Classification of Viruses• 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.

Virus Cell

Structural PartsProtein, Nucleic Acid Core

Nucleus, Cytoplasm, organelles, Membranes

Nucleic Acid Either DNA or RNA Both DNA and RNA

Reproduction Requires a Host CellBy Mitosis and Meiosis

Cellular Respiration No Yes

Cystallization Yes No

General Viral Information

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