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Notes: Viruses SB3d. Compare and contrast viruses with living organisms. Characteristics of Living Things: Made of cells Grow & develop Use energy Respond to stimuli Can reproduce If something lacks any of these traits then it is considered NONLIVING. Viruses are considered Nonliving because: Viruses are not made of cells Viruses do not use energy Viruses do not grow & develop Viruses can only reproduce when they are inside of a living host cell Viruses can form crystals and remain viable for a long time Virus Structure Viruses are infectious particles that contain nucleic acid inside a protein coat. Virus Structure The nucleic acid can be DNA or RNA • (they don’t have both like living cells) The protein coat is called a capsid. • The shape of a virus is determined by the proteins in its capsid May also have a viral envelope formed from the host cell membrane. • It may be covered with spikes used to recognize and attach to cells it infects. Virus Structure Look in your book at page 575 Reproduction in Viruses Viral Replication- the process by which viruses reproduce. Reproduction in Viruses Lysogenic Cycle: 1. virus attaches to a host cell 2. virus injects its DNA or RNA into the host cell 3. viral DNA is added to the host cell’s DNA 4. viral DNA is copied along with the cells DNA 5. viral DNA is passed on to new daughter cells At some point the DNA activates itself and enters the lytic cycle. Reproduction in Viruses Lytic Cycle: Virus attaches Virus injects DNA host cell begins making new viral parts from viral mRNA New viruses are formed Cell is destroyed and new viruses are released from the cell These viruses then go and infects new cells Many viruses cause disease Difficult to cure because they replicate very quickly Undergo frequent mutations making it hard to find a way to destroy the virus Medications often attack & harm healthy cells The best defense we have are vaccines- a piece of the virus is injected into you and your body produces antibodies against it.