Viruses Essential Questions: What is the structure of a virus and how do viruses cause infection?

The Sneeze: How Germs are Spread

What is a virus? Tiny, non living particle that can cause disease

What is a virus? Viruses are NOT considered to be living organisms because they do NOT meet ALL the characteristics of life

Naming Viruses Often named for the disease they cause or the tissue they infect (NO LATIN NAMES!!!) Examples: rabies virus and polio virus

Bacteriophage Virus that infects bacteria Bacteriophages can infect E. coli bacteria making it harmful to humans.

Viral Structure Basic structure includes: Nucleic Acid Core Protein Coat (Capsid) Viral Envelope (found in some viruses) which covers the capsid

Viral Structure

Nucleic Acid Core Contains genetic material which is either DNA or RNA The DNA or RNA is only instructions for making more copies of the virus –nothing else!!

Viral Shapes are determined by the capsid. Polyhedral Cylindrical Spherical Polyhedral Head w/Tail Ex) Polio Virus Ex) Tobacco Mosaic Virus Ex) Influenza Virus (Flu) Ex) Bacteriophage

Importance of Shape Allows virus to recognize, attach, and invade a specific host cell. Some are species specific. Ex: Bacteriophage only infects bacteria Some are also cell-type specific. Ex: HIV virus only infects human immune cells

Attachment and Entry

Viral Infection Once inside the host cell, two different processes may occur: Some replicate immediately, killing the host cell (Lytic Cycle) Others replicate in a way that does not kill the host cell immediately (Lysogenic Cycle)

Turn to a Partner Explain the diagram below. Use the words attachment and entry, lytic and lysogenic cycles.

Lytic Cycle In a lytic infection, a virus enters a cell, makes copies of itself, and causes the cell to burst. Individuals get sick relatively quickly after infection EX: Influenza (flu) & common cold

Lytic Cycle Animation

Lytic Cycle Attachment Entry/ Host DNA disintegrates Viral Parts are Replicated Assembly Lysis and Release

Lytic Cycle Attachment: Virus recognizes and attaches to host cell Entry: Viral DNA is injected into cell/ Host DNA is disintegrated Replication: Viral genes and capsids are copied Assembly: viral genes are put together inside protein coats Lysis and Release: cell bursts and releases viral particles

Lytic Cycle Bacteriophage Bacterial DNA Nucleic acid Bacterial host cell Bacteriophage Bacterial DNA B. A. C. D. E. Lytic Cycle

Lytic Cycle Bacteriophage Bacterial DNA Nucleic acid Bacterial host cell Bacteriophage Bacterial DNA B. Entry The bacteriophage injects its nucleic acid into the bacterial cell. A. Attachment C. Replication D. Assembly E. Lysis and Release The host’s metabolic machinery makes viral nucleic acid and proteins. New virus particles are assembled. The host cell breaks open and releases new virus particles. Lytic Cycle

Lysogenic Infection Unlike lytic viruses, lysogenic viruses do not lyse (burst and kill) the host cell right away. Instead a lysogenic virus remains inactive for a period of time.

Lysogenic Cycle Animation

Lysogenic Cycle Attachment & Entry Provirus formation DNA Replication Cell Division Lytic Cycle

Lysogenic Cycle Attachment & Entry Provirus formation: Viral DNA is embedded into host DNA DNA Replication: Viral DNA is copied along with host DNA Cell Division: Cell divides with host DNA Lytic Cycle: At any time, the Viral DNA leaves the host DNA and enters the lytic cycle

Lytic and Lysogenic Cycles

Turn to a Partner

Viral Diseases

HIV/AIDS HIV is a Retrovirus: Virus that contains RNA instead of DNA Does not kill the host immediately. (Lytic or Lysogenic??) HIV turns into AIDS when the virus goes into the Lytic Cycle

Polio virus infects and kills cells of the nervous system, producing paralysis. Human papillomavirus infects cells in the outermost layer of the skin, producing an abnormal growth known as a wart.

Other Viral Diseases Common Colds & Influenza Smallpox  high fever, head and body aches, sometimes vomiting, severe rash Herpes simplex  Cold sores, fever blisters

Treatment/Prevention Unlike bacterial diseases, viral diseases cannot be treated with antibiotics. WHY?? The best way to protect against most viral diseases is to prevent them with vaccines. Once the viral disease has been contracted, only symptoms of the infection can be treated. ANTIBIOTICS DO NOT WORK AGAINST VIRUSES!!!

Viruses http://www.youtube.com/watch?v=Rpj0emEGShQ

Comparison BACTERIA VIRUS Living, Prokaryotic Cells Reproduction: Binary Fission (asexual) Conjugation (sexual) Treatments: Anitbiotics Causes: Strep Throat Tuberculosis Lyme Disease Bacterial meningitis Tetanus Non-living Reproduction: MUST infect Host Cell Lytic Cycle Lysogenic Cycle Treatments: Symptoms ONLY! Vaccines Causes: Common cold Influenza (Flu) AIDS Warts Hepatitis A, B, C Polio Chicken pox