Viruses

What are Viruses?

• Viruses are particles of nucleic acid, protein, and in some cases lipids.

• All viruses enter living cells• According to science, viruses first developed

from living cells.

Discovery!• A plant disease in the

1800’s caused tobacco leaves to produce pale spots.

• A kind of juice containing particles was the cause of this disease.

• In 1897, Martinus Beijerinck classified the particles as viruses. The Latin word meaning poison.

Structure of a Virus

• A typical virus is made up of a core containing DNA or RNA with a protein coat instead of a cell wall.

• The protein coat or capsid allows the virus to enter a cell.

• The capsid is what tricks the cell to let the virus in.

T4 Bacteriophage Tobacco Mosaic Virus Influenza Virus

Viral Infection

• Viruses are very specific to the cells they infect. e.g. animal virus to animal; plant virus to plant; bacterial virus to bacteria.

• Viruses that infect bacteria are called bacteriophages. e.g. T4 and Lambda

• There are 2 different types of viral infection: Lytic and Lysogenic. They have their own cycles which are interchangeable.

Lytic Cycle Explained

• Virus or bacteriophage attaches to the cell. • The virus then injects its own DNA into the cell

creating RNA which takes over the infected cell.• The virus then causes the infected cell to make

copies of the itself.• When the copies of the virus have been made they

lyse or break out of the cell wall.• The viruses then go to infect other cells.

Lysogenic Cycle Explained

• After the virus’ DNA is injected, it is attached to the infected cells DNA.

• The virus’ DNA can replicate through cell division many times.

• Then the virus can actually separate its DNA from the infected cells DNA.

• From this point, the Lysogenic Cycle can jump to the Lytic Cycle but only in certain situations.

Lysogenic to Lytic

• After the viruses DNA separates from the cells DNA, the lysogenic cycle can jump to the lytic cycle.

• Under certain conditions can the lysogenic cycle transition to the lytic cycle.

Human Viruses

• Examples of human viruses are: Polio, measles, HIV, influenza, yellow fever, and more.

• The virus attacks some areas of the human cell causing the symptoms of the virus.

• Most viruses follow the lytic and lysogenic cycles of bacteriophages

Vaccines

• A vaccine is a preparation of weakened or killed virus or viral protein.

• The vaccine stimulates the immune system and may result in permanent immunity of the disease.

• Vaccines should be given before an infection and not after.

Oncogenic Viruses

• Oncogenic viruses cause a disruption among the control of cell growth and division.

• Oncogenic viruses cause cancer.• These viruses only cause cancer in animals

and humans.

Retroviruses

• Retroviruses are viruses that have RNA as their genetic information.

• Retroviruses have their genetic information copied backward. RNA to DNA instead of DNA to RNA. e.g. HIV

Prions

• In 1972 an American scientist names Stanley Prusiner discovered prions.

• Prions are particles that contain no DNA or RNA only protein particles.

• However, these protein particles are infectious and cause several diseases in humans and animals.

• e.g. Mad cow Disease

Viruses: Are they “Alive?”

• According to science, viruses are not “alive.”• Viruses are not cells and cannot reproduce on

its own.• Viruses, on the other hand, make copies of

themselves, regulate gene expression, and evolve only when infecting a living cell.

• What do you think? Would you consider a virus “alive?” Share your opinion.