Random fact Certain types of cancers have been linked to cancer viruses. Burkitt's lymphoma, cervical cancer, liver cancer, T-cell leukemia and Kaposi sarcoma are examples of cancers that have been associated with different types of viral infections. The majority of viral infections however, do not cause cancer.

A disease is destroying some of your crops!!! Leaves of the diseased plants are covered with large bleached spots that form a pattern (mosaic-ish) The disease progresses and turns the leaves yellow, they wither and fall off, killing the plants. How can you find out what is killing your plants?

What you would do as a scientist Take some leaves Crush them into a juice Place a few drops on the surface of healthy leaves A few days later the mosaic pattern is now on the healthy leaves The disease must be in the juice! It must be a microorganism but you can’t see it or filter it Even though you can’t see it, you know there is a disease-causing particle in the juice – you give it the Latin name for poison Viruses

Viruses

Virus basics Even smaller than one-celled bacteria Cannot be seen under a light microscope Many identified using electron microscopes (invented in 1930s) Noncellular particle Made up of genetic material and protein Can invade living cells

Structure of a virus Composed of a core of nucleic acid surrounded by a protein coat (capsid) Capsid protects the nucleic acid core (DNA or RNA – never both)

Bacteriophages Viruses that invade bacteria More complex structure than other viruses Head region composed of a capsid, nucleic acid core, and tail Capsid Head DNA/RNA Tail fiber Tail

Types of viruses Range from rod-shaped, to helical, to many sided cube-like The tobacco mosaic virus is about 300mn The virus that causes polio is about 20 nm Vary in size: 10 – 400 nm 1 nm = 1/1 000 000 000m

Specificity of a virus Usually, specific viruses will infect specific organisms Ex. A plant virus cannot infect an animal Some viruses will only infect humans The virus that causes rabies infects all mammals and some birds Viruses can also infect species of animals that are closely related Ex. A virus that infects mice can also infect rats Viruses can infect virtually every kind of organism, including mammals, birds, insects, and plants.

Reproduction of viruses Lytic Virus Must invade, or infect a living host When they invade they cause the cell to lyse, or burst (bacteriophage) With the bacteriophage, its tail fibers attach to the surface of a bacterium and injects its DNA into the cell The host cell cannot recognize the foreign DNA and uses it as a template which causes the production of molecules that destroy the host cell

E. coli When E. coli, bacterium in human intestine, is infected by a bacteriophage, the sequence of infection, growth, and replication can happen in only 25 minutes.

Lysogenic cycle Another way a virus infects a cell It does not reproduce and lyse the host cell (it stays hidden) The DNA of the virus enters the cell and is inserted into the DNA of the host cell Once in the host cell’s DNA, the viral DNA is known as a prophage The prophage can remain part of the DNA of the host cell for many generations The prophage can block other viruses from entering the cell and even add useful DNA to the host cell’s DNA as long as it stays in the prophage state Factors such as sudden changes in temperature and availability of nutrients can return the virus to the lytic stage https://www.youtube.com/watch?v=PHp6iYDi9ko

Lytic & Lysogenic

Viruses are parasites They depend on their hosts for respiration, nutrition and all of the other function that occur in living things Their dependence causes harm to the host

So are viruses living? Living things must be made up of cells and be able to live independently  no they are not But, when they infect a cell, they can grow, reproduce, regulate gene expression , and even evolve  so, yes they are?