Viruses

Virus – Latin for “toxin or poison”

Particles of nucleic acid, protein, and sometimes lipids

Enter living cells and use the cell to produce more viruses

Are viruses living?

To be classified as living one has to:

– Be made of cells– Reproduce independently– Have a genetic code– Obtain and use energy– Respond to environment– Change over time

Do viruses match these criteria:

– No– No– Yes– No– No– Yes

Viruses

Viruses have a core of DNA or RNA surrounded by a protein coat

Protein coat is called a capsid– Enables virus to enter host cell

Can live outside of a host in a crystallized form

Virus Structures: Helical - plant viruses

Tobacco Mosaic Virus – infects tobacco plants

Virus Structures: Icosohedral (spherical) – animal viruses

Influenza virus

Virus Structures: Complex

T4 Bacteriophage - infect bacteria

Virus Structures: Complex

Poxvirus (smallpox)

Viral Infections

Bacteriophages can cause two types of viral infections– Lytic – Virus DNA enters cell, makes new copies of

virus, and causes lysis Lysis = cell death, cell bursts open

– Lysogenic – virus inserts DNA into DNA of host cell so it can replicate with the host DNA

Lytic and Lysogenic Infections

Retroviruses

Virus RNA is translated into DNA The DNA copy is then inserted into the host

cell’s DNA Often remains dormant before directing

production of new viruses Causes cell death retro- means “backward”

Human Immunodeficiency Virus (HIV) - retrovirus

Virus Classification

Viruses are classified according to1. the type of nucleic acid (DNA or RNA)

2. Whether the nucleic acid is single or double stranded

3. Whether the virus has an envelope

4. By the type of infections that occur

There are about 1,550 known virus species and about 30,000 known strains

Modes of transmission

Vertical transmission (mother to child) - the baby is born with the virus. Ex. HIV or Hepatitis B

Horizontal transmission (person to person) - most common– Blood exchange, sexual activity, saliva exchange,

breathing (aerosol affect), insect vectors, or contaminated food or water

Diseases Caused by Viruses

Common Cold Chicken Pox

Diseases Caused by Viruses (cont’d)

Herpes Simplex Hepatitis B (liver)

Diseases Caused by Viruses (cont’d)

Polio (paralysis) Ebola (hemorrhage)

Vaccinations

Help prevent infection Can consist of live or killed viruses Vaccines build up the body’s immune response

for when the actual virus enters the body

Cow pox vaccination 1749

Acquired immunization

Artificial injection of a small amount of virus

Body’s immune response makes antibodies