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In ancient Rome, ‘virus’ meant “poison” or “venomous”. This name was passed down when these extremely small particles were discovered in the late 1800’s. This is the image of a bacterium being attacked by viral particles. You can see the contrast in size easily. Find a lower-case ‘i’ on your notes page…. • Viruses are extremely small non-cellular, infectious agents composed of genetic material (DNA or RNA) enclosed in a protein coat. • You will see Bacteriophages (bottom left) in many examples.... Bacteriophage Viruses Flu Viruses Viral Structure Capsid • A virus has an inner core of nucleic acid, either RNA or DNA and an outer protein coat called a capsid. Nucleic acid Capsid Nucleic acid Envelope • Some larger viruses, (human flu viruses), have an additional layer surrounding their capsids, called an envelope (usually made of membrane remnants of previously infected cells). Vertebrate immune systems can detect certain viral proteins, HOWEVER the genes for viral proteins tend to mutate at very high frequencies, so viruses can often elude immune fighters For instance Influenza mutates often, that is why a new vaccine is created each year. All Living Things, as a species…: • Are made of cells, and possess a system of organization • Require, can metabolize, and use energy. • Grow and develop. • Reproduce. • Can adapt to changing environments. * viruses do not metabolize, grow, nor are they composed of cells. Also, they don’t possess a way to reproduce without a host. However, THAT is true of many parasites. • Viruses are PATHOGENS- infectious, diseasecausing agents that invade target organisms and multiply inside them or on them. • Disease follows when the pathogens take over metabolic activities of other cells- damaging body tissues and interfering with their normal functioning. Can a Virus Infect Any Cell? Nope, not if it can’t get in! So…how does it get into the host cell? It works like a lock and key. If the key doesn’t match a lock, you can’t open the door The “lock” of the cell is called a receptor site. If the shape of the virus does not match the host’s receptor site, the virus can not inject it’s nucleic acid or enter the cell. Virus INJECTION ENDOCYTOSIS Lytic Cycle: Faster cycle that quickly destroys the host. Steps of the Lytic Cycle: 1. Virus attaches to host 2. Virus injects nucleic acid 3. Viral nucleic acid takes over and re-programs host’s DNA 4. New viruses form 5. Lysis – cell bursts and releases new viruses Lysogenic Cycle: Slow cycle in which the viral DNA hides in the host cell • A lysongenic cycle begins in the same way as a lytic cycle. Virus attaches to a host cell Viral nucleic acid integrates itself into the host’s DNA Viral nucleic acid enters the host Lysogenic Cycle: Slow cycle in which the viral DNA hides in the host cell At any time, this provirus can enter the Lytic cycle…. This part can go on for years…. The host cell will divide, and each time it replicates it’s DNA, it also makes a copy of the viral nucleic acid. RNA Viruses