BIO-121 Lab 12 - The Viruses PDF

Lab 12 – The Viruses BIO-121 28.5 Viruses, Viroids, and Prions Viruses are acellular structures that require a living cell to reproduce (obligate parasites). Viruses must use the host cell’s replication machinery, such as ribosomes and certain...

Lab 12 – The Viruses BIO-121 28.5 Viruses, Viroids, and Prions Viruses are acellular structures that require a living cell to reproduce (obligate parasites). Viruses must use the host cell’s replication machinery, such as ribosomes and certain enzymes. Viroids are strands of RNA that can reproduce inside a cell. Prions are protein molecules that cause other proteins to become prions. Viral Size and Structure Smaller than bacteria (0.03 to 0.2 micrometer) Variety of shapes such as helix, sphere, polyhedron, and more complex forms. Two main components to all viruses. Capsid (outer portion comprised of proteins). Encloses nucleic acid core and sometimes additional viral enzymes. May be surrounded by a lipid envelope. May have spikes for attachment to a host cell. Nucleic acid core (DNA or RNA). Genome can be single- or double-stranded DNA or single- or double-stranded RNA. Viruses (photos) (a): ©Biophoto Associates/Science Viral Reproduction 1 Viruses infect almost every type of organism on earth. Bacteriophages infect bacteria. Some infect only plants or animals or humans. Viruses are specific to a particular host. Spikes on a virus configure exactly to receptor molecules on the membrane of a potential host cell. The reproductive cycle of a typical enveloped animal RNA virus has six steps. Life Cycle of an Animal Virus (RNA) Viral Reproduction 2 Latency. Some animal viruses can lie dormant within a host cell. Helps avoid detection from host immune system. Example: Herpesvirus and retroviruses. New viruses are not produced, but the viral genome is reproduced. Stress may stimulate the latent virus to enter the biosynthesis stage. Viral Reproduction 3 Retroviruses. Genome is RNA. Also contain the enzyme reverse transcriptase, which transcribes RNA into DNA. This DNA is called cDNA (copy DNA). cDNA can incorporate into the host cell genome (and becomes a provirus). It is resistant to antiviral medication. It is able to escape detection by the host immune system. Viral Diseases of Humans Common Cold and Influenza. Colds are caused mainly by rhinoviruses. Symptoms: runny nose, mild fever, fatigue. Duration: around a week. Flu is caused by influenza viruses. Symptoms: high fever, chills, body aches, severe fatigue. Duration: several weeks possible; can be fatal. Antigens on cold and flu viruses can change. Antigenic drift—small changes in the virus. Antigenic shift—new combinations of surface spikes; likely to cause illness, even pandemics. Antigenic Drift and Antigenic Shift Measles Measles. Very contagious human disease. Spread by respiratory route. 7- to 12-day incubation period before flu-like symptoms and rash appear. 10% to 15% fatality rate in less-developed countries. MMR vaccine protects against measles (and mumps and rubella). Herpesviruses Herpesviruses. Remain latent much of the time. Herpesviruses that cause disease in humans. Herpes simplex type 1: cold sores and fever blisters; very common. Herpes simplex type 2: genital herpes. Varicella-zoster: chickenpox and shingles. Epstein–Barr virus: infectious mononucleosis; very common. Antiviral Drugs Because viruses use the machinery of host cells for viral replication, it is difficult to develop drugs that affect viral replication without harming host cells. Some antiviral drugs can inhibit viral enzymes. Antiviral drugs can target viral replication enzymes. Other antiviral drugs may affect virus attachment or assembly. Often, no drugs are available for viral infections. Antibiotics are not effective against viruses. Viroids and Prions 1 Viroids and prions are also acellular pathogens. Viroids are naked, circular RNA molecules that do not code for proteins. Viroid RNA is much smaller than that of most viral genomes. Infects only plant cells and causes disease. Examples: potato spindle tuber and apple scar skin. Viroids and Prions 2 Prions are proteinaceous infectious particles. Normal proteins change their shape into abnormal prion shape; this causes other normal proteins to change their shape. Causes degenerative diseases in nervous system. Prions passed through ingestion of infected tissues. Scrapie—sheep. Bovine spongiform encephalopathy (“mad cow disease”)—cattle; can pass to humans. Creutzfeldt–Jakob disease—humans. Kuru—human to human transmission through cannibalism. Chronic wasting disease—deer, elk, and moose.