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The Immune System • Objectives: • 1) Know the difference between viruses and bacteria. • 2) Know the different ways that bacteria and viruses attack the body. Viruses The Structure of Viruses How Viruses cause Disease Virus Size Viral Structure • Viruses differ widely in structure and size. • General structure of a virus is genetic material, DNA or RNA surrounded by a protein coat called a capsid. How viruses enter cells • Viruses have to bind to receptors on the outside of cells. • Cells won’t knowingly let viruses in, so the virus tricks the cell into letting it in by using receptors that are meant to let in something else. (Nutrients, for example) Viruses also inject their genetic material into the cell. • Here a bacteriophage (a virus that infect bacteria) injects its genetic material into a bacteria. If viruses infect cells then how is it that all cells are not infected? • Viruses only infect certain types of cells. This is known as their host range. • Viruses can only get into cells that have the proper receptors on the outside. Viral Infections Viruses need to infect cells to reproduce. • Lytic Infection: A virus enters a cell, uses the cell’s “machinery” to make copies of itself and then causes the cell to burst (Lyse) • Lysogenic Infection: The virus’s genetic material (DNA) bonds with the cell’s DNA and replicates along with the cell until it enters the lytic cycle. The viral DNA bonded in the cell’s DNA is a prophage Which infection do you think is worse? Are viruses alive? • What are the traits of living things? • How many of these traits do viruses have? • Do you think viruses are alive? Why or Why not? Characteristics of Living Things • • • • • • • • • Pg 18 in your text, Living Things: 1) Are based on Universal Genetic Code 2) Grow and develop 3) Respond to their environment 4) Reproduce (sexual/asexual) 5) Maintain homeostasis 6) Have a metabolism 7) Are made up of cells 8) As a group, evolve. Are viruses alive? • Think about what traits viruses have, do you think viruses have enough of the traits of living things to be considered alive?