenic cycle”


1. Attachment
-

same as lytic cycle


2. Synthesis


3. Assembly


4. Release


Temperate Phages


Temperate Phages are
bacteriophages

that do not
destroy host cell.


They inject their nucleic acid into host cell and integrate
it into the host DNA


Bacteria cells will replicate with the viral DNA
incorporated into the daughter cell


eventually
lytic

cycle can begin due to some kind of environmental
trigger

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.


Images from:


http://stemcells.nih.gov/info/2006report/2006Chapter4.htm


http://news.haverford.edu/blogs/nicu/2010/06/20/the
-
worth
-
of
-
viral
-
infection/