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2.2 Viruses, Viroids and Prions

Viruses

What is a virus?

Viruses Put to Work

  • small, non-living, infectious particles
  • contains genetic material in the form of DNA or RNA, which is sourrounded by a capsid
  • viruses do not create waste, do not use or produce energy, and cannot grow or reproduce on their own

Examples

(Swine Flue Virus Particles)

  • viruses are responsible for many diseases
  • common diseases, such as the common cold and chicken pox, produce mild symptoms
  • severe diseases, such as AIDS and rabies, produce more serious symptoms and even death
  • some viruses even play a role in cancer

Significance of Viruses

(Rabies)

  • Viruses can be beneficial in an ecosystem.
  • genes are introduced to a cell to replace a defective gene.
  • Viruses can be used as tools to insert drugs or DNA into cells.
  • Viruses can enter specific cells to insert DNA into the chromosomes of the cells that they infect, this is done by placing drugs inside virus capsules.
  • Virus Capsule: Where the DNA and RNA makeup is stored
  • Gene Therapy can be used to treat diseases
  • Gene therapy is slowly making successes in fighting parkinsons disease, cancer and hereditary blindness
  • viruses are significant because of their ability to spread
  • epidemic- a large rapidly spreading outbreak of a disease in a certain area
  • pandemic- when an epidemic spreads all over the world
  • viruses control the population of organisms

Infectious Cycles

Infectious cycle- process by which a virus infects, replicates and destroys the host cell

  • viruses do not carry out life functions like living cells
  • become active when their genetic material enters and takes control of living cell

Phage Infection - any of a group of viruses that infect bacteria

Vaccinations and Human Health

  • vaccines-mixtures containing weakened forms or parts of virus
  • when injected, trigger response from immune system without infection
  • exposure creates form of chemical "memory" which allows immune system to react quickly if the person ever comes in contact with real virus

HIV Infected Cells

Viroids and Prions

  • lysis- rupturing of cell; can occur when newly made viruses released from host cell
  • when assembly complete, lysis occurs as host cell ruptures
  • this releases approx. 100-200 new viruses into surroundings
  • host cell is destroyed
  • whole cycle can take less than hour
  • in this state viral DNA stays in dormant state called lysogeny for years
  • bacterium grows and divides, making copies of the virus DNA in chromosome
  • when cell environment changes, viral DNA becomes active, sperates from bacteria, and enters lytic cycle

Transduction

Transmission

  • air
  • direct physical contact
  • biting insects or animals
  • exchange of body fluids
  • enter through body injuries
  • rare occasion when viral DNA separates from bacteria, taking a small piece of bacteria DNA with it, incorporating it into viral DNA
  • the virus then carries this DNA and inserts it into other bacteria when infecting it

Viroids

  • Viroids are plant pathogens
  • They are circular RNA (ribonucleic) particles that are much simpler than a virus
  • Like viruses, they can reproduce only within a host cell
  • Unlike viruses, they do not manufacture any proteins
  • They only make a single RNA molecule
  • Viroid-infected plants are responsible for crop failures that lose millions of dollars in agricultural revenue each year
  • plants they infect include potatoes, cucumbers, tomatoes and avocados
  • There have not been any human diseases known to be caused by viroids

Prions

  • infectious particles smaller than viruses that contain no nucleic acids (no DNA or RNA)
  • Prions: proteinacious infectious particles
  • cause a number or rare diseases in animals
  • Abnormally shaped
  • Can be found in the brain and nervous tissues.
  • Interaction of normal and infectious prions can affect normal ones
  • If an infectious tissue were to be eaten by another mammal the prion would enter the normal mammals blood stream and go to it's brain affecting that mammal

Classifying Viruses

Characteristics

  • Vary in structure, but all consist of an RNA or DNA molecule surrounded by a capsid
  • Some viruses surrounded by an envelope, which is created when a virus leaves the host cell and the host cell membrane wraps around virus
  • Capsid vs. Envelope: capsid is a protein coat of the genetic material (DNA or RNA) of the virus and a envelope is a protective covering of the protein capsid

Origin

  • There have been several hypothesis...
  • 1) Originated as small infectious cells that over time lost their cytoplasm and their ability to reproduce outside a living cell
  • 2) Originated as escaped fragments of DNA(deoxyribonucleic acid) or RNA(ribonucleic acid) that were once a formed part of living cells
  • 3) Virus-like particles existed before the first cell.

Classfication and Phylogeny

  • Viruses are non-living, but reproduce
  • Orders, Families, Genera and Species
  • Classified by: size, shape, and type of genetic material
  • 4000 virus species have been classified
  • Viruses: can infect only single host species or few closely related hosts
  • Species of Virus: infect only one organ system, or a single tissue or cell type in their host (ie. HIV)
  • Bacteriophages: Do not enter a host, but inject their DNA into bacterium, and protein capsule remains outside the cell.
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