Viruses

Objective

To compare the structures and functions of viruses to cells and describe the role of viruses in causing diseases and conditions, such as AIDS, common colds, smallpox, influenza and warts.

Virus Facts

• The smallest viruses are merely 20 nm in diameter.

• You could place millions of them on the head of a pin.

Structure of Viruses• Viruses are diverse in shape and size. Some

look like lunar landing craft, others like spiked balls and some are polyhedral.

T4 BacteriophageTobacco Mosaic

VirusInfluenza

VirusHead

Tail sheath

DNA

Tail fiber

RNACapsid

Surfaceproteins Membrane

envelope

RNA

Capsidproteins

Structure of Viruses

• Viruses are basically composed of a protein shell called a capsid, with a nucleic acid core inside.– What is a nucleic acid?

•DNA and RNA

Virus Facts

• Many biologists do not consider them to be alive.– Why?

•Viruses are NOT cells!!•Reproduction

Virus Facts

• Viruses cannot live independently.

• They are intracellular parasites.• They have to invade a living

cell and take over in order to reproduce.

Virus Facts

• Viruses recognize their host by a “lock and key” mechanism between the receptor site on the host cell and the proteins on the exterior of the virus.

Virus Facts:Reproduction

• Two stages– Lytic cycle

•Where it destroys the host cell immediately.

•Examples….Cold and Flu

Reproduction Cont.

• Lysogenic cycle– Where virus can remain attached

to the host DNA for a long time.•HIV•Cold sores•Warts•Herpes

Virus Characteristics:Reproduction

• Viral Replication

1.1.

Virus Characteristics:Reproduction

• 1. Attachment: Virus attaches to a host cell.

• 2. Entry: Genetic material is injected into the host cell.

Virus Characteristics:Reproduction

• 3. Replication: Viral parts are produced by the host cell’s organelles.

• 4. Assembly: Viral parts are assembled into viruses.

Virus Characteristics:Reproduction

• 5. Release: Viruses are released from the host cell. These new viruses can infect other cells.

Vaccines

• Vaccines are killed or weakened forms of a virus.

• Vaccines are available for certain viral diseases, such as polio, rubella, measles, rabies, smallpox and mumps.

Vaccines Cont.

• Although vaccines prevent contraction of a viral disease, we have very little defense against a viral infection once it occurs.

• Antibiotics are NOT effective against viruses!

Questions

• Why is a virus not considered a living organism?

• Why could a virus not infect another virus?

• Why can viruses NOT be grown in a laboratory petri dish like we can grow bacteria?

Virus and CellCharacteristics

• See the chart on p. 483

Conclusion Questions1. Do you think viruses should be considered a form of

life? Defend your opinion with evidence.

2. Describe virus replication. What must be involved in order for a virus to reproduce itself?

3. Describe the typical structure of a virus.

4. Discuss ways that viruses disrupt the equilibrium of animal and plant cells. List five diseases that occur in animals, and two diseases that occur in plants that are caused by viruses.

5. You have been diagnosed with the flu. Explain why a doctor will not provide you with a prescription for antibiotics cure your infection.

p 482-483

p 480-481p 478

p 488

p 486, 488

Living Things Share the Following Characteristics

See textbook, p. 16• Living things are made up of units called cells.• Living things reproduce.• Living things are based on a universal genetic code.• Living things grow and develop.• Living things obtain and use materials and energy.• Living things respond to their environment.• Living things maintain a stable internal environment.• Taken as a group, living things change over time.