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Chapter 20: Viruses SB3d. Compare and contrast viruses with living organisms.

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Presentation on theme: "Chapter 20: Viruses SB3d. Compare and contrast viruses with living organisms."— Presentation transcript:

1 Chapter 20: Viruses SB3d. Compare and contrast viruses with living organisms.

2 Is a virus alive? Viruses are not considered living because they are missing key characteristics of living organisms. –Need a host cell to reproduce –Does not maintain homeostasis –Does not carry out metabolic activities –Does not have organelles or cytoplasm

3 2 Categories of Viruses Virulent - fast acting Ex. Stomach flu -Go through the lytic cycle Temperate - have to incubate Ex. Chickenpox -Go through the lysogenic cycle before entering the lytic cycle

4 Viral Structure Nucleic Acid –Either DNA or RNA (not both) Capsid or Protein coat –DNA or RNA is contained in a capsid which is composed of protein Some viruses have an envelope –a lipid membrane that surrounds the entire virus particle –Many serious viruses have an envelope Some viruses have tail fibers –These fibers act as syringes to inject viral DNA into its bacterial host –These viruses are often called bacteriophages.

5 Viral Structure cont. Examples: –Influenza is an enveloped virus encmed/targets/illus/ilt/T012833A.gif

6 Shapes of Viruses Icosahedral –is a geometric shape with twenty triangular flat sides. –Ex. Polio virus, a type of picornavirus 36/q4323/ch18_0_b.jpg

7 Virus Shapes cont. Helical –Shaped like a cork screw –Ex. Tobacco Mosaic Virus (TMV)

8 Virus classification Based on three categories –Enveloped or not –DNA or RNA –Helical or Icosahedral

9 Viral Reproduction Differs based on genetic material All viral reproduction takes place inside the hosts cell (obligate intracellular parasite) –DNA viruses use their DNA to make RNA which directs the production of new viruses –Some DNA viruses combine their DNA with the hosts DNA to direct new virus synthesis –Example of DNA virus: Herpes, the disease that causes chickenpox

10 Viral Reproduction cont. RNA viruses use their RNA to direct the host cells ribosomes to make proteins that make new viruses EX. Rabies, a type of rhabdovirus Retroviruses are RNA viruses –They replicate by using an enzyme called reverse transcriptase –Reverse transcriptase uses the viral RNA to make a DNA template to make a new virus (mutates rapidly)

11 Retroviruses Type of RNA virus Retroviruses are RNA viruses –They replicate by using an enzyme called reverse transcriptase –Reverse transcriptase uses the viral RNA as a template to make viral DNA –The viral DNA is integrated into the host DNA to make new viruses Ex. HIV

12 Bacteriophages Viruses that infect bacteria They have been a significant tool in studying viral reproduction

13 The Lytic Cycle

14 Lysogenic Cycle

15 Treatment of viral disease Viruses are difficult to treat because they are not living organisms Viruses are unaffected by antibiotics –They lack all of the physical features that antibiotics act on Vaccines and antiretroviral drugs are the only treatments –Antiretroviral drugs do not kill the virus, they interfere with its reproduction Ex. AZT is used for HIV

16 Viroids Not viruses Composed only of one loop of RNA Cause disease in plants –Ex. Orange blight, potato blight, etc.

17 Prions Prions are not viruses Prions are simply malformed proteins that clump on to neurons and disrupt their function Ex. Mad Cow disease (BSE) Ex. Creutzfeld-Jacob disease (CJ disease) –All prion diseases cause neural degeneration


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