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Viruses, Viroids , and Prions

Viruses, Viroids , and Prions. SBI3U0. Homework. Pg 59 #1-3, 5-11, 15. Viruses. Many pathogens are not actually cells They are viruses Common examples are; Influenza HIV – human immunodeficiency virus Hepatitis Ebola Human papillomavirus (HPV)

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Viruses, Viroids , and Prions

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  1. Viruses, Viroids, and Prions SBI3U0

  2. Homework • Pg 59 • #1-3, 5-11, 15

  3. Viruses • Many pathogens are not actually cells • They are viruses • Common examples are; • Influenza • HIV – human immunodeficiency virus • Hepatitis • Ebola • Human papillomavirus (HPV) • If viruses aren’t cells, what are they? How are they infectious?

  4. Viruses • Viruses are technically not alive • They have no metabolism, do not maintain homeostasis, do not replicate on their own, etc... • They CAN replicate, but only by taking over a host cell • But they are composed of many of the same molecules as living organisms • A DNA or RNA (ribonucleic acid) molecule • A protein casing called a capsid • Viruses are infectious particles containing genetic material inside of a protein coat

  5. Viruses • All viruses are infectious • Most are species specific, they only infect certain types of cells • Viruses are generally very easily transmitted, this leads to epidemics and pandemics • Epidemic • A large-scale outbreak of a disease • Located in a specific geographic region • Pandemic • An epidemic that occurs over a very wide geographic area

  6. Common Viruses

  7. The Origin of Viruses • The origin of viruses is somewhat of a mystery, there are several hypotheses about their origin • Some think they were infectious cells that over time lost their cytoplasm and ability to replicate without a host cell • Some think they are “escaped” DNA/RNA molecules that were once part of living cells • A recent hypothesis suggests that viruses are ancient, and their ancestors existed before the first cells • An ancient virus ancestor may even be the precursor to living cells as we know them

  8. Structure • There are several types of viral structures • All contain DNA/RNA encased in a capsid tobacco mosaic adenovirus HIV • HIV is encased in an envelope • Made of the lipid bilayer of the host cell

  9. Bacteriophage • A bacteriophage is a virus that infects only bacteria • Much of the early research on viruses was concerning bacteriophages • These bacteria have an interesting structure because the capsid does not enter the cell like most other viruses • The bacteriophage injects its DNA into the bacterial cell

  10. Virus Infection Cycles • When a virus infects a cell, the viral DNA/RNA is brought into the cell • Lytic cycle • The viral DNA/RNA uses the cells machinery to produce several copies of the virus • Lysis occurs when the viruses rupture the cell membrane and go off to infect other cells • Lysogenic cycle • The viral DNA can be incorporated into the host cells DNA • As the host cell replicates, the viral DNA is past on to more and more cells • At some point the viral DNA separates and starts the lytic cycle

  11. Vaccines • Several vaccines have been developed over the years to help the immune system combat viral infections • This has led to the reduction of deaths due to measels, mumps, and several other viral diseases to zero (or near zero) • A vaccine usually contains viral components, or altered viral components (so they aren’t infectious) injected into the body • This allows the immune system to recognize the viral components and create antibodies to recognize them • Thus, when the virus enters the body, the immune system can respond quickly and combat it • However, some viruses have structures that do not allow for easy vaccine creation (HIV), and some are constantly changing (influenza)

  12. Antiserum • Another method of protection against viruses (and other infectious and poisonous agents) is antiserums • Antiserum is often created from the blood of an individual (human or animal) that is immune to the disease/poison • Serum can contain antibodies, T-cells, or both • T-cells are often not used in humans due to the risk of graft-versus-host disease • Antiserum confers a temporary immunity to the disease/poison • Can last up to a few months

  13. Gene Therapy • Since viruses are good at incorporating their DNA into a host organism’s DNA, they can be used for gene therapy • The DNA of the virus is altered to contain the desired genes (and so that they aren’t infectious) • The virus is then used to infect the host and add the desired genes into the host cell’s genome • Uses: • Cures for disease • Mass production of chemicals (ie: insulin) • Creating hybrid species (genetically modified crops)

  14. Viroids and Prions • Viriods • Discovered in 1971 • Small sections of RNA (~220 bases long, compared to ~2000 for the smallest viral DNA) • Infect plants only • Do not code for proteins • Thought to interrupt the normal function of RNA (involved in producing protein from the DNA code) • Prions • Misfolded proteins • Cause normal proteins to refold into the infectious conformation • Misfolded proteins form aggregates, especially in nerve cells • Leads to Mad Cow disease, and Creutzfeldt-Jakob disease in humans

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