Transcript Classification of Viruses

Classification of Viruses
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Are Virus's Living Things?
They are not classified as living organisms because they do not have a
cellular structure.
They do not have any of the structures that are found in living cells.
They consist of strands of DNA or RNA surrounded by a protein coat called
a capsid.
What are They?
Virus’s are little more than mobile genes that infect cells and cause them to
manufacture more viruses.
The capsid protects the genetic material and helps attach the virus to the
host cell
Classifying Viruses
First observed in 1935.
More than 160 groups have been identified.
They are classified mainly by the types of diseases they cause.
Different groups have different shapes.
• Viral Shapes - Polyhedral
• The Polio Virus responsible for Polio
• T4 Phage that infects E.coli
• Viral Shapes - Spherical
• An example of the AIDS virus
• Viral Shapes - Cylindrical
• The tobacco mosaic virus
Reproduction
• Viruses reproduce by using other organisms.
• They attach to the host cell and inject their DNA
into the cell.
• Virus DNA causes the cell’s metabolism to
replicate more virus DNA
• New viruses are replicated and eventually burst
out of the cell. When the cell membrane breaks
open it is often referred to as lyses.
• Once it breaks open the host cell dies.
Viral Diseases
• There are a number of ways that viruses can infect cells.
• Retroviruses (RNA viruses) use an enzyme (reverse transcriptase)
to cause the host cell to copy the viral RNA into DNA.
• This new DNA instructs the cell to manufacture more viruses.
• Another method is for the viral DNA to be incorporated into the host
DNA.
• It remains attached as the host cell goes through many cell divisions
until it eventually completed its cycle.
• While attached to the host cell it is called a provirus.
• Many diseases such as AIDS and cold sores can remain inactive as
proviruses until they are triggered to complete their cycle.
• People test positive for aids virus without having the symptoms of
the disease.
• People who are susceptible to cold sores seem to have them come
and go at different times.
Using Viruses
• Useful pieces of genes can be copied by
using viruses as a vector.
• The genes are combined with the virus
DNA and when they infect cells multiple
copies of the gene are made as the
viruses replicate
• The multiplied genes can then be
harvested.
Origin of Viruses
• Viruses are composed of genes so
therefore they developed after cells.
• It is believed that they originated as
fragments of genetic material that broke
off from the parent chromosome.
• They survived as parasitic organisms on
similar types of cells.
General Viral Information
Virus
Cell
Structural Parts
Protein, Nucleic Acid
Core
Nucleus, Cytoplasm,
organelles,
Membranes
Nucleic Acid
Either DNA or RNA
Both DNA and RNA
Reproduction
Requires a Host Cell
By Mitosis and
Meiosis
Cellular Respiration
No
Yes
Cystallization
Yes
No
Viruses: non-living or alive?
• A virus hijacks its host's cell machinery to create more virus
particles completing the life cycle. It is the ultimate parasite!
• Viruses are somewhere between the living and non-living.
They can reproduce and show inheritance, but are
dependent upon their hosts, and in many ways can be treated
like ordinary molecules (they can be crystallized!).
• Whether or not they are "alive", they are obligate parasites,
and have no form which can reproduce independent of their
host.
• Like most parasites they have a specific host range,
sometimes specific to one species (or even limited cell types
of one species) and sometimes more general.
Your Turn
• Read pgs. 122 – 126
• Page 126
Questions 1,2,3,5