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Viruses Characteristics of Viruses • Virus – a small infectious agent of genetic material encased in protein • Not made of cells • Uses the host cell’s energy and machinery to copy itself • Cannot reproduce on their own • Non-living, not considered biotic • Cannot metabolize their own energy • Not considered microorganisms • Different structure than cells • Smaller than cells • Contain few specialized structures • Genetic material can be DNA or RNA • Capsid – the protein coat encasing the genetic material of a virus • Primary Types • Helical • Ex) Tobacco mosaic • Polyhedral • Ex) Adenovirus • Enveloped • Lipid layer from host cell • Bacteriophage – a virus that infects bacteria • Complex virus • Structure: protein tail with fibers • Irregularly shaped • Contain double-stranded DNA Biology 15.1 – Viruses Viruses Reproduction of Viruses • Parts of a virus enter the host cell • Metabolic machinery is hijacked • Viral DNA or RNA instructs the host cell to make copies of the virus • Viral proteins and genetic material are reproduced • Lytic pathway – a type of viral reproduction where the virus lyses, or breaks open the host cell membrane in order to release the newly replicated viruses • New viruses leave host cell to infect other nearby cells • Lysis – viruses burst the cell membrane as they leave • Results in death of host cell • Lysogenic pathway – a type of viral reproduction involving the incorporation of the viral genes into the host cell chromosomes • Host cell passively becomes genetic viral factory • Viral genes copied each time cell replicates via mitosis Biology 15.1 – Viruses Viruses Diseases Caused by Viruses • Helper T cells – type of white blood cells that play an important role in the immune system • HIV (human immunodeficiency virus) – a virus which causes AIDS and results in the loss of T cell and immune system functioning • Helper T cells become useless in fighting disease • AIDS (acquired immune deficiency syndrome) – a syndrome characterized by the loss of the immune system and caused by HIV, the human immunodeficiency virus • Die from secondary infections • No vaccine • Limiting and avoiding transmission of virus can prevent infection • HIV exits by budding from the host cell • Envelope of virus fuses to plasma membrane of host cell • Virus exits cell wrapped in host cell membrane • Initial infection: number of helper T cells drops • Large amount of HIV genome produced • Clinical latency: virus enters lysogenic pathway • Viral genomes lay mostly dormant • Lytic cycle resumes • Steady decrease in helper T cells • More HIV viruses are produced • More immune cells infected • When enough helper T cells are destroyed, the patient is diagnosed with AIDS Biology 15.1 – Viruses Viruses • Influenza – a viral disease commonly known as “the flu” • Symptoms of fever, fatigue, respiratory infections • May cause death • Caused by a variety of influenza viruses • Strains mutate often • Receiving seasonal vaccine can help prevent • Washing hands frequently and avoiding contact with those infected can reduce transmission • Common cold – an infection caused by a variety of viruses, most commonly rhinoviruses • Symptoms similar to the flu but less intense • Currently no vaccine • Reduce transmission by washing hands frequently and avoid contact with those infected • Hepatitis A – a virus that causes inflammation of the liver • Produces a jaundice, or yellow, appearance of skin and eyes • Virus is carried through infected food or water • Vaccine can prevent • Transmission can be limited by washing hands and food and using clean water Biology 15.1 – Viruses