Lesson 7: Viruses

What are viruses?• Viruses: small, non-living, infectious

particles containing genetic material in the form of DNA or RNA with a protein capsule, called the capsid.

• Non-cellular particle

• Over 4000 viruses species classified but scientists believe that there are millions

• Highly specific about what they invade (ex: plant viruses infect only plant cells); specialize in the cell types they infect.

Features of Viruses:• Viruses have no cytoplasm

• Less than 0.1µm in diameter (100s of thousands of viruses could fit inside a human cell)

• Cannot grow or reproduce on their own

• Do not produce energy; do not create waste

• Take control of the cell that they infect

Viruses – Living or Non-Living?

• Considered non-living because:– Do not metabolize energy– Do not perform cellular respiration– Cannot grow– Cannot reproduce on their own (act

as parasites)

Shapes of Viruses

Viral Diseases

• Mild viral diseases such as the common cold or chicken pox, or more severe viral diseases such as AIDS or cholera, are all infectious. They can spread quickly and cause an epidemic (outbreak confined to a geographical region) or a pandemic (widespread, global epidemic).

Bacteriophages:

• Viruses infect single host species or a few closely related hosts. Ex: human immunodeficiency virus (HIV) infects certain types of immune system cells.

• Viruses that infect bacteria are called bacteriophages.

Vaccination• Vaccinations are such preventions against viruses.

• Vaccines are mixtures that contain weakened forms or parts of dangerous virus.

• When vaccines are injected into an individual’s body, they trigger a response by the immune system without causing an infection.

• This exposure creates a form of chemical ‘memory’ that allows the immune system to reach quickly if the individual ever comes in contact with the real virus.

Infectious Cycles

• Viruses must invade a living host cell to reproduce

• There are two ways to do this– 1. Lytic Cycle– 2. Lysogenic Cycle

Homework:

• Why are viruses considered to be non-living?

• What characteristics do viruses share with all living things?

• Which viral diseases are quite common and associated with the winter season?

• Explain the relationship between a virus’s dormant period in a cell and the appearance of cold sores.

Research:

• The human influence virus, H1N1 (2009- swine flu), was declared a pandemic by the World Health Organization (WHO). What criteria does the WHO use to designate a disease outbreak as ‘pandemic’? How many deaths are thought to have resulted from this pandemic? How many countries have reported cases of H1N1? How did Canada respond to this outbreak?

Research:

• Visit the WHO website and choose any other viral disease and research its cause, symptoms, prevention and treatment (if any).

• WATCH: http://www.khanacademy.org/video/viruses?topic=biology