Viruses

Intro to Viruses Movie

I.General info: A. Viruses are pathogens (cause disease) that affect organisms in all 6 kingdoms B. Don’t belong to any kingdom because they are not considered living organisms C. Acellular (non-living)

II.Why are viruses acellular? A. Don’t grow or carry on metabolism B. Do not come directly from other viruses C. Lack cytoplasm and organelles D. Have only 1 type of nucleic acid (DNA or RNA); cells have both E. Have only a few enzymes F. Can survive crystallization

DRAW Fig , pg. 483

III. Viral Characteristics A. All viruses are small; only visible through electron microscope B. Consists of two parts: 1. Nucleic acid core - can be either DNA or RNA, but never both 2. Capsid - outer protein coat

Nucleic Acid Core and Capsid

IV.Capsid A. Determines the shape of the virus B. Allows the virus to recognize, bind to and enter specific cells C. Highly specific D. 3 Basic Capsid Shapes:

1. Geometric shape (e.g. polyhedral) Ex. Adenovirus (respiratory virus) and HIV Adenovirus

HIV virus Reverse transcriptase Protein coat membrane RNA Receptors

2. Helical or rod-shaped (ex. tobacco mosaic virus)

3. Combined shape (ex. bacteriophage)

T4 Bacteriophage Tobacco Mosaic Virus Influenza Virus Head Tail sheath DNA Tail fiber RNA Capsid Surface proteins Membrane envelope RNA Capsid proteins DRAW FIG 19-9; pg. 479

V.Viral Infection A. A virus can reproduce only by infecting living cells B. Once inside, they use the cell’s machinery to replicate and produce more viruses C. This happens in one of two ways:

1. Lytic Infection – virus enters the cell, makes copies of itself, then kills the cell by causing it to burst a. Attachment - virus attaches itself to the receptor site of host cell b. Injection - virus use enzymes to enter the cell wall of host, then injects its nucleic acid

c. Replication - viral nucleic acid is replicated by the cell’s machinery and produces proteins that form the viral enzymes and capsids 1) Retroviruses have a special enzyme called reverse transcriptase*. *This enzyme converts RNA to DNA before replication begins.

d. d. Assembly - Viral parts are assembled to form new complete viral units e. Lysis - New viruses break out of host cell, killing it and attacking new cells

2. Lysogenic Infection a. a. Attachment and Infection - same as lytic b. Splicing - The viral DNA is spliced into the DNA of the host.

c. c. Cellular duplication - During normal cell division the viral DNA is also duplicated and passed on with the cell’s own DNA d. Activation - An environmental stress stimulates the activation of the viral DNA and stages 3-5 of the lytic cycle begin (ex. cold sore outbreak) DRAW FIGURE on page 481

VI. Harmful Effects A. A. Viral diseases in human Viruses cause disease by disrupting an organism’s homeostasis Examples: Rabies, mumps, measles, AIDS, herpes (chickenpox & shingles), influenza (some), polio, common cold, conjuctivitis (pink eye), warts, smallpox, hepatitis, mononucleosis, HPV, some cancers & many more diseases

Mumps – infect saliva gland cells

Measles – infect nasal and throat cells

Shingles - infects nerve cells

Polio – infects nerves that control muscles

“Tree Man” “Tree Man”HPV

B. Viral diseases in livestock 1. Foot-and-mouth disease, Rous sarcoma, West Nile, avian influenza (bird flu)

C. C. Viroids and Prions (virus-like particles) Viroids are single-stranded RNA molecules w/no capsid that cause disease in plants a. a. Disrupt metabolism and stunt plant growth

2. 2. Prions “protein infectious particles” contain no DNA or RNA; cause disease in animals a. a. Abnormally folded proteins b. b. Cause the formation of protein clumps called amyloids c. c. Ex: BSA (mad cow disease), scrapie (sheep) and Creutzfeldt-Jakob disease (Kuru people)

VII. Beneficial Effects A. A. Can be used to insert desirable genetic information into cells Genetically engineered organisms like tulip coloration Deliver correct genes into human cells (cure for cystic fibrosis?) B. B. Possible antibiotic (kills bacteria)

VIII. Combating Viral Infections A. A. Antiviral Drugs Work by interfering with virus replication stage These are not cures B. B. Vaccines C. C. Organism’s immune system