Viruses

• Viruses: small, non-living, infectious particles containing genetic material in the form of DNA or RNA with a protein capsule, called the capsid.
• • Non-cellular particle
  • Over 4000 viruses species classified but scientists believe that there are millions
  • Highly specific about what they invade (ex: plant viruses infect only plant cells); specialize in the cell types they infect.
• • Viruses have no cytoplasm
  • Less than 0.1µm in diameter (100s of thousands of viruses could fit inside a human cell)
  • Cannot grow or reproduce on their own
  • Do not produce energy; do not create waste
  • Take control of the cell that they infect 
• • Considered non-living because:
  • Do not metabolize energy
  • Do not perform cellular respiration
  • Cannot grow
  • Cannot reproduce on their own (act as parasites)
• Virus Shapes: Helical, Polyhedral, Spherical, Complex
• Mild viral diseases such as the common cold or chicken pox, or more severe viral diseases such as AIDS or cholera, are all infectious. They can spread quickly and cause an epidemic (outbreak confined to a geographical region) or a pandemic (widespread, global epidemic).
• Bacteriophages: Viruses infect single host species or a few closely related hosts. Ex: human immunodeficiency virus (HIV) infects certain types of immune system cells.
• • Vaccinations are preventions against viruses. 
  • Vaccines are mixtures that contain weakened forms or parts of dangerous virus. 
  • When vaccines are injected into an individual’s body, they trigger a response by the immune system without causing an infection. 
  • This exposure creates a form of chemical ‘memory’ that allows the immune system to reach quickly if the individual ever comes in contact with the real virus. 
• Viruses do not carry out life functions like living cells do. They become active only when their genetic material has entered and taken control of a living cell. The process by which a virus infects a host cell, replicates, and destroys the host cell is called an infectious cycle.
• Lysis: the rupturing of a cell; can occur when newly made viruses are released

  from a host cell
• lysogeny: a state of dormancy in which

  viral DNA may remain within a host cell’s

  chromosome for many cell cycle generations
• Gene Therapy: A method of treating disease in which genes are introduced into cells to replace, supplement, or repair a defective gene.
• Viruses can enter specific cells, and some can insert their own DNA into the chromosomes of the cells they infect.

  Scientists can therefore use viruses to deliver drugs or genes to targeted cells. They place drugs inside virus capsules or replace the viral DNA with DNA they want to insert into a host cell.

  This technology is still relatively new, but it is being used effectively in some applications and holds great promise in others.