IN: ► Discuss the following two questions with your group. What is a virus? (Come up with a definition.) Are viruses alive? (Be prepared to defend your.

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Presentation transcript:

IN: ► Discuss the following two questions with your group. What is a virus? (Come up with a definition.) Are viruses alive? (Be prepared to defend your answer.)

Characteristics of Life ► Are made of one or more cells ► Reproduce ► Grow and develop ► Obtain and use energy ► Respond to their environment

Viruses = “poison” (Latin); infectious particles

Viruses ► Not living (???) organisms; not in 6 kingdoms ► Lack cell structures for growth and reproduction ► Must replicate in a living host ► Smaller than bacteria; need electron microscope to see

Structure ► Nucleic Acid: DNA or RNA, single or double stranded ► Wrapped in a protein coat called a capsid ► Some have an envelope (taken from host cell membrane)

Symmetry ► Icosahedral – 20 + sides ► Helical (Spiral) ► Complex (e.g. head & tail or enveloped) enveloped)

Classification ►B►B►B►Based on… ►1►1►1►1. RNA or DNA ►2►2►2►2. Capsid (size, symmetry, presence of envelope)

DNA Viruses 1. DNA RNA proteins 2. Join to host DNA RNA proteins

RNA Viruses 1.make proteins directly from RNA 2.RNA (reverse transcriptase) DNA RNA proteins - seen in retroviruses

Reproduction ► Obligate intracellular parasites, gain control of host cell’s machinery

1.Lytic Cycle (all viruses) - virus attaches to host cell, injects nucleic acid -DNA/RNA replicates utilizing host cell’s enzymes -host ruptures, releasing 100’s of viruses.

The Lytic cycle

Lysogenic Cycle (some viruses – HIV, herpes)  Virus attaches to host cell, injects nucleic acid  Viral nucleic acid inserts into host’s DNA (provirus)  Provirus replicates whenever the host cell reproduces  No new viruses are produced at this time  The provirus does not harm the host cell  Upon “activation” (e.g. radiation, weakened state, chemicals), provirus takes over the cell and resumes lytic cycle.

The Lysogenic Cycle

Diseases ► Spread by air, human contact, insects, animals, and food ► Common diseases: flu, cold, chickenpox, measles, herpes, hepatitis A ► Serious Diseases: Smallpox, polio, AIDS, hepatitis C ► Some Viruses (e.g. hepatitis B, human papillomavirus)  1. Carry oncogenes (genes that cause cancer) or  2. Stimulate host oncogenes ► Few antiviral drugs ► Immune system provides protection; e.g. white blood cells, lymohocytes, fever, interferons (interfere with synthesis of viral proteins) ► Vaccines are made from weakened form of virus; stimulate immune response

Viral Diseases ► Warts ► Herpes ► Chicken pox ► Smallpox ► The Common Cold ► Influenza (flu) ► Rabies ► AIDS ► Cancer chickenpox shingles herpes smallpox

Viroids ► Recently isolated ► Viroids = naked RNA segments with no protein coat or envelope, smallest known particles to replicate; infect plants

Prions ► Prions = infectious proteins with no nucleic acid, clump together inside of cell and eventually destroy it; ► Cause slow, progressive disease; e.g. kuru, “mad cow” disease, Creutzfelt-Jakob disease

Remember Viruses !!!! ► ► Not considered living things by many scientists……. ► ► 1. No Growth ► ► 2. No Homeostasis ► ► 3. No Metabolism ► ► 4. Reproduce only in a host cell ► ► 5. Only Protein and Nucleic Acid

Studying Viruses 1. Virus: “poison” Latin); infectious particles 2. Wendell Stanley was the first to isolate a virus in 1935 by crystallization